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Journal articles on the topic 'Spin qubits'
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1
Takeda, Kenta, Akito Noiri, Takashi Nakajima, Takashi Kobayashi, and Seigo Tarucha. "Quantum error correction with silicon spin qubits." Nature 608, no. 7924 (2022): 682–86. http://dx.doi.org/10.1038/s41586-022-04986-6.
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AbstractFuture large-scale quantum computers will rely on quantum error correction (QEC) to protect the fragile quantum information during computation1,2. Among the possible candidate platforms for realizing quantum computing devices, the compatibility with mature nanofabrication technologies of silicon-based spin qubits offers promise to overcome the challenges in scaling up device sizes from the prototypes of today to large-scale computers3–5. Recent advances in silicon-based qubits have enabled the implementations of high-quality one-qubit and two-qubit systems6–8. However, the demonstration of QEC, which requires three or more coupled qubits1, and involves a three-qubit gate9–11 or measurement-based feedback, remains an open challenge. Here we demonstrate a three-qubit phase-correcting code in silicon, in which an encoded three-qubit state is protected against any phase-flip error on one of the three qubits. The correction to this encoded state is performed by a three-qubit conditional rotation, which we implement by an efficient single-step resonantly driven iToffoli gate. As expected, the error correction mitigates the errors owing to one-qubit phase-flip, as well as the intrinsic dephasing mainly owing to quasi-static phase noise. These results show successful implementation of QEC and the potential of a silicon-based platform for large-scale quantum computing.
2
Tahan, Charles. "Opinion: Democratizing Spin Qubits." Quantum 5 (November 18, 2021): 584. http://dx.doi.org/10.22331/q-2021-11-18-584.
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I've been building Powerpoint-based quantum computers with electron spins in silicon for 20 years. Unfortunately, real-life-based quantum dot quantum computers are harder to implement. Materials, fabrication, and control challenges still impede progress. The way to accelerate discovery is to make and measure more qubits. Here I discuss separating the qubit realization and testing circuitry from the materials science and on-chip fabrication that will ultimately be necessary. This approach should allow us, in the shorter term, to characterize wafers non-invasively for their qubit-relevant properties, to make small qubit systems on various different materials with little extra cost, and even to test spin-qubit to superconducting cavity entanglement protocols where the best possible cavity quality is preserved. Such a testbed can advance the materials science of semiconductor quantum information devices and enable small quantum computers. This article may also be useful as a light and light-hearted introduction to quantum dot spin qubits.
3
Aldeghi, Michele, Rolf Allenspach, and Gian Salis. "Modular nanomagnet design for spin qubits confined in a linear chain." Applied Physics Letters 122, no. 13 (2023): 134003. http://dx.doi.org/10.1063/5.0139670.
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On-chip micromagnets enable electrically controlled quantum gates on electron spin qubits. Extending the concept to a large number of qubits is challenging in terms of providing large enough driving gradients and individual addressability. Here, we present a design aimed at driving spin qubits arranged in a linear chain and strongly confined in directions lateral to the chain. Nanomagnets are placed laterally to the one side of the qubit chain, one nanomagnet per two qubits. The individual magnets are “U”-shaped, such that the magnetic shape anisotropy orients the magnetization alternately toward and against the qubit chain even if an external magnetic field is applied along the qubit chain. The longitudinal and transversal stray field components serve as addressability and driving fields. Using micromagnetic simulations, we calculate driving and dephasing rates and the corresponding qubit quality factor. The concept is validated with spin-polarized scanning electron microscopy of Fe nanomagnets fabricated on silicon substrates, finding excellent agreement with micromagnetic simulations. Several features required for a scalable spin qubit design are met in our approach: strong driving and weak dephasing gradients, reduced crosstalk and operation at low external magnetic fields.
4
WU, YIN-ZHONG, WEI-MIN ZHANG, and CHOPIN SOO. "QUANTUM COMPUTATION BASED ON ELECTRON SPIN QUBITS WITHOUT SPIN-SPIN INTERACTION." International Journal of Quantum Information 03, supp01 (2005): 155–62. http://dx.doi.org/10.1142/s0219749905001341.
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Using electron spin states in a unit cell of three semiconductor quantum dots as qubit states, a scalable quantum computation scheme is advocated without invoking qubit-qubit interactions. Single electron tunneling technology and coherent quantum-dot cellular automata architecture are used to generate an ancillary charge entangled state which is then converted into spin entangled state. Without using charge measurement and ancillary qubits, we demonstrate universal quantum computation based on free electron spin and coherent quantum-dot cellular automata.
5
Schönenberger, Christian. "Andreev‐Qubit‐Qubit‐Kopplung auf Distanz." Physik in unserer Zeit 56, no. 2 (2025): 60–61. https://doi.org/10.1002/piuz.202570205.
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Mikroskopische Andreev‐Qubits lassen sich nun kohärent über makroskopische Distanzen koppeln, was die Erzeugung von verschränkten Zuständen und allgemeinen Zwei‐Qubit‐Operationen ermöglicht. Dies konnte kürzlich sowohl für Andreev‐Paar‐Qubits als auch für Andreev‐Spin‐Qubits demonstriert werden.
6
Yamamoto, Satoru, Shigeaki Nakazawa, Kenji Sugisaki, et al. "Adiabatic quantum computing with spin qubits hosted by molecules." Physical Chemistry Chemical Physics 17, no. 4 (2015): 2742–49. http://dx.doi.org/10.1039/c4cp04744c.
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Hu, Rui-Zi, Rong-Long Ma, Ming Ni, et al. "Flopping-mode spin qubit in a Si-MOS quantum dot." Applied Physics Letters 122, no. 13 (2023): 134002. http://dx.doi.org/10.1063/5.0137259.
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Spin qubits based on silicon metal-oxide semiconductor (Si-MOS) quantum dots (QDs) are promising platforms for large-scale quantum computers. To control spin qubits in QDs, electric dipole spin resonance (EDSR) has been most commonly used in recent years. By delocalizing an electron across a double quantum dots charge state, “flopping-mode” EDSR has been realized in Si/SiGe QDs. Here, we demonstrate a flopping-mode spin qubit in a Si-MOS QD via Elzerman single-shot readout. When changing the detuning with a fixed drive power, we achieve s-shape spin resonance frequencies, an order of magnitude improvement in the spin Rabi frequencies, and virtually constant spin dephasing times. Our results offer a route to large-scale spin qubit systems with higher control fidelity in Si-MOS QDs.
8
Bahari, Iskandar, Timothy P. Spiller, Shane Dooley, Anthony Hayes, and Francis McCrossan. "Collapse and revival of entanglement between qubits coupled to a spin coherent state." International Journal of Quantum Information 16, no. 02 (2018): 1850017. http://dx.doi.org/10.1142/s021974991850017x.
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We extend the study of the Jayne–Cummings (JC) model involving a pair of identical two-level atoms (or qubits) interacting with a single mode quantized field. We investigate the effects of replacing the radiation field mode with a composite spin, comprising [Formula: see text] qubits, or spin-1/2 particles. This model is relevant for physical implementations in superconducting circuit QED, ion trap and molecular systems. For the case of the composite spin prepared in a spin coherent state, we demonstrate the similarities of this set-up to the qubits-field model in terms of the time evolution, attractor states and in particular the collapse and revival of the entanglement between the two qubits. We extend our analysis by taking into account an effect due to qubit imperfections. We consider a difference (or “mismatch”) in the dipole interaction strengths of the two qubits, for both the field mode and composite spin cases. To address decoherence due to this mismatch, we then average over this coupling strength difference with distributions of varying width. We demonstrate in both the field mode and the composite spin scenarios that increasing the width of the “error” distribution increases suppression of the coherent dynamics of the coupled system, including the collapse and revival of the entanglement between the qubits.
9
Ferraro, Elena, and Marco De Michielis. "Bandwidth-Limited and Noisy Pulse Sequences for Single Qubit Operations in Semiconductor Spin Qubits." Entropy 21, no. 11 (2019): 1042. http://dx.doi.org/10.3390/e21111042.
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Spin qubits are very valuable and scalable candidates in the area of quantum computation and simulation applications. In the last decades, they have been deeply investigated from a theoretical point of view and realized on the scale of few devices in the laboratories. In semiconductors, spin qubits can be built confining the spin of electrons in electrostatically defined quantum dots. Through this approach, it is possible to create different implementations: single electron spin qubit, singlet–triplet spin qubit, or a three-electron architecture, e.g., the hybrid qubit. For each qubit type, we study the single qubit rotations along the principal axis of Bloch sphere including the mandatory non-idealities of the control signals that realize the gate operations. The realistic transient of the control signal pulses are obtained by adopting an appropriate low-pass filter function. In addition. the effect of disturbances on the input signals is taken into account by using a Gaussian noise model.
10
Vlasov, Alexander Yu. "Quantum circuits and Spin(3n) groups." Quantum Information and Computation 15, no. 3&4 (2015): 235–59. http://dx.doi.org/10.26421/qic15.3-4-3.
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All quantum gates with one and two qubits may be described by elements of Spin groups due to isomorphisms Spin(3)\isomSU(2) and Spin(6)\isomSU(4). However, the group of n-qubit gates SU(2^n) for n>2 has bigger dimension than Spin(3n). A quantum circuit with one- and two-qubit gates may be used for construction of arbitrary unitary transformation SU(2^n). Analogously, the `$Spin(3n)$ circuits' are introduced in this work as products of elements associated with one- and two-qubit gates with respect to the above-mentioned isomorphisms. The matrix tensor product implementation of the Spin(3n) group together with relevant models by usual quantum circuits with 2n qubits are investigated in such a framework. A certain resemblance with well-known sets of non-universal quantum gates (e.g., matchgates, noninteracting-fermion quantum circuits) related with Spin(2n) may be found in presented approach. Finally, a possibility of the classical simulation of such circuits in polynomial time is discussed.
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RAO, K. RAMA KOTESWARA, and ANIL KUMAR. "ENTANGLEMENT IN A 3-SPIN HEISENBERG-XY CHAIN WITH NEAREST-NEIGHBOR INTERACTIONS, SIMULATED IN AN NMR QUANTUM SIMULATOR." International Journal of Quantum Information 10, no. 04 (2012): 1250039. http://dx.doi.org/10.1142/s0219749912500396.
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The evolution of entanglement in a 3-spin chain with nearest-neighbor Heisenberg-XY interactions for different initial states is investigated here. In an NMR experimental implementation, we generate multipartite entangled states starting from initial separable pseudo-pure states by simulating nearest-neighbor XY interactions in a 3-spin linear chain of nuclear spin qubits. For simulating XY interactions, we follow algebraic method of Zhang et al. [Phys. Rev. A72 (2005) 012331]. Bell state between end qubits has been generated by using only the unitary evolution of the XY Hamiltonian. For generating W-state and GHZ-state a single qubit rotation is applied on second and all the three qubits, respectively after the unitary evolution of the XY Hamiltonian.
12
Hu, Rui-Zi, Rong-Long Ma, Ming Ni, et al. "An Operation Guide of Si-MOS Quantum Dots for Spin Qubits." Nanomaterials 11, no. 10 (2021): 2486. http://dx.doi.org/10.3390/nano11102486.
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In the last 20 years, silicon quantum dots have received considerable attention from academic and industrial communities for research on readout, manipulation, storage, near-neighbor and long-range coupling of spin qubits. In this paper, we introduce how to realize a single spin qubit from Si-MOS quantum dots. First, we introduce the structure of a typical Si-MOS quantum dot and the experimental setup. Then, we show the basic properties of the quantum dot, including charge stability diagram, orbital state, valley state, lever arm, electron temperature, tunneling rate and spin lifetime. After that, we introduce the two most commonly used methods for spin-to-charge conversion, i.e., Elzerman readout and Pauli spin blockade readout. Finally, we discuss the details of how to find the resonance frequency of spin qubits and show the result of coherent manipulation, i.e., Rabi oscillation. The above processes constitute an operation guide for helping the followers enter the field of spin qubits in Si-MOS quantum dots.
13
Wang, Yu, Yi Chen, Hong T. Bui, et al. "An atomic-scale multi-qubit platform." Science 382, no. 6666 (2023): 87–92. http://dx.doi.org/10.1126/science.ade5050.
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Individual electron spins in solids are promising candidates for quantum science and technology, where bottom-up assembly of a quantum device with atomically precise couplings has long been envisioned. Here, we realized atom-by-atom construction, coherent operations, and readout of coupled electron-spin qubits using a scanning tunneling microscope. To enable the coherent control of “remote” qubits that are outside of the tunnel junction, we complemented each electron spin with a local magnetic field gradient from a nearby single-atom magnet. Readout was achieved by using a sensor qubit in the tunnel junction and implementing pulsed double electron spin resonance. Fast single-, two-, and three-qubit operations were thereby demonstrated in an all-electrical fashion. Our angstrom-scale qubit platform may enable quantum functionalities using electron spin arrays built atom by atom on a surface.
14
Tanuma, Yuri, Anastasios Stergiou, Andreja Bužan Bobnar, et al. "Robust coherent spin centers from stable azafullerene radicals entrapped in cycloparaphenylene rings." Nanoscale 13, no. 47 (2021): 19946–55. http://dx.doi.org/10.1039/d1nr06393f.
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Stable and abundant spin-1/2 species from azafullerene (C59N˙) supramolecularly hosted in [10]cycloparaphenylene nanohoops are operated as stable qubits, with possibility of qubit wiring via intermediate polymerized spin-redistributed states.
15
Knaut, C. M., A. Suleymanzade, Y. C. Wei, et al. "Entanglement of nanophotonic quantum memory nodes in a telecom network." Nature 629, no. 8012 (2024): 573–78. http://dx.doi.org/10.1038/s41586-024-07252-z.
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AbstractA key challenge in realizing practical quantum networks for long-distance quantum communication involves robust entanglement between quantum memory nodes connected by fibre optical infrastructure1–3. Here we demonstrate a two-node quantum network composed of multi-qubit registers based on silicon-vacancy (SiV) centres in nanophotonic diamond cavities integrated with a telecommunication fibre network. Remote entanglement is generated by the cavity-enhanced interactions between the electron spin qubits of the SiVs and optical photons. Serial, heralded spin-photon entangling gate operations with time-bin qubits are used for robust entanglement of separated nodes. Long-lived nuclear spin qubits are used to provide second-long entanglement storage and integrated error detection. By integrating efficient bidirectional quantum frequency conversion of photonic communication qubits to telecommunication frequencies (1,350 nm), we demonstrate the entanglement of two nuclear spin memories through 40 km spools of low-loss fibre and a 35-km long fibre loop deployed in the Boston area urban environment, representing an enabling step towards practical quantum repeaters and large-scale quantum networks.
16
Hill, Charles D., Eldad Peretz, Samuel J. Hile, et al. "A surface code quantum computer in silicon." Science Advances 1, no. 9 (2015): e1500707. http://dx.doi.org/10.1126/sciadv.1500707.
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The exceptionally long quantum coherence times of phosphorus donor nuclear spin qubits in silicon, coupled with the proven scalability of silicon-based nano-electronics, make them attractive candidates for large-scale quantum computing. However, the high threshold of topological quantum error correction can only be captured in a two-dimensional array of qubits operating synchronously and in parallel—posing formidable fabrication and control challenges. We present an architecture that addresses these problems through a novel shared-control paradigm that is particularly suited to the natural uniformity of the phosphorus donor nuclear spin qubit states and electronic confinement. The architecture comprises a two-dimensional lattice of donor qubits sandwiched between two vertically separated control layers forming a mutually perpendicular crisscross gate array. Shared-control lines facilitate loading/unloading of single electrons to specific donors, thereby activating multiple qubits in parallel across the array on which the required operations for surface code quantum error correction are carried out by global spin control. The complexities of independent qubit control, wave function engineering, and ad hoc quantum interconnects are explicitly avoided. With many of the basic elements of fabrication and control based on demonstrated techniques and with simulated quantum operation below the surface code error threshold, the architecture represents a new pathway for large-scale quantum information processing in silicon and potentially in other qubit systems where uniformity can be exploited.
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Koiller, Belita, Xuedong Hu, Rodrigo B. Capaz, Adriano S. Martins, and Sankar Das Sarma. "Silicon-based spin and charge quantum computation." Anais da Academia Brasileira de Ciências 77, no. 2 (2005): 201–22. http://dx.doi.org/10.1590/s0001-37652005000200002.
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Silicon-based quantum-computer architectures have attracted attention because of their promise for scalability and their potential for synergetically utilizing the available resources associated with the existing Si technology infrastructure. Electronic and nuclear spins of shallow donors (e.g. phosphorus) in Si are ideal candidates for qubits in such proposals due to the relatively long spin coherence times. For these spin qubits, donor electron charge manipulation by external gates is a key ingredient for control and read-out of single-qubit operations, while shallow donor exchange gates are frequently invoked to perform two-qubit operations. More recently, charge qubits based on tunnel coupling in P+2 substitutional molecular ions in Si have also been proposed. We discuss the feasibility of the building blocks involved in shallow donor quantum computation in silicon, taking into account the peculiarities of silicon electronic structure, in particular the six degenerate states at the conduction band edge. We show that quantum interference among these states does not significantly affect operations involving a single donor, but leads to fast oscillations in electron exchange coupling and on tunnel-coupling strength when the donor pair relative position is changed on a lattice-parameter scale. These studies illustrate the considerable potential as well as the tremendous challenges posed by donor spin and charge as candidates for qubits in silicon.
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Cuccoli, Alessandro, Davide Nuzzi, Ruggero Vaia, and Paola Verrucchi. "Using solitons for manipulating qubits." International Journal of Quantum Information 12, no. 02 (2014): 1461013. http://dx.doi.org/10.1142/s0219749914610139.
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Many proposals for quantum devices are based on qubits that are physically realized by the spin magnetic moment of some quantum object. In this case, one of the most often adopted strategies for manipulating qubits is that of using external magnetic fields. However, selectively applying a field just to one qubit may be a practically unattainable goal, as it is, for instance, in most solid-state based setups. In this work, we present a proposal for using nonlinear excitations of solitonic type to accomplish the above task. Our scheme entails the generation of a dynamical soliton in a classical spin-chain which is locally coupled with one qubit: as the soliton runs through, the qubit behaves, due to its interaction with the chain, as if it were subject to a magnetic field with a time dependence that follows from the soliton's features. We here present results for the time evolution of the qubit density-matrix induced by the overall dynamics of the above scheme.
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Xue, Xiao, Maximilian Russ, Nodar Samkharadze, et al. "Quantum logic with spin qubits crossing the surface code threshold." Nature 601, no. 7893 (2022): 343–47. http://dx.doi.org/10.1038/s41586-021-04273-w.
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AbstractHigh-fidelity control of quantum bits is paramount for the reliable execution of quantum algorithms and for achieving fault tolerance—the ability to correct errors faster than they occur1. The central requirement for fault tolerance is expressed in terms of an error threshold. Whereas the actual threshold depends on many details, a common target is the approximately 1% error threshold of the well-known surface code2,3. Reaching two-qubit gate fidelities above 99% has been a long-standing major goal for semiconductor spin qubits. These qubits are promising for scaling, as they can leverage advanced semiconductor technology4. Here we report a spin-based quantum processor in silicon with single-qubit and two-qubit gate fidelities, all of which are above 99.5%, extracted from gate-set tomography. The average single-qubit gate fidelities remain above 99% when including crosstalk and idling errors on the neighbouring qubit. Using this high-fidelity gate set, we execute the demanding task of calculating molecular ground-state energies using a variational quantum eigensolver algorithm5. Having surpassed the 99% barrier for the two-qubit gate fidelity, semiconductor qubits are well positioned on the path to fault tolerance and to possible applications in the era of noisy intermediate-scale quantum devices.
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Miao, Kevin C., Joseph P. Blanton, Christopher P. Anderson, et al. "Universal coherence protection in a solid-state spin qubit." Science 369, no. 6510 (2020): 1493–97. http://dx.doi.org/10.1126/science.abc5186.
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Decoherence limits the physical realization of qubits, and its mitigation is critical for the development of quantum science and technology. We construct a robust qubit embedded in a decoherence-protected subspace, obtained by applying microwave dressing to a clock transition of the ground-state electron spin of a silicon carbide divacancy defect. The qubit is universally protected from magnetic, electric, and temperature fluctuations, which account for nearly all relevant decoherence channels in the solid state. This culminates in an increase of the qubit’s inhomogeneous dephasing time by more than four orders of magnitude (to >22 milliseconds), while its Hahn-echo coherence time approaches 64 milliseconds. Requiring few key platform-independent components, this result suggests that substantial coherence improvements can be achieved in a wide selection of quantum architectures.
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Koh, C. Y. "Entanglement and quantum spin glass." International Journal of Modern Physics B 28, no. 20 (2014): 1430012. http://dx.doi.org/10.1142/s0217979214300126.
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The paper reviews the entanglement behavior of a 2-qubit system in a quantum spin glass using the Heisenberg XX model and the interaction of the system with a spin glass bath environment. In the first part, we study the entanglement (concurrence) for a 3- and 4-qubit with nearest neighbor interaction. With a fixed mean and varying standard deviation for the J coupling, the concurrence is numerically plotted with temperature for the different configurations. A general formula for the concurrence is given for n qubits at low temperature. In the second part, we study the concurrence of a 2-qubit system coupled to a spin glass bath environment with n = 2 to ≥ 4 qubits. The bath sites are coupled with random J coupling and varying applied magnetic field. A general formula for concurrence is given for mean J = 0 and B = 0 for n bath sites. For small random J and magnetic field B, a steady state is obtained with an approximate concurrence of 0.5, showing that the entanglement is preserved.
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Neyens, Samuel, Otto K. Zietz, Thomas F. Watson, et al. "Probing single electrons across 300-mm spin qubit wafers." Nature 629, no. 8010 (2024): 80–85. http://dx.doi.org/10.1038/s41586-024-07275-6.
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AbstractBuilding a fault-tolerant quantum computer will require vast numbers of physical qubits. For qubit technologies based on solid-state electronic devices1–3, integrating millions of qubits in a single processor will require device fabrication to reach a scale comparable to that of the modern complementary metal–oxide–semiconductor (CMOS) industry. Equally important, the scale of cryogenic device testing must keep pace to enable efficient device screening and to improve statistical metrics such as qubit yield and voltage variation. Spin qubits1,4,5 based on electrons in Si have shown impressive control fidelities6–9 but have historically been challenged by yield and process variation10–12. Here we present a testing process using a cryogenic 300-mm wafer prober13 to collect high-volume data on the performance of hundreds of industry-manufactured spin qubit devices at 1.6 K. This testing method provides fast feedback to enable optimization of the CMOS-compatible fabrication process, leading to high yield and low process variation. Using this system, we automate measurements of the operating point of spin qubits and investigate the transitions of single electrons across full wafers. We analyse the random variation in single-electron operating voltages and find that the optimized fabrication process leads to low levels of disorder at the 300-mm scale. Together, these results demonstrate the advances that can be achieved through the application of CMOS-industry techniques to the fabrication and measurement of spin qubit devices.
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Rogers, Ciaran, Deepak Asthana, Adam Brookfield, et al. "Modelling Conformational Flexibility in a Spectrally Addressable Molecular Multi-Qubit Model System." Angewandte Chemie Internation Edition 61, no. 45 (2022): e202207947. https://doi.org/10.1002/anie.202207947.
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Dipolar coupled multi-spin systems have the potential to be used as molecular qubits. Herein we report the synthesis of a molecular multi-qubit model system with three individually addressable, weakly interacting, spin 1=2 centres of differing g-values. We use pulsed Electron Paramagnetic Resonance (EPR) techniques to characterise and separately address the individual electron spin qubits; CuII, Cr7Ni ring and a nitroxide, to determine the strength of the inter-qubit dipolar interaction. Orientation selective Relaxation-Induced Dipolar Modulation Enhancement (os-RIDME) detecting across the CuII spectrum revealed a strongly correlated CuII-Cr7Ni ring relationship; detecting on the nitroxide resonance measured both the nitroxide and CuII or nitroxide and Cr7Ni ring correlations, with switchability of the interaction based on differing relaxation dynamics, indicating a handle for implementing EPR-based quantum information processing (QIP) algorithms.
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Mani, Tomoyasu. "Molecular qubits based on photogenerated spin-correlated radical pairs for quantum sensing." Chemical Physics Reviews 3, no. 2 (2022): 021301. http://dx.doi.org/10.1063/5.0084072.
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Photogenerated spin-correlated radical pairs (SCRPs) in electron donor–bridge–acceptor (D–B–A) molecules can act as molecular qubits and inherently spin qubit pairs. SCRPs can take singlet and triplet spin states, comprising the quantum superposition state. Their synthetic accessibility and well-defined structures, together with their ability to be prepared in an initially pure, entangled spin state and optical addressability, make them one of the promising avenues for advancing quantum information science. Coherence between two spin states and spin selective electron transfer reactions form the foundation of using SCRPs as qubits for sensing. We can exploit the unique sensitivity of the spin dynamics of SCRPs to external magnetic fields for sensing applications including resolution-enhanced imaging, magnetometers, and magnetic switch. Molecular quantum sensors, if realized, can provide new technological developments beyond what is possible with classical counterparts. While the community of spin chemistry has actively investigated magnetic field effects on chemical reactions via SCRPs for several decades, we have not yet fully exploited the synthetic tunability of molecular systems to our advantage. This review offers an introduction to the photogenerated SCRPs-based molecular qubits for quantum sensing, aiming to lay the foundation for researchers new to the field and provide a basic reference for researchers active in the field. We focus on the basic principles necessary to construct molecular qubits based on SCRPs and the examples in quantum sensing explored to date from the perspective of the experimentalist.
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Huang, Jonathan Y., Rocky Y. Su, Wee Han Lim, et al. "High-fidelity spin qubit operation and algorithmic initialization above 1 K." Nature 627, no. 8005 (2024): 772–77. http://dx.doi.org/10.1038/s41586-024-07160-2.
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AbstractThe encoding of qubits in semiconductor spin carriers has been recognized as a promising approach to a commercial quantum computer that can be lithographically produced and integrated at scale1–10. However, the operation of the large number of qubits required for advantageous quantum applications11–13 will produce a thermal load exceeding the available cooling power of cryostats at millikelvin temperatures. As the scale-up accelerates, it becomes imperative to establish fault-tolerant operation above 1 K, at which the cooling power is orders of magnitude higher14–18. Here we tune up and operate spin qubits in silicon above 1 K, with fidelities in the range required for fault-tolerant operations at these temperatures19–21. We design an algorithmic initialization protocol to prepare a pure two-qubit state even when the thermal energy is substantially above the qubit energies and incorporate radiofrequency readout to achieve fidelities up to 99.34% for both readout and initialization. We also demonstrate single-qubit Clifford gate fidelities up to 99.85% and a two-qubit gate fidelity of 98.92%. These advances overcome the fundamental limitation that the thermal energy must be well below the qubit energies for the high-fidelity operation to be possible, surmounting a main obstacle in the pathway to scalable and fault-tolerant quantum computation.
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Hays, M., V. Fatemi, D. Bouman, et al. "Coherent manipulation of an Andreev spin qubit." Science 373, no. 6553 (2021): 430–33. http://dx.doi.org/10.1126/science.abf0345.
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Two promising architectures for solid-state quantum information processing are based on electron spins electrostatically confined in semiconductor quantum dots and the collective electrodynamic modes of superconducting circuits. Superconducting electrodynamic qubits involve macroscopic numbers of electrons and offer the advantage of larger coupling, whereas semiconductor spin qubits involve individual electrons trapped in microscopic volumes but are more difficult to link. We combined beneficial aspects of both platforms in the Andreev spin qubit: the spin degree of freedom of an electronic quasiparticle trapped in the supercurrent-carrying Andreev levels of a Josephson semiconductor nanowire. We performed coherent spin manipulation by combining single-shot circuit–quantum-electrodynamics readout and spin-flipping Raman transitions and found a spin-flip time TS = 17 microseconds and a spin coherence time T2E = 52 nanoseconds. These results herald a regime of supercurrent-mediated coherent spin-photon coupling at the single-quantum level.
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Vahapoglu, Ensar, James P. Slack-Smith, Ross C. C. Leon, et al. "Single-electron spin resonance in a nanoelectronic device using a global field." Science Advances 7, no. 33 (2021): eabg9158. http://dx.doi.org/10.1126/sciadv.abg9158.
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Spin-based silicon quantum electronic circuits offer a scalable platform for quantum computation, combining the manufacturability of semiconductor devices with the long coherence times afforded by spins in silicon. Advancing from current few-qubit devices to silicon quantum processors with upward of a million qubits, as required for fault-tolerant operation, presents several unique challenges, one of the most demanding being the ability to deliver microwave signals for large-scale qubit control. Here, we demonstrate a potential solution to this problem by using a three-dimensional dielectric resonator to broadcast a global microwave signal across a quantum nanoelectronic circuit. Critically, this technique uses only a single microwave source and is capable of delivering control signals to millions of qubits simultaneously. We show that the global field can be used to perform spin resonance of single electrons confined in a silicon double quantum dot device, establishing the feasibility of this approach for scalable spin qubit control.
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Royer, Baptiste, Arne L. Grimsmo, Nicolas Didier, and Alexandre Blais. "Fast and high-fidelity entangling gate through parametrically modulated longitudinal coupling." Quantum 1 (May 11, 2017): 11. http://dx.doi.org/10.22331/q-2017-05-11-11.
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We investigate an approach to universal quantum computation based on the modulation of longitudinal qubit-oscillator coupling. We show how to realize a controlled-phase gate by simultaneously modulating the longitudinal coupling of two qubits to a common oscillator mode. In contrast to the more familiar transversal qubit-oscillator coupling, the magnitude of the effective qubit-qubit interaction does not rely on a small perturbative parameter. As a result, this effective interaction strength can be made large, leading to short gate times and high gate fidelities. We moreover show how the gate infidelity can be exponentially suppressed with squeezing and how the entangling gate can be generalized to qubits coupled to separate oscillators. Our proposal can be realized in multiple physical platforms for quantum computing, including superconducting and spin qubits.
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APPN Editorial Office. "Highlights from the Asia Pacific Region." Asia Pacific Physics Newsletter 02, no. 02 (2013): 29–46. http://dx.doi.org/10.1142/s2251158x13000271.
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Quantum information technologies hold the promise of greatly outperforming traditional approaches in, e.g., cryptography, metrology and simulation. However, the ultimate goal of realizing scalable quantum computing has so far remained elusive, largely owing to the formidable difficulty in "wiring up" suitable quantum bits (qubits). In recent years, individual nitrogen-vacancy (NV-) defects in diamond have emerged as one of the most promising candidates for a solidstate qubit for two reasons. First, they possess the longest observed room-temperature coherence time of an electron spin (the qubit) to date; second, their spin can be initialized and measured with a nanoscale resolution using optical techniques under ambient conditions. However, interconnecting different NV- centres remains a big challenge. This problem is further exacerbated by the need for a large spatial separation between adjacent qubits, required for individual qubit addressability.
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Mądzik, Mateusz T., Thaddeus D. Ladd, Fay E. Hudson, et al. "Controllable freezing of the nuclear spin bath in a single-atom spin qubit." Science Advances 6, no. 27 (2020): eaba3442. http://dx.doi.org/10.1126/sciadv.aba3442.
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The quantum coherence and gate fidelity of electron spin qubits in semiconductors are often limited by nuclear spin fluctuations. Enrichment of spin-zero isotopes in silicon markedly improves the dephasing time T2*, which, unexpectedly, can extend two orders of magnitude beyond theoretical expectations. Using a single-atom 31P qubit in enriched 28Si, we show that the abnormally long T2* is due to the freezing of the dynamics of the residual 29Si nuclei, caused by the electron-nuclear hyperfine interaction. Inserting a waiting period when the electron is controllably removed unfreezes the nuclear dynamics and restores the ergodic T2* value. Our conclusions are supported by a nearly parameter-free modeling of the 29Si nuclear spin dynamics, which reveals the degree of backaction provided by the electron spin. This study clarifies the limits of ergodic assumptions in nuclear bath dynamics and provides previously unidentified strategies for maximizing coherence and gate fidelity of spin qubits in semiconductors.
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Boerkamp, Martijn. "Six-qubit silicon quantum processor sets a record." Physics World 35, no. 12 (2022): 6i. http://dx.doi.org/10.1088/2058-7058/35/12/06.
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Piot, N., B. Brun, V. Schmitt, et al. "A single hole spin with enhanced coherence in natural silicon." Nature Nanotechnology, September 22, 2022. http://dx.doi.org/10.1038/s41565-022-01196-z.
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AbstractSemiconductor spin qubits based on spin–orbit states are responsive to electric field excitations, allowing for practical, fast and potentially scalable qubit control. Spin electric susceptibility, however, renders these qubits generally vulnerable to electrical noise, which limits their coherence time. Here we report on a spin–orbit qubit consisting of a single hole electrostatically confined in a natural silicon metal-oxide-semiconductor device. By varying the magnetic field orientation, we reveal the existence of operation sweet spots where the impact of charge noise is minimized while preserving an efficient electric-dipole spin control. We correspondingly observe an extension of the Hahn-echo coherence time up to 88 μs, exceeding by an order of magnitude existing values reported for hole spin qubits, and approaching the state-of-the-art for electron spin qubits with synthetic spin–orbit coupling in isotopically purified silicon. Our finding enhances the prospects of silicon-based hole spin qubits for scalable quantum information processing.
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Hu, Ye-Bin, Rong Chen, Guo-Qing Yan, and Xing-Yu Zhu. "Long-range entanglement between spin qubits in quantum dots by virtual photon process." Modern Physics Letters A, July 7, 2023. http://dx.doi.org/10.1142/s0217732323500530.
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Electron spin qubits in silicon quantum dots are an attractive candidate for large-scalable quantum computation. An essential step for quantum information processing based on spin qubits is to realize the spatially separated two-qubit gate and entanglement with high fidelity. Here, we consider two spin qubits coupled to a common superconducting resonator in circuit quantum electrodynamics. We investigate the long-range two-qubit iSWAP gate mediated by virtual microwave photons using a Gaussian smoothing pulse. We show that the entangling gate fidelity can reach [Formula: see text] under realistic experimental conditions and analyze the factors limiting gate fidelity. Moreover, we numerically demonstrate the generation of remote Bell entangled states of spin qubits with high fidelity. In addition, this spin–resonator architecture can be used to implement quantum algorithms using our scheme. These results pave the way for quantum information processing with spin qubits.
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Noiri, Akito, Kenta Takeda, Takashi Nakajima, et al. "A shuttling-based two-qubit logic gate for linking distant silicon quantum processors." Nature Communications 13, no. 1 (2022). http://dx.doi.org/10.1038/s41467-022-33453-z.
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AbstractControl of entanglement between qubits at distant quantum processors using a two-qubit gate is an essential function of a scalable, modular implementation of quantum computation. Among the many qubit platforms, spin qubits in silicon quantum dots are promising for large-scale integration along with their nanofabrication capability. However, linking distant silicon quantum processors is challenging as two-qubit gates in spin qubits typically utilize short-range exchange coupling, which is only effective between nearest-neighbor quantum dots. Here we demonstrate a two-qubit gate between spin qubits via coherent spin shuttling, a key technology for linking distant silicon quantum processors. Coherent shuttling of a spin qubit enables efficient switching of the exchange coupling with an on/off ratio exceeding 1000, while preserving the spin coherence by 99.6% for the single shuttling between neighboring dots. With this shuttling-mode exchange control, we demonstrate a two-qubit controlled-phase gate with a fidelity of 93%, assessed via randomized benchmarking. Combination of our technique and a phase coherent shuttling of a qubit across a large quantum dot array will provide feasible path toward a quantum link between distant silicon quantum processors, a key requirement for large-scale quantum computation.
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Yoneda, J., W. Huang, M. Feng, et al. "Coherent spin qubit transport in silicon." Nature Communications 12, no. 1 (2021). http://dx.doi.org/10.1038/s41467-021-24371-7.
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AbstractA fault-tolerant quantum processor may be configured using stationary qubits interacting only with their nearest neighbours, but at the cost of significant overheads in physical qubits per logical qubit. Such overheads could be reduced by coherently transporting qubits across the chip, allowing connectivity beyond immediate neighbours. Here we demonstrate high-fidelity coherent transport of an electron spin qubit between quantum dots in isotopically-enriched silicon. We observe qubit precession in the inter-site tunnelling regime and assess the impact of qubit transport using Ramsey interferometry and quantum state tomography techniques. We report a polarization transfer fidelity of 99.97% and an average coherent transfer fidelity of 99.4%. Our results provide key elements for high-fidelity, on-chip quantum information distribution, as long envisaged, reinforcing the scaling prospects of silicon-based spin qubits.
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Srinivasa, V., J. M. Taylor, and J. R. Petta. "Cavity-Mediated Entanglement of Parametrically Driven Spin Qubits via Sidebands." PRX Quantum 5, no. 2 (2024). http://dx.doi.org/10.1103/prxquantum.5.020339.
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We consider a pair of quantum dot-based spin qubits that interact via microwave photons in a superconducting cavity and that are also parametrically driven by separate external electric fields. For this system, we formulate a model for spin qubit entanglement in the presence of mutually off-resonant qubit and cavity frequencies. We show that the sidebands generated via the driving fields enable highly tunable qubit-qubit entanglement using only ac control and without requiring the qubit and cavity frequencies to be tuned into simultaneous resonance. The model we derive can be mapped to a variety of qubit types, including detuning-driven one-electron spin qubits in double quantum dots and three-electron resonant exchange qubits in triple quantum dots. The high degree of nonlinearity inherent in spin qubits renders these systems particularly favorable for parametric drive-activated entanglement. We determine multiple common resonance conditions for the two driven qubits and the cavity and identify experimentally relevant parameter regimes that enable the implementation of entangling gates with suppressed sensitivity to cavity photon occupation and decay. The parametrically driven sideband resonance approach that we describe provides a promising route toward scalability and modularity in spin-based quantum information processing through drive-enabled tunability that can also be implemented in micromagnet-free electron and hole systems for spin-photon coupling. Published by the American Physical Society 2024
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Mifune, Shion, Taro Kanao, and Tetsufumi Tanamoto. "Effects of dissipation in reservoir computing using a spin qubit array." Japanese Journal of Applied Physics, March 12, 2025. https://doi.org/10.35848/1347-4065/adbf9e.
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Abstract Reservoir computing (RC) is one of the hottest research topic as an application of many physical devices because the device characteristics can be used directly in computing sequences. Quantum RC is also a promising candidate for application in small-number qubit systems. Here, we propose a quantum RC based on the spin qubit system that reflects the status of the spin qubits in experiments comprising a one-dimensional qubit array. Spin qubits are coupled via the Heisenberg interaction, and data sequences are inputted to one of the spin qubits via pulsed rotations. By introducing dissipation, we obtained a relatively good performance in the quantum RC.
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Thorvaldson, I., D. Poulos, C. M. Moehle, et al. "Grover’s algorithm in a four-qubit silicon processor above the fault-tolerant threshold." Nature Nanotechnology, February 20, 2025. https://doi.org/10.1038/s41565-024-01853-5.
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Abstract Spin qubits in silicon are strong contenders for the realization of a practical quantum computer. Single- and two-qubit gates have shown fidelities above the fault-tolerant threshold, and entanglement of three qubits has been achieved. Furthermore, high-fidelity operation of two-qubit algorithms is possible. Here we implement a four-qubit silicon processor with all control fidelities above the fault-tolerant threshold. We demonstrate a three-qubit Grover’s search algorithm with a ~95% probability of finding the marked state. To this end, we fabricate the processor from three phosphorus atoms precision-patterned into isotopically pure silicon. We define three phosphorus nuclear spin qubits and one electron spin qubit. The long coherence times of the qubits enable single-qubit fidelities above 99.9% for all qubits. Moreover, the efficient single-pulse multi-qubit operation enabled by the electron–nuclear hyperfine interaction facilitates controlled-Z gates with above 99% fidelity between all pairs of nuclear spins when using the electron as an ancilla. These control fidelities, combined with high-fidelity non-demolition readout of all nuclear spins, allows the creation of a three-qubit Greenberger–Horne–Zeilinger state with 96.2% fidelity. Looking ahead, coupling neighbouring nuclear spin registers, as the one shown here, via electron–electron exchange may enable larger, yet fault-tolerant, quantum processors.
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Wang Ning, Wang Bao-Chuan, and Guo Guo-Ping. "New progress in silicon-based semiconductor quantum computation." Acta Physica Sinica, 2022, 0. http://dx.doi.org/10.7498/aps.71.20221900.
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Spin qubits in silicon-based semiconductor quantum dots have become one of the prominent candidates for realizing fault-tolerant quantum computing due to their long coherence time, good controllability, and compatibility with modern advanced integrated circuit manufacturing processes. In recent years, thanks to the remarkable progress made in silicon-based materials, structure of quantum dot and its fabrication process, and qubit manipulation technology, high-fidelity state preparation and readout, single- and two-qubit gates have been demonstrated for silicon spin qubits. The control fidelities for single- and two-qubit gates all exceed 99%—fault tolerance threshold required by the surface code known for its exceptionally high tolerance to errors. In this paper, we briefly introduce the basic concepts of silicon-based semiconductor quantum dots, discuss the state-of-art technologies used to improve the fidelities of single- and two-qubit gates, and finally highlight the research directions that need to be focused on.<br />The paper is organized as follows. Firstly, we introduce three major types of quantum dots (QD) devices fabricated on different silicon-based substrate, including Si/SiGe heterojunction and Si/SiO<sub>2</sub>. The spin degree of electron or nuclear hosted in QD can be encoded to spin qubits. Electron spin qubit can be thermal initialized to ground state utilizing electron reservoirs and read out by spin-charge conversion mechanism energy-selective readout (Elzerman readout) with reservoirs or Pauli spin blockade (PSB) needless for a reservoir. Additionally, high fidelity single-shot readout has been demonstrated using radio-frequency gate reflectometry combined with PSB, which has unique advantages in large-scale qubit array. To coherent control the spin qubits, electron dipole renounce (ESR) or electron dipole spin resonance (EDSR) for electron and nuclear magnetic resonance (NMR) for nuclear are introduced. With help of isotope purification greatly improving the dephasing time of qubit and fast single-qubit manipulation based on EDSR, fidelity above 99.9 percent can be reached. For the two-qubit gates based on exchange interaction between electron spins, the strength of interaction <em>J</em> combined with Zeeman energy difference Δ<em>E</em><sub><em>Z</em></sub> determines the energy levels of system, which lead to the different two-qubit gates, such as controlled-Z (CZ), controlled-Rotation (CROT) and the square root of the SWAP gate ($\sqrt{\text { SWAP }}$) gates. In order to improve the fidelity of two-qubit gates, a series of key technologies are used in the experiments, not only isotope purification but also symmetry operation, careful Hamiltonian engineering and gate set tomography. Fidelity of two-qubit gates exceeding 99 percent has been demonstrated for electron spin qubits in Si/SiGe quantum dots and nuclear spin qubits in donors. These progresses have pushed the silicon-based spin qubits platform to constitute a major stepping stone towards fault-tolerant quantum computation. Finally, we discuss the next step for spin qubits, that is, how to effectively expand the number of qubits and there are still many problems to be explored and solved.
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van Riggelen, F., W. I. L. Lawrie, M. Russ, et al. "Phase flip code with semiconductor spin qubits." npj Quantum Information 8, no. 1 (2022). http://dx.doi.org/10.1038/s41534-022-00639-8.
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AbstractThe fault-tolerant operation of logical qubits is an important requirement for realizing a universal quantum computer. Spin qubits based on quantum dots have great potential to be scaled to large numbers because of their compatibility with standard semiconductor manufacturing. Here, we show that a quantum error correction code can be implemented using a four-qubit array in germanium. We demonstrate a resonant SWAP gate and by combining controlled-Z and controlled-S−1 gates we construct a Toffoli-like three-qubit gate. We execute a two-qubit phase flip code and find that we can preserve the state of the data qubit by applying a refocusing pulse to the ancilla qubit. In addition, we implement a phase flip code on three qubits, making use of a Toffoli-like gate for the final correction step. Both the quality and quantity of the qubits will require significant improvement to achieve fault-tolerance. However, the capability to implement quantum error correction codes enables co-design development of quantum hardware and software, where codes tailored to the properties of spin qubits and advances in fabrication and operation can now come together to advance semiconductor quantum technology.
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Van, Riggelen-Doelman Floor, Chien-An Wang, Snoo Sander L. De, et al. "Coherent spin qubit shuttling through germanium quantum dots." May 16, 2024. https://doi.org/10.5281/zenodo.11203148.
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Raw data on which the figures in the publication 'Coherent spin qubit shuttling through germanium quantum dots' are based and the python scripts which are used to generated the figures. Abstract of the paper: Quantum links can interconnect qubit registers and are therefore essential in networked quantum computing. Semiconductor quantum dot qubits have seen significant progress in the high-fidelity operation of small qubit registers but establishing a compelling quantum link remains a challenge. Here, we show that a spin qubit can be shuttled through multiple quantum dots while preserving its quantum information. Remarkably, we achieve these results using hole spin qubits in germanium, despite the presence of strong spin-orbit interaction. We accomplish the shuttling of spin basis states over effective lengths beyond 300 μm and demonstrate the coherent shuttling of superposition states over effective lengths corresponding to 9 μm, which we can extend to 49 μm by incorporating dynamical decoupling. These findings indicate qubit shuttling as an effective approach to route qubits within registers and to establish quantum links between registers.
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Landig, A. J., J. V. Koski, P. Scarlino, et al. "Virtual-photon-mediated spin-qubit–transmon coupling." Nature Communications 10, no. 1 (2019). http://dx.doi.org/10.1038/s41467-019-13000-z.
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Abstract Spin qubits and superconducting qubits are among the promising candidates for realizing a solid state quantum computer. For the implementation of a hybrid architecture which can profit from the advantages of either approach, a coherent link is necessary that integrates and controllably couples both qubit types on the same chip over a distance that is several orders of magnitude longer than the physical size of the spin qubit. We realize such a link with a frequency-tunable high impedance SQUID array resonator. The spin qubit is a resonant exchange qubit hosted in a GaAs triple quantum dot. It can be operated at zero magnetic field, allowing it to coexist with superconducting qubits on the same chip. We spectroscopically observe coherent interaction between the resonant exchange qubit and a transmon qubit in both resonant and dispersive regimes, where the interaction is mediated either by real or virtual resonator photons.
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Vahapoglu, E., J. P. Slack-Smith, R. C. C. Leon, et al. "Coherent control of electron spin qubits in silicon using a global field." npj Quantum Information 8, no. 1 (2022). http://dx.doi.org/10.1038/s41534-022-00645-w.
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AbstractSilicon spin qubits promise to leverage the extraordinary progress in silicon nanoelectronic device fabrication over the past half century to deliver large-scale quantum processors. Despite the scalability advantage of using silicon technology, realising a quantum computer with the millions of qubits required to run some of the most demanding quantum algorithms poses several outstanding challenges, including how to control many qubits simultaneously. Recently, compact 3D microwave dielectric resonators were proposed as a way to deliver the magnetic fields for spin qubit control across an entire quantum chip using only a single microwave source. Although spin resonance of individual electrons in the globally applied microwave field was demonstrated, the spins were controlled incoherently. Here we report coherent Rabi oscillations of single electron spin qubits in a planar SiMOS quantum dot device using a global magnetic field generated off-chip. The observation of coherent qubit control driven by a dielectric resonator establishes a credible pathway to achieving large-scale control in a spin-based quantum computer.
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van Riggelen-Doelman, Floor, Chien-An Wang, Sander L. de Snoo, et al. "Coherent spin qubit shuttling through germanium quantum dots." Nature Communications 15, no. 1 (2024). http://dx.doi.org/10.1038/s41467-024-49358-y.
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AbstractQuantum links can interconnect qubit registers and are therefore essential in networked quantum computing. Semiconductor quantum dot qubits have seen significant progress in the high-fidelity operation of small qubit registers but establishing a compelling quantum link remains a challenge. Here, we show that a spin qubit can be shuttled through multiple quantum dots while preserving its quantum information. Remarkably, we achieve these results using hole spin qubits in germanium, despite the presence of strong spin-orbit interaction. In a minimal quantum dot chain, we accomplish the shuttling of spin basis states over effective lengths beyond 300 microns and demonstrate the coherent shuttling of superposition states over effective lengths corresponding to 9 microns, which we can extend to 49 microns by incorporating dynamical decoupling. These findings indicate qubit shuttling as an effective approach to route qubits within registers and to establish quantum links between registers.
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Chu, Ning, Xin Zhang, Rong-Long Ma, et al. "Single-qubit anisotropy induced by micromagnet in Si-MOS quantum dot." Applied Physics Express, January 15, 2025. https://doi.org/10.35848/1882-0786/adaad7.
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Abstract Synthesized spin-orbit coupling (SSOC) is crucial for the operation of spin qubits in silicon quantum dot, as it address the challenge posed by the inherently weak intrinsic spin-orbit coupling (ISOC) in silicon. Here, we investigate the anisotropic properties of single spin qubit in silicon metal-oxide-semiconductor (Si-MOS) quantum dot and provide experimental evidence for the control of SSOC. Additionally, we experimentally demonstrate that tuning the operating point away from the conventional configuration can enhance the quality factor of the spin qubit. These findings lay a foundation for the realization of high-quality tunable spin-orbit qubits.
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Qiao, Haifeng, Yadav P. Kandel, John S. Van Dyke, et al. "Floquet-enhanced spin swaps." Nature Communications 12, no. 1 (2021). http://dx.doi.org/10.1038/s41467-021-22415-6.
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AbstractThe transfer of information between quantum systems is essential for quantum communication and computation. In quantum computers, high connectivity between qubits can improve the efficiency of algorithms, assist in error correction, and enable high-fidelity readout. However, as with all quantum gates, operations to transfer information between qubits can suffer from errors associated with spurious interactions and disorder between qubits, among other things. Here, we harness interactions and disorder between qubits to improve a swap operation for spin eigenstates in semiconductor gate-defined quantum-dot spins. We use a system of four electron spins, which we configure as two exchange-coupled singlet–triplet qubits. Our approach, which relies on the physics underlying discrete time crystals, enhances the quality factor of spin-eigenstate swaps by up to an order of magnitude. Our results show how interactions and disorder in multi-qubit systems can stabilize non-trivial quantum operations and suggest potential uses for non-equilibrium quantum phenomena, like time crystals, in quantum information processing applications. Our results also confirm the long-predicted emergence of effective Ising interactions between exchange-coupled singlet–triplet qubits.
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Feng, MengKe, Lin Htoo Zaw, and Teck Seng Koh. "Two-qubit sweet spots for capacitively coupled exchange-only spin qubits." npj Quantum Information 7, no. 1 (2021). http://dx.doi.org/10.1038/s41534-021-00449-4.
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AbstractThe implementation of high fidelity two-qubit gates is a bottleneck in the progress toward universal quantum computation in semiconductor quantum dot qubits. We study capacitive coupling between two triple quantum dot spin qubits encoded in the S = 1/2, Sz = −1/2 decoherence-free subspace—the exchange-only (EO) spin qubits. We report exact gate sequences for CPHASE and CNOT gates, and demonstrate theoretically, the existence of multiple two-qubit sweet spots (2QSS) in the parameter space of capacitively coupled EO qubits. Gate operations have the advantage of being all-electrical, but charge noise that couple to electrical parameters of the qubits cause decoherence. Assuming noise with a 1/f spectrum, two-qubit gate fidelities and times are calculated, which provide useful information on the noise threshold necessary for fault-tolerance. We study two-qubit gates at single and multiple parameter 2QSS. In particular, for two existing EO implementations—the resonant exchange (RX) and the always-on exchange-only (AEON) qubits—we compare two-qubit gate fidelities and times at positions in parameter space where the 2QSS are simultaneously single-qubit sweet spots (1QSS) for the RX and AEON. These results provide a potential route to the realization of high fidelity quantum computation.
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Geyer, Simon, Bence Hetényi, Stefano Bosco, et al. "Anisotropic exchange interaction of two hole-spin qubits." Nature Physics, May 6, 2024. http://dx.doi.org/10.1038/s41567-024-02481-5.
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AbstractSemiconductor spin qubits offer the potential to employ industrial transistor technology to produce large-scale quantum computers. Silicon hole spin qubits benefit from fast all-electrical qubit control and sweet spots to counteract charge and nuclear spin noise. However, the demonstration of a two-qubit interaction has remained an open challenge. One missing factor is an understanding of the exchange coupling in the presence of a strong spin–orbit interaction. Here we study two hole-spin qubits in a silicon fin field-effect transistor, the workhorse device of today’s semiconductor industry. We demonstrate electrical tunability of the exchange splitting from above 500 MHz to close-to-off and perform a conditional spin-flip in 24 ns. The exchange is anisotropic because of the spin–orbit interaction. Upon tunnelling from one quantum dot to the other, the spin is rotated by almost 180 degrees. The exchange Hamiltonian no longer has the Heisenberg form and can be engineered such that it enables two-qubit controlled rotation gates without a trade-off between speed and fidelity. This ideal behaviour applies over a wide range of magnetic field orientations, rendering the concept robust with respect to variations from qubit to qubit, indicating that it is a suitable approach for realizing a large-scale quantum computer.
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Künne, Matthias, Alexander Willmes, Max Oberländer, et al. "The SpinBus architecture for scaling spin qubits with electron shuttling." Nature Communications 15, no. 1 (2024). http://dx.doi.org/10.1038/s41467-024-49182-4.
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AbstractQuantum processor architectures must enable scaling to large qubit numbers while providing two-dimensional qubit connectivity and exquisite operation fidelities. For microwave-controlled semiconductor spin qubits, dense arrays have made considerable progress, but are still limited in size by wiring fan-out and exhibit significant crosstalk between qubits. To overcome these limitations, we introduce the SpinBus architecture, which uses electron shuttling to connect qubits and features low operating frequencies and enhanced qubit coherence. Device simulations for all relevant operations in the Si/SiGe platform validate the feasibility with established semiconductor patterning technology and operation fidelities exceeding 99.9%. Control using room temperature instruments can plausibly support at least 144 qubits, but much larger numbers are conceivable with cryogenic control circuits. Building on the theoretical feasibility of high-fidelity spin-coherent electron shuttling as key enabling factor, the SpinBus architecture may be the basis for a spin-based quantum processor that meets the scalability requirements for practical quantum computing.
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Aruachan, Katy, Yamil Colón, Daniel Aravena, and Felipe Herrera. "Semi-Empirical Haken-Strobl Model for Molecular Spin Qubits." New Journal of Physics, August 22, 2023. http://dx.doi.org/10.1088/1367-2630/acf2bd.
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Abstract Understanding the physical processes that determine the relaxation T 1 and dephasing T 2 times of molecular spin qubits is critical for envisioned applications in quantum metrology and information processing. Recent spin-echo T 1 measurements of solid-state molecular spin qubits have stimulated the development of quantum mechanical models for predicting intrinsic spin qubit timescales using first-principles electronic structure methods. We develop an alternative semi-empirical approach to construct Redfield quantum master equations for molecular spin qubits using a stochastic Haken-Strobl model for a central spin with a fluctuating gyromagnetic tensor due to spin-lattice interaction and a fluctuating local magnetic field due to interactions with other lattice spins. Using two vanadium-based spin qubits as case study, we compute qubit population and decoherence timescales as a function of temperature and magnetic field using a bath spectral density parametrized with a small number of T 1 measurements. The theory quantitatively agrees with experimental data over a range of conditions beyond those used to parametrize the model, demonstrating the generalization potential of the method. The ability of the model to describe the temperature dependence of the ratio T 2/T 1 is discussed and possible applications for designing novel molecule-based quantum magnetometers are suggested.