Superconducting flux qubits are promising candidates for the physical realization of a scalable quantum processor. Indeed, these circuits may have both a small decoherence rate and a large anharmonicity. These properties enable the application of fast quantum gates with high fidelity and reduce scaling limitations due to frequency crowding. The major difficulty of flux qubits' design consists of controlling precisely their transition energy - the so-called qubit gap - while keeping long and reproducible relaxation times. Solving this problem is challenging and requires extremely good control of e-beam lithography, oxidation parameters of the junctions and sample surface. Here we present measurements of a large batch of flux qubits and demonstrate a high level of reproducibility and control of qubit gaps, relaxation times and pure echo dephasing times. These results open the way for potential applications in the fields of quantum hybrid circuits and quantum computation.Superconducting flux qubits are promising candidates for the physical realization of a scalable quantum processor. Indeed, these circuits may have both a small decoherence rate and a large anharmonicity. These properties enable the application of fast quantum gates with high fidelity and reduce scaling limitations due to frequency crowding. The major difficulty of flux qubits' design consists of controlling precisely their transition energy - the so-called qubit gap - while keeping long and reproducible relaxation times. Solving this problem is challenging and requires extremely good control of e-beam lithography, oxidation parameters of the junctions and sample surface. Here we present measurements of a large batch of flux qubits and demonstrate a high level of reproducibility and control of qubit gaps, relaxation times and pure echo dephasing times. These results open the way for potential applications in the fields of quantum hybrid circuits and quantum computation.

arXiv:2207.01427Jul 4, 20220 citations

The Flux Qubit Revisited

Fei Yan, Simon Gustavsson, Archana Kamal +10 more

The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). Here, we revisit the design and fabrication of the superconducting flux qubit, achieving a planar device with broad frequency tunability, strong anharmonicity, high reproducibility, and relaxation times in excess of $40\,\mu$ s at its flux-insensitive point. Qubit relaxation times $T_1$ across 22 qubits are consistently matched with a single model involving resonator loss, ohmic charge noise, and 1/f flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, resulting in $T_2\approx 85\,\mu$ s, approximately the $2T_1$ limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting $T_2$ in contemporary qubits based on transverse qubit-resonator interaction.The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). Here, we revisit the design and fabrication of the superconducting flux qubit, achieving a planar device with broad frequency tunability, strong anharmonicity, high reproducibility, and relaxation times in excess of $40\,\mu$s at its flux-insensitive point. Qubit relaxation times $T_1$ across 22 qubits are consistently matched with a single model involving resonator loss, ohmic charge noise, and 1/f flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, resulting in $T_2\approx 85\,\mu$s, approximately the $2T_1$ limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting $T_2$ in contemporary qubits based on transverse qubit-resonator interaction.

Aug 25, 20156 citations

The flux qubit revisited to enhance coherence and reproducibility

Fei Yan, Simon Gustavsson, Archana Kamal +12 more

Abstract The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). Here, we revisit the design and fabrication of the superconducting flux qubit, achieving a planar device with broad-frequency tunability, strong anharmonicity, high reproducibility and relaxation times in excess of 40 μs at its flux-insensitive point. Qubit relaxation times T 1 across 22 qubits are consistently matched with a single model involving resonator loss, ohmic charge noise and 1/f -flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal-photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, resulting in T 2 ≈85 μs, approximately the 2 T 1 limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting T 2 in contemporary qubits based on transverse qubit–resonator interaction.Abstract The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). Here, we revisit the design and fabrication of the superconducting flux qubit, achieving a planar device with broad-frequency tunability, strong anharmonicity, high reproducibility and relaxation times in excess of 40 μs at its flux-insensitive point. Qubit relaxation times T 1 across 22 qubits are consistently matched with a single model involving resonator loss, ohmic charge noise and 1/f -flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal-photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, resulting in T 2 ≈85 μs, approximately the 2 T 1 limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting T 2 in contemporary qubits based on transverse qubit–resonator interaction.

arXiv:1508.06299Nature CommunicationsNov 3, 2016582 citations

Universal Fast-Flux Control of a Coherent, Low-Frequency Qubit

Helin Zhang, Srivatsan Chakram, Tanay Roy +6 more

The heavy-fluxonium circuit is a promising building block for superconducting quantum processors due to its long relaxation and dephasing time at the flux-frustration point. However, the suppressed charge matrix elements and low transition frequency make it challenging to perform fast single-qubit gates using standard protocols. We report on new protocols for reset, fast coherent control,and readout that allow high-quality operation of the qubit with a 14 MHz transition frequency, an order of magnitude lower in energy than the ambient thermal energy scale. We utilize higher levels of the fluxonium to read out the qubit state and to initialize the qubit with 97% fidelity corresponding to cooling it to 190 μK. Instead of using standard microwave pulses, we control the qubit only with fast-flux pulses, generating control fields much larger than the qubit frequency. We develop a universal set of gates based on nonadiabatic Landau-Zener transitions that act in 20-60 ns, less than the single-qubit Larmor period. We measure qubit coherence of T1,T2e∼300 μs for a fluxonium in a 2D architecture and realize single-qubit gates with an average gate fidelity of 99.8% as characterized by randomized benchmarking.The heavy-fluxonium circuit is a promising building block for superconducting quantum processors due to its long relaxation and dephasing time at the flux-frustration point. However, the suppressed charge matrix elements and low transition frequency make it challenging to perform fast single-qubit gates using standard protocols. We report on new protocols for reset, fast coherent control,and readout that allow high-quality operation of the qubit with a 14 MHz transition frequency, an order of magnitude lower in energy than the ambient thermal energy scale. We utilize higher levels of the fluxonium to read out the qubit state and to initialize the qubit with 97% fidelity corresponding to cooling it to 190 μK. Instead of using standard microwave pulses, we control the qubit only with fast-flux pulses, generating control fields much larger than the qubit frequency. We develop a universal set of gates based on nonadiabatic Landau-Zener transitions that act in 20-60 ns, less than the single-qubit Larmor period. We measure qubit coherence of T1,T2e∼300 μs for a fluxonium in a 2D architecture and realize single-qubit gates with an average gate fidelity of 99.8% as characterized by randomized benchmarking.

Jan 15, 2021104 citations

High-Coherence Fluxonium Qubit

Long B. Nguyen, Yen‐Hsiang Lin, Aaron Somoroff +3 more

We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfying T 2 > 100 μs (T 2 > 400 μs in one device).Moreover, given the state-of-the-art values of the surface loss tangent and the 1=f flux-noise amplitude, the coherence time can be further improved beyond 1 ms.Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit-over 10 2 here-must be minimized for best qubit coherence.We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfying T 2 > 100 μs (T 2 > 400 μs in one device).Moreover, given the state-of-the-art values of the surface loss tangent and the 1=f flux-noise amplitude, the coherence time can be further improved beyond 1 ms.Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit-over 10 2 here-must be minimized for best qubit coherence.We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.

arXiv:1810.11006Physical Review XNov 25, 2019290 citations

Toward the Fluxonium Quantum Processor

Long B. Nguyen

This thesis reports recent achievements toward scalable quantum computing with fluxonium, a superconducting artificial atom with rich energy spectrum and selection rules similar to those found in natural atoms. We show how such spectral properties can be harnessed to protect the qubit from energy relaxation and dephasing. At half-integer flux quantum bias, we show that fluxonium’s |0〉→ |1〉qubit transition has high coherence by design, with T1, T2≈500 μs in one device, the highest reported in superconducting circuits so far. Yet, the qubit exhibits the same level of addressability found in more conventional superconducting qubits (Tgate<50 ns). In addition, a controlled-Z gate can be implemented by sending a short2π-pulse at a frequency near the |1〉→|2〉transition of the target qubit. Preliminary results suggest that this gate can be used to entangle two fluxonium qubits with high fidelity. We also discuss experimental techniques employed to characterize the qubits, and present a perspective on future fluxonium-based quantum technologies.This thesis reports recent achievements toward scalable quantum computing with fluxonium, a superconducting artificial atom with rich energy spectrum and selection rules similar to those found in natural atoms. We show how such spectral properties can be harnessed to protect the qubit from energy relaxation and dephasing. At half-integer flux quantum bias, we show that fluxonium’s |0〉→ |1〉qubit transition has high coherence by design, with T1, T2≈500 μs in one device, the highest reported in superconducting circuits so far. Yet, the qubit exhibits the same level of addressability found in more conventional superconducting qubits (Tgate<50 ns). In addition, a controlled-Z gate can be implemented by sending a short2π-pulse at a frequency near the |1〉→|2〉transition of the target qubit. Preliminary results suggest that this gate can be used to entangle two fluxonium qubits with high fidelity. We also discuss experimental techniques employed to characterize the qubits, and present a perspective on future fluxonium-based quantum technologies.

Jan 1, 20201 citation

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), and pure echo dephasing times (

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). These results open the way for potential applications in the fields of quantum hybrid circuits and quantum computation.

40\,\mu

s at its flux-insensitive point. Qubit relaxation times

T_1

across 22 qubits are consistently matched with a single model involving resonator loss, ohmic charge noise, and 1/f flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, resulting in

T_2\approx 85\,\mu

s, approximately the

2T_1

limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting

T_2

in contemporary qubits based on transverse qubit-resonator interaction.