Journal articles: 'Transman qubit'

1

Said, T., A. Chouikh, K. Essammouni, and M. Bennai. "Implementing N-quantum phase gate via circuit QED with qubit–qubit interaction." Modern Physics Letters B 30, no. 05 (2016): 1650050. http://dx.doi.org/10.1142/s0217984916500500.

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We propose a method for realizing a quantum phase gate of one qubit simultaneously controlling [Formula: see text] target qubits based on the qubit–qubit interaction. We show how to implement the proposed gate with one transmon qubit simultaneously controlling [Formula: see text] transmon qubits in a circuit QED driven by a strong microwave field. In our scheme, the operation time of this phase gate is independent of the number [Formula: see text] of qubits. On the other hand, this gate can be realized in a time of nanosecond-scale much smaller than the decoherence time and dephasing time both being the time of microsecond-scale. Numerical simulation of the occupation probabilities of the second excited lever shows that the scheme could be achieved efficiently within current technology.

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2

Yuan, Wei-Ping, Zhi-Cheng He, Sai Li, and Zheng-Yuan Xue. "Fast Reset Protocol for Superconducting Transmon Qubits." Applied Sciences 13, no. 2 (2023): 817. http://dx.doi.org/10.3390/app13020817.

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For larger-scale quantum information processing, qubit reset plays an important role, as the coherent times for qubits are limited. However, previous schemes require either long reset times or a complex pulse calibration technique, leading to low efficiency in qubit reset. Here, we propose a fast and simple reset protocol for superconducting transmon qubits based on the coupler-coupled qubits architecture. In this setup, a mixing pulse is used to transfer the qubit excitation to the combined excitation of a low-qulity coupler and readout resonator, which will quickly decay to their respectively ground states, leading to efficient qubit reset to the ground state. Our numerical results show that the residual population of the qubit’s excited state can be suppressed to 0.04% within 28 ns; the reset time will be 283 ns if photon depletion of the readout resonator is required. Thus, our protocol provides a promising way for the high-efficiency superconducting qubit reset.

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3

Sun, Xiaopei, Bing Li, Enna Zhuo, et al. "Realization of superconducting transmon qubits based on topological insulator nanowires." Applied Physics Letters 122, no. 15 (2023): 154001. http://dx.doi.org/10.1063/5.0140079.

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Topological-material-based Josephson junctions have the potential to be used to host Majorana zero modes and to construct topological qubits. For operating the topological qubits at an appropriate timescale to avoid decoherence and quasiparticle poisoning, one would eventually go to the time domain and embed the topological qubits into quantum electrodynamic circuits. Here, we constructed a topological-insulator-nanowire-based transmon qubit and demonstrated its strong coupling to a coplanar waveguide resonator. The flux-tunable spectrum and Rabi oscillations with a qubit lifetime [Formula: see text] of [Formula: see text] were observed. Such a hybrid platform, containing topological materials and quantum electrodynamic circuits, can further be used to study the physical properties such as Majorana zero modes in topological quantum circuits.

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4

Tao, Rui, Xiao-Tao Mo, Zheng-Yuan Xue, and Jian Zhou. "Practical one-step synthesis of multipartite entangled states on superconducting circuits." International Journal of Quantum Information 17, no. 07 (2019): 1950051. http://dx.doi.org/10.1142/s0219749919500515.

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Quantum entanglement is an important resource for quantum information processing tasks. However, realistic multipartite entangled state production is very difficult. In this paper, we propose an efficient single-step scheme for generating many body Greenberger–Horne–Zeilinger (GHZ) states on superconducting circuits by using a superconducting transmission-line resonator (TLR) interact with [Formula: see text] superconducting transmon qubits. The distinct merit of our proposal is that it does not require the qubit-resonator coupling strengths to be the same, which is usually impractical experimentally, and thus is one of the main reasons for entanglement generation infidelity in previous single-step schemes. The removing of the uniform interaction requirement is achieved by modulating the qubits splitting frequencies with ac microwave fields, which results in tunable individual qubit-resonator coupling strength, and thus effective uniform qubit–qubit interaction Hamiltonian can be obtained. Since microwave control is conventional nowadays, our proposal can be directly tested experimentally, which makes previous multipartite entangled states generation schemes more efficient.

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5

Kubo, Kentaro, and Hayato Goto. "Fast parametric two-qubit gate for highly detuned fixed-frequency superconducting qubits using a double-transmon coupler." Applied Physics Letters 122, no. 6 (2023): 064001. http://dx.doi.org/10.1063/5.0138699.

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High-performance two-qubit gates have been reported with superconducting qubits coupled via a single-transmon coupler (STC). Most of them are implemented for qubits with a small detuning since reducing residual ZZ coupling for highly detuned qubits by an STC is challenging. In terms of the frequency crowding and crosstalk, however, highly detuned qubits are desirable. Here, we numerically demonstrate a high-performance parametric gate for highly detuned fixed-frequency qubits using a recently proposed tunable coupler called a double-transmon coupler (DTC). Applying an ac flux pulse, we can perform a maximally entangling universal gate ([Formula: see text]) with an average fidelity over 99.99% and a short gate time of about 24 ns. This speed is comparable to resonance-based gates for slightly detuned tunable qubits. Moreover, using a dc flux pulse alternatively, we can achieve another kind of entangling gate called a CZ gate with an average fidelity over 99.99% and a gate time of about 18 ns. Given the flexibility and feasible settings, we can expect that the DTC will contribute towards realizing a high-performance quantum computer in the near future.

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6

Dong, Yuqian, Yong Li, Wen Zheng, et al. "Measurement of Quasiparticle Diffusion in a Superconducting Transmon Qubit." Applied Sciences 12, no. 17 (2022): 8461. http://dx.doi.org/10.3390/app12178461.

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Quasiparticles, especially the ones near the Josephson junctions in the superconducting qubits, are known as an important source of decoherence. By injecting quasiparticles into a quantum chip, we characterized the diffusion feature by measuring the energy relaxation time and the residual excited-state population of a transmon qubit. From the extracted transition rates, we phenomenologically modeled the quasiparticle diffusion in a superconducting circuit that contained “hot” nonequilibrium quasiparticles in addition to low-energy ones.

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7

Ye, Yangsen, Sirui Cao, Yulin Wu, et al. "Realization of High-Fidelity Controlled-Phase Gates in Extensible Superconducting Qubits Design with a Tunable Coupler." Chinese Physics Letters 38, no. 10 (2021): 100301. http://dx.doi.org/10.1088/0256-307x/38/10/100301.

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High-fidelity two-qubit gates are essential for the realization of large-scale quantum computation and simulation. Tunable coupler design is used to reduce the problem of parasitic coupling and frequency crowding in many-qubit systems and thus thought to be advantageous. Here we design an extensible 5-qubit system in which center transmon qubit can couple to every four near-neighboring qubits via a capacitive tunable coupler and experimentally demonstrate high-fidelity controlled-phase (CZ) gate by manipulating central qubit and one near-neighboring qubit. Speckle purity benchmarking and cross entropy benchmarking are used to assess the purity fidelity and the fidelity of the CZ gate. The average purity fidelity of the CZ gate is 99.69±0.04% and the average fidelity of the CZ gate is 99.65±0.04%, which means that the control error is about 0.04%. Our work is helpful for resolving many challenges in implementation of large-scale quantum systems.

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8

Bultink, C. C., T. E. O’Brien, R. Vollmer, et al. "Protecting quantum entanglement from leakage and qubit errors via repetitive parity measurements." Science Advances 6, no. 12 (2020): eaay3050. http://dx.doi.org/10.1126/sciadv.aay3050.

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Protecting quantum information from errors is essential for large-scale quantum computation. Quantum error correction (QEC) encodes information in entangled states of many qubits and performs parity measurements to identify errors without destroying the encoded information. However, traditional QEC cannot handle leakage from the qubit computational space. Leakage affects leading experimental platforms, based on trapped ions and superconducting circuits, which use effective qubits within many-level physical systems. We investigate how two-transmon entangled states evolve under repeated parity measurements and demonstrate the use of hidden Markov models to detect leakage using only the record of parity measurement outcomes required for QEC. We show the stabilization of Bell states over up to 26 parity measurements by mitigating leakage using postselection and correcting qubit errors using Pauli-frame transformations. Our leakage identification method is computationally efficient and thus compatible with real-time leakage tracking and correction in larger quantum processors.

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9

Groszkowski, Peter, and Jens Koch. "Scqubits: a Python package for superconducting qubits." Quantum 5 (November 17, 2021): 583. http://dx.doi.org/10.22331/q-2021-11-17-583.

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scqubits is an open-source Python package for simulating and analyzing superconducting circuits. It provides convenient routines to obtain energy spectra of common superconducting qubits, such as the transmon, fluxonium, flux, cos(2ϕ) and the 0-π qubit. scqubits also features a number of options for visualizing the computed spectral data, including plots of energy levels as a function of external parameters, display of matrix elements of various operators as well as means to easily plot qubit wavefunctions. Many of these tools are not limited to single qubits, but extend to composite Hilbert spaces consisting of coupled superconducting qubits and harmonic (or weakly anharmonic) modes. The library provides an extensive suite of methods for estimating qubit coherence times due to a variety of commonly considered noise channels. While all functionality of scqubits can be accessed programatically, the package also implements GUI-like widgets that, with a few clicks can help users both create relevant Python objects, as well as explore their properties through various plots. When applicable, the library harnesses the computing power of multiple cores via multiprocessing. scqubits further exposes a direct interface to the Quantum Toolbox in Python (QuTiP) package, allowing the user to efficiently leverage QuTiP's proven capabilities for simulating time evolution.

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10

Ahmad, Halima Giovanna, Caleb Jordan, Roald van den Boogaart, et al. "Investigating the Individual Performances of Coupled Superconducting Transmon Qubits." Condensed Matter 8, no. 1 (2023): 29. http://dx.doi.org/10.3390/condmat8010029.

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The strong requirement for high-performing quantum computing led to intensive research on novel quantum platforms in the last decades. The circuital nature of Josephson-based quantum superconducting systems powerfully supports massive circuital freedom, which allowed for the implementation of a wide range of qubit designs, and an easy interface with the quantum processing unit. However, this unavoidably introduces a coupling with the environment, and thus to extra decoherence sources. Moreover, at the time of writing, control and readout protocols mainly use analogue microwave electronics, which limit the otherwise reasonable scalability in superconducting quantum circuits. Within the future perspective to improve scalability by integrating novel control energy-efficient superconducting electronics at the quantum stage in a multi-chip module, we report on an all-microwave characterization of a planar two-transmon qubits device, which involves state-of-the-art control pulses optimization. We demonstrate that the single-qubit average gate fidelity is mainly limited by the gate pulse duration and the quality of the optimization, and thus does not preclude the integration in novel hybrid quantum-classical superconducting devices.

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11

Dheer, Vihaan. "The optimization of flux trajectories for the adiabatic controlled-Z gate on split-tunable transmons." AIP Advances 12, no. 9 (2022): 095306. http://dx.doi.org/10.1063/5.0087364.

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In a system of two tunable-frequency qubits, it is well-known that adiabatic tuning into strong coupling-interaction regions between the qubit subspace and the rest of the Hilbert space can be used to generate an effective controlled-Z rotation. We address the problem of determining a preferable adiabatic trajectory along which the qubit frequency is tuned and apply this to the flux-tunable transmon model. The especially minimal anharmonic nature of these quantum processors makes them good candidates for qubit control using non-computational states as long as higher-level leakage is properly addressed. While the statement of this method has occurred multiple times in the literature, there have been few discussions on which trajectories may be used. We present a generalized method for optimizing parameterized families of possible flux trajectories and provide examples of use on five test families of one and two parameters.

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12

Stefanazzi, Leandro, Kenneth Treptow, Neal Wilcer, et al. "The QICK (Quantum Instrumentation Control Kit): Readout and control for qubits and detectors." Review of Scientific Instruments 93, no. 4 (2022): 044709. http://dx.doi.org/10.1063/5.0076249.

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We introduce a Xilinx RF System-on-Chip (RFSoC)-based qubit controller (called the Quantum Instrumentation Control Kit, or QICK for short), which supports the direct synthesis of control pulses with carrier frequencies of up to 6 GHz. The QICK can control multiple qubits or other quantum devices. The QICK consists of a digital board hosting an RFSoC field-programmable gate array, custom firmware, and software and an optional companion custom-designed analog front-end board. We characterize the analog performance of the system as well as its digital latency, important for quantum error correction and feedback protocols. We benchmark the controller by performing standard characterizations of a transmon qubit. We achieve an average gate fidelity of [Formula: see text]. All of the schematics, firmware, and software are open-source.

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13

Stefanazzi, Leandro, Kenneth Treptow, Neal Wilcer, et al. "The QICK (Quantum Instrumentation Control Kit): Readout and control for qubits and detectors." Review of Scientific Instruments 93, no. 4 (2022): 044709. http://dx.doi.org/10.1063/5.0076249.

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We introduce a Xilinx RF System-on-Chip (RFSoC)-based qubit controller (called the Quantum Instrumentation Control Kit, or QICK for short), which supports the direct synthesis of control pulses with carrier frequencies of up to 6 GHz. The QICK can control multiple qubits or other quantum devices. The QICK consists of a digital board hosting an RFSoC field-programmable gate array, custom firmware, and software and an optional companion custom-designed analog front-end board. We characterize the analog performance of the system as well as its digital latency, important for quantum error correction and feedback protocols. We benchmark the controller by performing standard characterizations of a transmon qubit. We achieve an average gate fidelity of [Formula: see text]. All of the schematics, firmware, and software are open-source.

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14

Stefanazzi, Leandro, Kenneth Treptow, Neal Wilcer, et al. "The QICK (Quantum Instrumentation Control Kit): Readout and control for qubits and detectors." Review of Scientific Instruments 93, no. 4 (2022): 044709. http://dx.doi.org/10.1063/5.0076249.

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We introduce a Xilinx RF System-on-Chip (RFSoC)-based qubit controller (called the Quantum Instrumentation Control Kit, or QICK for short), which supports the direct synthesis of control pulses with carrier frequencies of up to 6 GHz. The QICK can control multiple qubits or other quantum devices. The QICK consists of a digital board hosting an RFSoC field-programmable gate array, custom firmware, and software and an optional companion custom-designed analog front-end board. We characterize the analog performance of the system as well as its digital latency, important for quantum error correction and feedback protocols. We benchmark the controller by performing standard characterizations of a transmon qubit. We achieve an average gate fidelity of [Formula: see text]. All of the schematics, firmware, and software are open-source.

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15

Sharafiev, Aleksei, Mathieu L. Juan, Oscar Gargiulo, et al. "Visualizing the emission of a single photon with frequency and time resolved spectroscopy." Quantum 5 (June 10, 2021): 474. http://dx.doi.org/10.22331/q-2021-06-10-474.

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At the dawn of Quantum Physics, Wigner and Weisskopf obtained a full analytical description (a photon portrait) of the emission of a single photon by a two-level system, using the basis of frequency modes (Weisskopf and Wigner, "Zeitschrift für Physik", 63, 1930). A direct experimental reconstruction of this portrait demands an accurate measurement of a time resolved fluorescence spectrum, with high sensitivity to the off-resonant frequencies and ultrafast dynamics describing the photon creation. In this work we demonstrate such an experimental technique in a superconducting waveguide Quantum Electrodynamics (wQED) platform, using single transmon qubit and two coupled transmon qubits as quantum emitters. In both scenarios, the photon portraits agree quantitatively with the predictions of the input-output theory and qualitatively with Wigner-Weisskopf theory. We believe that our technique allows not only for interesting visualization of fundamental principles, but may serve as a tool, e.g. to realize multi-dimensional spectroscopy in waveguide Quantum Electrodynamics.

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16

Linke, Norbert M., Dmitri Maslov, Martin Roetteler, et al. "Experimental comparison of two quantum computing architectures." Proceedings of the National Academy of Sciences 114, no. 13 (2017): 3305–10. http://dx.doi.org/10.1073/pnas.1618020114.

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We run a selection of algorithms on two state-of-the-art 5-qubit quantum computers that are based on different technology platforms. One is a publicly accessible superconducting transmon device (www.research.ibm.com/ibm-q) with limited connectivity, and the other is a fully connected trapped-ion system. Even though the two systems have different native quantum interactions, both can be programed in a way that is blind to the underlying hardware, thus allowing a comparison of identical quantum algorithms between different physical systems. We show that quantum algorithms and circuits that use more connectivity clearly benefit from a better-connected system of qubits. Although the quantum systems here are not yet large enough to eclipse classical computers, this experiment exposes critical factors of scaling quantum computers, such as qubit connectivity and gate expressivity. In addition, the results suggest that codesigning particular quantum applications with the hardware itself will be paramount in successfully using quantum computers in the future.

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17

Cai, Han, Qi-Chun Liu, Chang-Hao Zhao, Ying-Shan Zhang, Jian-She Liu, and Wei Chen. "Construction of two-qubit logical gates by transmon qubits in a three-dimensional cavity." Chinese Physics B 27, no. 8 (2018): 084207. http://dx.doi.org/10.1088/1674-1056/27/8/084207.

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18

Navez, P., A. G. Balanov, S. E. Savel’ev, and A. M. Zagoskin. "Quantum electrodynamics of non-demolition detection of single microwave photon by superconducting qubit array." Journal of Applied Physics 133, no. 10 (2023): 104401. http://dx.doi.org/10.1063/5.0137747.

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By consistently applying the formalism of quantum electrodynamics, we developed a comprehensive theoretical framework describing the interaction of single microwave photons with an array of superconducting transmon qubits in a waveguide cavity resonator. In particular, we analyze the effects of microwave photons on the array’s response to a weak probe signal exciting the resonator. The study reveals that high quality factor cavities provide a better spectral resolution of the response, while cavities with moderate quality factors allow better sensitivity for a single-photon detection. Remarkably, our analysis showed that a single-photon signal can be detected by even a sole qubit in a cavity under the realistic range of system parameters. We also discuss how the quantum properties of the microwave radiation and electrodynamical properties of resonators affect the response of qubits’ array. Our results provide an efficient theoretical background for informing the development and design of quantum devices consisting of arrays of qubits, especially for those using a cavity where an explicit expression for the transmission or reflection is required.

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Dinerstein, Alec, Caroline S. Gorham, and Eugene F. Dumitrescu. "The hybrid topological longitudinal transmon qubit." Materials for Quantum Technology 1, no. 2 (2021): 021001. http://dx.doi.org/10.1088/2633-4356/abfbc9.

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20

Kannan, B., D. L. Campbell, F. Vasconcelos, et al. "Generating spatially entangled itinerant photons with waveguide quantum electrodynamics." Science Advances 6, no. 41 (2020): eabb8780. http://dx.doi.org/10.1126/sciadv.abb8780.

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Realizing a fully connected network of quantum processors requires the ability to distribute quantum entanglement. For distant processing nodes, this can be achieved by generating, routing, and capturing spatially entangled itinerant photons. In this work, we demonstrate the deterministic generation of such photons using superconducting transmon qubits that are directly coupled to a waveguide. In particular, we generate two-photon N00N states and show that the state and spatial entanglement of the emitted photons are tunable via the qubit frequencies. Using quadrature amplitude detection, we reconstruct the moments and correlations of the photonic modes and demonstrate state preparation fidelities of 84%. Our results provide a path toward realizing quantum communication and teleportation protocols using itinerant photons generated by quantum interference within a waveguide quantum electrodynamics architecture.

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21

Ding, Cheng-Yun, Li-Na Ji, Tao Chen, and Zheng-Yuan Xue. "Path-optimized nonadiabatic geometric quantum computation on superconducting qubits." Quantum Science and Technology 7, no. 1 (2021): 015012. http://dx.doi.org/10.1088/2058-9565/ac3621.

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Abstract Quantum computation based on nonadiabatic geometric phases has attracted a broad range of interests, due to its fast manipulation and inherent noise resistance. However, it is limited to some special evolution paths, and the gate-times are typically longer than conventional dynamical gates, resulting in weakening of robustness and more infidelities of the implemented geometric gates. Here, we propose a path-optimized scheme for geometric quantum computation (GQC) on superconducting transmon qubits, where high-fidelity and robust universal nonadiabatic geometric gates can be implemented, based on conventional experimental setups. Specifically, we find that, by selecting appropriate evolution paths, the constructed geometric gates can be superior to their corresponding dynamical ones under different local errors. Numerical simulations show that the fidelities for single-qubit geometric phase, π/8 and Hadamard gates can be obtained as 99.93%, 99.95% and 99.95%, respectively. Remarkably, the fidelity for two-qubit control-phase gate can be as high as 99.87%. Therefore, our scheme provides a new perspective for GQC, making it more promising in the application of large-scale fault-tolerant quantum computation.

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22

Maciejewski, Filip B., Zoltán Zimborás, and Michał Oszmaniec. "Mitigation of readout noise in near-term quantum devices by classical post-processing based on detector tomography." Quantum 4 (April 24, 2020): 257. http://dx.doi.org/10.22331/q-2020-04-24-257.

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We propose a simple scheme to reduce readout errors in experiments on quantum systems with finite number of measurement outcomes. Our method relies on performing classical post-processing which is preceded by Quantum Detector Tomography, i.e., the reconstruction of a Positive-Operator Valued Measure (POVM) describing the given quantum measurement device. If the measurement device is affected only by an invertible classical noise, it is possible to correct the outcome statistics of future experiments performed on the same device. To support the practical applicability of this scheme for near-term quantum devices, we characterize measurements implemented in IBM's and Rigetti's quantum processors. We find that for these devices, based on superconducting transmon qubits, classical noise is indeed the dominant source of readout errors. Moreover, we analyze the influence of the presence of coherent errors and finite statistics on the performance of our error-mitigation procedure. Applying our scheme on the IBM's 5-qubit device, we observe a significant improvement of the results of a number of single- and two-qubit tasks including Quantum State Tomography (QST), Quantum Process Tomography (QPT), the implementation of non-projective measurements, and certain quantum algorithms (Grover's search and the Bernstein-Vazirani algorithm). Finally, we present results showing improvement for the implementation of certain probability distributions in the case of five qubits.

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Andersen, Christian Kraglund, and Alexandre Blais. "Ultrastrong coupling dynamics with a transmon qubit." New Journal of Physics 19, no. 2 (2017): 023022. http://dx.doi.org/10.1088/1367-2630/aa5941.

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Guo, Yanbo, Guozhong Wang та Nianquan Jiang. "Generating χ-Type Four-Qubit Entangled States in Superconducting Transmon Qubit System". International Journal of Theoretical Physics 53, № 9 (2014): 3135–41. http://dx.doi.org/10.1007/s10773-014-2110-0.

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Sevriuk, V. A., W. Liu, J. Rönkkö, et al. "Initial experimental results on a superconducting-qubit reset based on photon-assisted quasiparticle tunneling." Applied Physics Letters 121, no. 23 (2022): 234002. http://dx.doi.org/10.1063/5.0129345.

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We present here our recent results on qubit reset scheme based on a quantum-circuit refrigerator (QCR). In particular, we use the photon-assisted quasiparticle tunneling through a superconductor–insulator–normal-metal–insulator–superconductor junction to controllably decrease the energy relaxation time of the qubit during the QCR operation. In our experiment, we use a transmon qubit with dispersive readout. The QCR is capacitively coupled to the qubit through its normal-metal island. We employ rapid, square-shaped QCR control voltage pulses with durations in the range of 2–350 ns and a variety of amplitudes to optimize the reset time and fidelity. Consequently, we reach a qubit ground-state probability of roughly 97% with 80-ns pulses starting from the first excited state. The qubit state probability is extracted from averaged readout signal, where the calibration is based on Rabi oscillations, thus not distinguishing the residual thermal population of the qubit.

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Weides, Martin P., Jeffrey S. Kline, Michael R. Vissers, et al. "Coherence in a transmon qubit with epitaxial tunnel junctions." Applied Physics Letters 99, no. 26 (2011): 262502. http://dx.doi.org/10.1063/1.3672000.

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Tsioutsios, I., K. Serniak, S. Diamond, et al. "Free-standing silicon shadow masks for transmon qubit fabrication." AIP Advances 10, no. 6 (2020): 065120. http://dx.doi.org/10.1063/1.5138953.

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Cherubim, Cleverson, Frederico Brito, and Sebastian Deffner. "Non-Thermal Quantum Engine in Transmon Qubits." Entropy 21, no. 6 (2019): 545. http://dx.doi.org/10.3390/e21060545.

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The design and implementation of quantum technologies necessitates the understanding of thermodynamic processes in the quantum domain. In stark contrast to macroscopic thermodynamics, at the quantum scale processes generically operate far from equilibrium and are governed by fluctuations. Thus, experimental insight and empirical findings are indispensable in developing a comprehensive framework. To this end, we theoretically propose an experimentally realistic quantum engine that uses transmon qubits as working substance. We solve the dynamics analytically and calculate its efficiency.

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Rosenblum, S., P. Reinhold, M. Mirrahimi, Liang Jiang, L. Frunzio, and R. J. Schoelkopf. "Fault-tolerant detection of a quantum error." Science 361, no. 6399 (2018): 266–70. http://dx.doi.org/10.1126/science.aat3996.

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A critical component of any quantum error–correcting scheme is detection of errors by using an ancilla system. However, errors occurring in the ancilla can propagate onto the logical qubit, irreversibly corrupting the encoded information. We demonstrate a fault-tolerant error-detection scheme that suppresses spreading of ancilla errors by a factor of 5, while maintaining the assignment fidelity. The same method is used to prevent propagation of ancilla excitations, increasing the logical qubit dephasing time by an order of magnitude. Our approach is hardware-efficient, as it uses a single multilevel transmon ancilla and a cavity-encoded logical qubit, whose interaction is engineered in situ by using an off-resonant sideband drive. The results demonstrate that hardware-efficient approaches that exploit system-specific error models can yield advances toward fault-tolerant quantum computation.

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Gao, Gui-Long, Gen-Chang Cai, Shou-Sheng Huang, Ming-Feng Wang, and Nian-Quan Jiang. "One-Step Generation of Multi-Qubit GHZ and W States in Superconducting Transmon Qubit System." Communications in Theoretical Physics 57, no. 2 (2012): 205–8. http://dx.doi.org/10.1088/0253-6102/57/2/07.

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Tsuchimoto, Yuta, and Martin Kroner. "Low-loss high-impedance circuit for quantum transduction between optical and microwave photons." Materials for Quantum Technology 2, no. 2 (2022): 025001. http://dx.doi.org/10.1088/2633-4356/ac5ac4.

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Abstract Quantum transducers between microwave and optical photons are essential for long-distance quantum networks based on superconducting qubits. An optically active self-assembled quantum dot molecule (QDM) is an attractive platform for the implementation of a quantum transducer because an exciton in a QDM can be efficiently coupled to both optical and microwave fields at the single-photon level. Recently, the transduction between microwave and optical photons has been demonstrated with a QDM integrated with a superconducting resonator. In this paper, we present a design of a QD-high impedance resonator device with a low microwave loss and an expected large single-microwave photon coupling strength of 100s of MHz. We integrate self-assembled QDs onto a high-impedance superconducting resonator using a transfer printing technique and demonstrate a low-microwave loss rate of 1.8 MHz and gate tunability of the QDs. The corresponding microwave photon decay time of 88 ns is longer than the time necessary for the optical-microwave transduction process as well as the transmon-resonator swap operation time. This feature will facilitate efficient quantum transduction between an optical and microwave qubit.

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Antony, Abhinandan, Martin V. Gustafsson, Guilhem J. Ribeill, et al. "Miniaturizing Transmon Qubits Using van der Waals Materials." Nano Letters 21, no. 23 (2021): 10122–26. http://dx.doi.org/10.1021/acs.nanolett.1c04160.

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33

Dial, Oliver, Douglas T. McClure, Stefano Poletto, et al. "Bulk and surface loss in superconducting transmon qubits." Superconductor Science and Technology 29, no. 4 (2016): 044001. http://dx.doi.org/10.1088/0953-2048/29/4/044001.

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Gambetta, Jay M., Conal E. Murray, Y. K. K. Fung, et al. "Investigating Surface Loss Effects in Superconducting Transmon Qubits." IEEE Transactions on Applied Superconductivity 27, no. 1 (2017): 1–5. http://dx.doi.org/10.1109/tasc.2016.2629670.

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Xu, Yilun, Gang Huang, Jan Balewski, et al. "Automatic Qubit Characterization and Gate Optimization with QubiC." ACM Transactions on Quantum Computing, April 13, 2022. http://dx.doi.org/10.1145/3529397.

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As the size and complexity of a quantum computer increases, quantum bit (qubit) characterization and gate optimization become complex and time-consuming tasks. Current calibration techniques require complicated and verbose measurements to tune up qubits and gates, which cannot easily expand to the large-scale quantum systems. We develop a concise and automatic calibration protocol to characterize qubits and optimize gates using QubiC , which is an open source FPGA (field-programmable gate array) based control and measurement system for superconducting quantum information processors. We propose multi-dimensional loss-based optimization of single-qubit gates and full XY-plane measurement method for the two-qubit CNOT gate calibration. We demonstrate the QubiC automatic calibration protocols are capable of delivering high-fidelity gates on the state-of-the-art transmon-type processor operating at the Advanced Quantum Testbed at Lawrence Berkeley National Laboratory. The single-qubit and two-qubit Clifford gate infidelities measured by randomized benchmarking are of 4.9(1.1) × 10 − 4 and 1.4(3) × 10 − 2 , respectively.

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Place, Alexander P. M., Lila V. H. Rodgers, Pranav Mundada, et al. "New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds." Nature Communications 12, no. 1 (2021). http://dx.doi.org/10.1038/s41467-021-22030-5.

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AbstractThe superconducting transmon qubit is a leading platform for quantum computing and quantum science. Building large, useful quantum systems based on transmon qubits will require significant improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. This indicates that relaxation likely originates from uncontrolled surfaces, interfaces, and contaminants. Previous efforts to improve qubit lifetimes have focused primarily on designs that minimize contributions from surfaces. However, significant improvements in the lifetime of two-dimensional transmon qubits have remained elusive for several years. Here, we fabricate two-dimensional transmon qubits that have both lifetimes and coherence times with dynamical decoupling exceeding 0.3 milliseconds by replacing niobium with tantalum in the device. We have observed increased lifetimes for seventeen devices, indicating that these material improvements are robust, paving the way for higher gate fidelities in multi-qubit processors.

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37

Wang, Z. T., Peng Zhao, Z. H. Yang, Ye Tian, H. F. Yu, and S. P. Zhao. "Escaping detrimental interactions with microwave-dressed transmon qubits." Chinese Physics Letters, June 27, 2023. http://dx.doi.org/10.1088/0256-307x/40/7/070304.

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Abstract Superconducting transmon qubits with fixed frequencies are widely used in many applications due to their advantages of better coherence and less control lines as compared to the frequency tunable qubits. However, any uncontrolled interactions with the qubits such as the two-level systems could lead to adverse impacts, degrading the qubit coherence and inducing crosstalk. To mitigate the detrimental effect from uncontrolled interactions between qubits and defect modes in fixedfrequency transmon qubits, we propose and demonstrate an active approach using an off-resonance microwave drive to dress the qubit and induce the ac-Stark shift on the qubit frequency. We show experimentally that the qubit frequency can be tuned well away from the defect mode so that the impact on qubit coherence is greatly reduced while maintaining the universal controls of the qubit initialization, readout, and single-qubit gate operations. Our approach provides an effective way for tuning the qubit frequency and suppressing the detrimental effect from the defect modes that happen to be located close to the qubit frequency.

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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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Zhang, Eric J., Srikanth Srinivasan, Neereja Sundaresan, et al. "High-performance superconducting quantum processors via laser annealing of transmon qubits." Science Advances 8, no. 19 (2022). http://dx.doi.org/10.1126/sciadv.abi6690.

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Scaling the number of qubits while maintaining high-fidelity quantum gates remains a key challenge for quantum computing. Presently, superconducting quantum processors with >50 qubits are actively available. For these systems, fixed-frequency transmons are attractive because of their long coherence and noise immunity. However, scaling fixed-frequency architectures proves challenging because of precise relative frequency requirements. Here, we use laser annealing to selectively tune transmon qubits into desired frequency patterns. Statistics over hundreds of annealed qubits demonstrate an empirical tuning precision of 18.5 MHz, with no measurable impact on qubit coherence. We quantify gate error statistics on a tuned 65-qubit processor, with median two-qubit gate fidelity of 98.7%. Baseline tuning statistics yield a frequency-equivalent resistance precision of 4.7 MHz, sufficient for high-yield scaling beyond 10 3 qubit levels. Moving forward, we anticipate selective laser annealing to play a central role in scaling fixed-frequency architectures.

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Vepsäläinen, Antti, Roni Winik, Amir H. Karamlou, et al. "Improving qubit coherence using closed-loop feedback." Nature Communications 13, no. 1 (2022). http://dx.doi.org/10.1038/s41467-022-29287-4.

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AbstractSuperconducting qubits are a promising platform for building a larger-scale quantum processor capable of solving otherwise intractable problems. In order for the processor to reach practical viability, the gate errors need to be further suppressed and remain stable for extended periods of time. With recent advances in qubit control, both single- and two-qubit gate fidelities are now in many cases limited by the coherence times of the qubits. Here we experimentally employ closed-loop feedback to stabilize the frequency fluctuations of a superconducting transmon qubit, thereby increasing its coherence time by 26% and reducing the single-qubit error rate from (8.5 ± 2.1) × 10−4 to (5.9 ± 0.7) × 10−4. Importantly, the resulting high-fidelity operation remains effective even away from the qubit flux-noise insensitive point, significantly increasing the frequency bandwidth over which the qubit can be operated with high fidelity. This approach is helpful in large qubit grids, where frequency crowding and parasitic interactions between the qubits limit their performance.

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Spring, Peter A., Shuxiang Cao, Takahiro Tsunoda, et al. "High coherence and low cross-talk in a tileable 3D integrated superconducting circuit architecture." Science Advances 8, no. 16 (2022). http://dx.doi.org/10.1126/sciadv.abl6698.

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We report high qubit coherence as well as low cross-talk and single-qubit gate errors in a superconducting circuit architecture that promises to be tileable to two-dimensional (2D) lattices of qubits. The architecture integrates an inductively shunted cavity enclosure into a design featuring nongalvanic out-of-plane control wiring and qubits and resonators fabricated on opposing sides of a substrate. The proof-of-principle device features four uncoupled transmon qubits and exhibits average energy relaxation times T 1 = 149(38) μs, pure echoed dephasing times T ϕ, e = 189(34) μs, and single-qubit gate fidelities F = 99.982(4)% as measured by simultaneous randomized benchmarking. The 3D integrated nature of the control wiring means that qubits will remain addressable as the architecture is tiled to form larger qubit lattices. Band structure simulations are used to predict that the tiled enclosure will still provide a clean electromagnetic environment to enclosed qubits at arbitrary scale.

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Wang, Chenlu, Xuegang Li, Huikai Xu, et al. "Towards practical quantum computers: transmon qubit with a lifetime approaching 0.5 milliseconds." npj Quantum Information 8, no. 1 (2022). http://dx.doi.org/10.1038/s41534-021-00510-2.

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AbstractHere we report a breakthrough in the fabrication of a long lifetime transmon qubit. We use tantalum films as the base superconductor. By using a dry etching process, we obtained transmon qubits with a best T1 lifetime of 503 μs. As a comparison, we also fabricated transmon qubits with other popular materials, including niobium and aluminum, under the same design and fabrication processes. After characterizing their coherence properties, we found that qubits prepared with tantalum films have the best performance. Since the dry etching process is stable and highly anisotropic, it is much more suitable for fabricating complex scalable quantum circuits, when compared to wet etching. As a result, the current breakthrough indicates that the dry etching process of tantalum film is a promising approach to fabricate medium- or large-scale superconducting quantum circuits with a much longer lifetime, meeting the requirements for building practical quantum computers.

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Kosen, Sandoko, Hang-Xi Li, Marcus Rommel, et al. "Building blocks of a flip-chip integrated superconducting quantum processor." Quantum Science and Technology, May 25, 2022. http://dx.doi.org/10.1088/2058-9565/ac734b.

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Abstract We have integrated single and coupled superconducting transmon qubits into flip-chip modules. Each module consists of two chips - one quantum chip and one control chip - that are bump-bonded together. We demonstrate time-averaged coherence times exceeding 90μs, single-qubit gate fidelities exceeding 99.9%, and two-qubit gate fidelities above 98.6%. We also present device design methods and discuss the sensitivity of device parameters to variation in interchip spacing. Notably, the additional flip-chip fabrication steps do not degrade the qubit performance compared to our baseline state-of-the-art in single-chip, planar circuits. This integration technique can be extended to the realisation of quantum processors accommodating hundreds of qubits in one module as it offers adequate input/output wiring access to all qubits and couplers.

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Asaad, Serwan, Christian Dickel, Nathan K. Langford, et al. "Independent, extensible control of same-frequency superconducting qubits by selective broadcasting." npj Quantum Information 2, no. 1 (2016). http://dx.doi.org/10.1038/npjqi.2016.29.

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Abstract A critical ingredient for realising large-scale quantum information processors will be the ability to make economical use of qubit control hardware. We demonstrate an extensible strategy for reusing control hardware on same-frequency transmon qubits in a circuit QED chip with surface-code-compatible connectivity. A vector switch matrix enables selective broadcasting of input pulses to multiple transmons with individual tailoring of pulse quadratures for each, as required to minimise the effects of leakage on weakly anharmonic qubits. Using randomised benchmarking, we compare multiple broadcasting strategies that each pass the surface-code error threshold for single-qubit gates. In particular, we introduce a selective broadcasting control strategy using five pulse primitives, which allows independent, simultaneous Clifford gates on arbitrary numbers of qubits.

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Babu, Aravind Plathanam, Jani Tuorila, and Tapio Ala-Nissila. "State leakage during fast decay and control of a superconducting transmon qubit." npj Quantum Information 7, no. 1 (2021). http://dx.doi.org/10.1038/s41534-020-00357-z.

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AbstractSuperconducting Josephson junction qubits constitute the main current technology for many applications, including scalable quantum computers and thermal devices. Theoretical modeling of such systems is usually done within the two-level approximation. However, accurate theoretical modeling requires taking into account the influence of the higher excited states without limiting the system to the two-level qubit subspace. Here, we study the dynamics and control of a superconducting transmon using the numerically exact stochastic Liouville–von Neumann equation approach. We focus on the role of state leakage from the ideal two-level subspace for bath induced decay and single-qubit gate operations. We find significant short-time state leakage due to the strong coupling to the bath. We quantify the leakage errors in single-qubit gates and demonstrate their suppression with derivative removal adiabatic gates (DRAG) control for a five-level transmon in the presence of decoherence. Our results predict the limits of accuracy of the two-level approximation and possible intrinsic constraints in qubit dynamics and control for an experimentally relevant parameter set.

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Lisenfeld, Jürgen, Alexander Bilmes, Anthony Megrant, et al. "Electric field spectroscopy of material defects in transmon qubits." npj Quantum Information 5, no. 1 (2019). http://dx.doi.org/10.1038/s41534-019-0224-1.

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AbstractSuperconducting integrated circuits have demonstrated a tremendous potential to realize integrated quantum computing processors. However, the downside of the solid-state approach is that superconducting qubits suffer strongly from energy dissipation and environmental fluctuations caused by atomic-scale defects in device materials. Further progress towards upscaled quantum processors will require improvements in device fabrication techniques, which need to be guided by novel analysis methods to understand and prevent mechanisms of defect formation. Here, we present a technique to analyse individual defects in superconducting qubits by tuning them with applied electric fields. This provides a spectroscopy method to extract the defects’ energy distribution, electric dipole moments, and coherence times. Moreover, it enables one to distinguish defects residing in Josephson junction tunnel barriers from those at circuit interfaces. We find that defects at circuit interfaces are responsible for about 60% of the dielectric loss in the investigated transmon qubit sample. About 40% of all detected defects are contained in the tunnel barriers of the large-area parasitic Josephson junctions that occur collaterally in shadow evaporation, and only $$\approx$$≈3% are identified as strongly coupled defects, which presumably reside in the small-area qubit tunnel junctions. The demonstrated technique provides a valuable tool to assess the decoherence sources related to circuit interfaces and to tunnel junctions that is readily applicable to standard qubit samples.

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Kounalakis, Marios, Yaroslav M. Blanter, and Gary A. Steele. "Synthesizing multi-phonon quantum superposition states using flux-mediated three-body interactions with superconducting qubits." npj Quantum Information 5, no. 1 (2019). http://dx.doi.org/10.1038/s41534-019-0219-y.

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AbstractMassive mechanical resonators operating at the quantum scale can enable a large variety of applications in quantum technologies as well as fundamental tests of quantum theory. Of crucial importance in that direction is both their integrability into state-of-the-art quantum platforms as well as the ability to prepare them in generic quantum states using well-controlled high-fidelity operations. Here, we propose a scheme for controlling a radio-frequency mechanical resonator at the quantum scale using two superconducting transmon qubits that can be integrated on the same chip. Specifically, we consider two qubits coupled via a capacitor in parallel to a superconducting quantum interference device (SQUID), which has a suspended mechanical beam embedded in one of its arms. Following a theoretical analysis of the quantum system, we find that this configuration, in combination with an in-plane magnetic field, can give rise to a tuneable three-body interaction in the single-photon strong-coupling regime, while enabling suppression of the stray qubit-qubit coupling. Using state-of-the-art parameters and qubit operations at single-excitation levels, we numerically demonstrate the possibility of ground-state cooling as well as high-fidelity preparation of mechanical quantum states and qubit-phonon entanglement, i.e. states having negative Wigner functions and obeying non-classical correlations. Our work significantly extends the quantum control toolbox of radio-frequency mechanical resonators and may serve as a promising architecture for integrating such mechanical elements with transmon-based quantum processors.

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Bera, Tanmoy, Sourav Majumder, Sudhir Kumar Sahu, and Vibhor Singh. "Large flux-mediated coupling in hybrid electromechanical system with a transmon qubit." Communications Physics 4, no. 1 (2021). http://dx.doi.org/10.1038/s42005-020-00514-y.

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AbstractControl over the quantum states of a massive oscillator is important for several technological applications and to test the fundamental limits of quantum mechanics. Addition of an internal degree of freedom to the oscillator could be a valuable resource for such control. Recently, hybrid electromechanical systems using superconducting qubits, based on electric-charge mediated coupling, have been quite successful. Here, we show a hybrid device, consisting of a superconducting transmon qubit and a mechanical resonator coupled using the magnetic-flux. The coupling stems from the quantum-interference of the superconducting phase across the tunnel junctions. We demonstrate a vacuum electromechanical coupling rate up to 4 kHz by making the transmon qubit resonant with the readout cavity. Consequently, thermal-motion of the mechanical resonator is detected by driving the hybridized-mode with mean-occupancy well below one photon. By tuning qubit away from the cavity, electromechanical coupling can be enhanced to 40 kHz. In this limit, a small coherent drive on the mechanical resonator results in the splitting of qubit spectrum, and we observe interference signature arising from the Landau-Zener-Stückelberg effect. With improvements in qubit coherence, this system offers a platform to realize rich interactions and could potentially provide full control over the quantum motional states.

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Premkumar, Anjali, Conan Weiland, Sooyeon Hwang, et al. "Microscopic relaxation channels in materials for superconducting qubits." Communications Materials 2, no. 1 (2021). http://dx.doi.org/10.1038/s43246-021-00174-7.

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AbstractDespite mounting evidence that materials imperfections are a major obstacle to practical applications of superconducting qubits, connections between microscopic material properties and qubit coherence are poorly understood. Here, we combine measurements of transmon qubit relaxation times (T1) with spectroscopy and microscopy of the polycrystalline niobium films used in qubit fabrication. By comparing films deposited using three different techniques, we reveal correlations between T1 and intrinsic film properties such as grain size, enhanced oxygen diffusion along grain boundaries, and the concentration of suboxides near the surface. Qubit and resonator measurements show signatures of two-level system defects, which we propose to be hosted in the grain boundaries and surface oxides. We also show that the residual resistance ratio of the polycrystalline niobium films can be used as a figure of merit for qubit lifetime. This comprehensive approach to understanding qubit decoherence charts a pathway for materials-driven improvements of superconducting qubit performance.

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Castellanos-Beltran, M. A., A. J. Sirois, L. Howe, et al. "Coherence-limited digital control of a superconducting qubit using a Josephson pulse generator at 3 K." Applied Physics Letters 122, no. 19 (2023). http://dx.doi.org/10.1063/5.0147692.

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Compared to traditional semiconductor control electronics (TSCE) located at room temperature, cryogenic single flux quantum (SFQ) electronics can provide qubit measurement and control alternatives that address critical issues related to scalability of cryogenic quantum processors. Single-qubit control and readout have been demonstrated recently using SFQ circuits coupled to superconducting qubits. Experiments where the SFQ electronics are co-located with the qubit have suffered from excess decoherence and loss due to quasiparticle poisoning of the qubit. A previous experiment by our group showed that moving the control electronics to the 3 K stage of the dilution refrigerator avoided this source of decoherence in a high-coherence three-dimensional transmon geometry. In this paper, we also generate the pulses at the 3 K stage but have optimized the qubit design and control lines for scalable two-dimensional transmon devices. We directly compare the qubit lifetime T1, coherence time T2*, and gate fidelity when the qubit is controlled by the Josephson pulse generator (JPG) circuit vs the TSCE setup. We find agreement within the daily fluctuations for T1 and T2*, and agreement within 10% for randomized benchmarking. We also performed interleaved randomized benchmarking on individual JPG gates demonstrating an average error per gate of 0.46% showing good agreement with what is expected based on the qubit coherence and higher-state leakage. These results are an order of magnitude improvement in gate fidelity over our previous work and demonstrate that a Josephson microwave source operated at 3 K is a promising component for scalable qubit control.

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Journal articles on the topic 'Transman qubit'

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