High-fidelity single- and two-qubit gates are essential building blocks for a fault-tolerant quantum computer. While there has been much progress in suppressing single-qubit gate errors in superconducting qubit systems, two-qubit gates still suffer from error rates that are orders of magnitude higher. One limiting factor is the residual ZZ-interaction, which originates from a coupling between computational states and higher-energy states. While this interaction is usually viewed as a nuisance, here we experimentally demonstrate that it can be exploited to produce a universal set of fast single- and two-qubit entangling gates in a coupled transmon qubit system. To implement arbitrary single-qubit rotations, we design a new protocol called the two-axis gate that is based on a three-part composite pulse. It rotates a single qubit independently of the state of the other qubit despite the strong ZZ-coupling. We achieve single-qubit gate fidelities as high as 99.1% from randomized benchmarking measurements. We then demonstrate both a CZ gate and a CNOT gate. Because the system has a strong ZZ-interaction, a CZ gate can be achieved by letting the system freely evolve for a gate time $t_g=53.8$ ns. To design the CNOT gate, we utilize an analytical microwave pulse shape based on the SWIPHT protocol for realizing fast, low-leakage gates. We obtain fidelities of 94.6% and 97.8% for the CNOT and CZ gates respectively from quantum progress tomography.High-fidelity single- and two-qubit gates are essential building blocks for a fault-tolerant quantum computer. While there has been much progress in suppressing single-qubit gate errors in superconducting qubit systems, two-qubit gates still suffer from error rates that are orders of magnitude higher. One limiting factor is the residual ZZ-interaction, which originates from a coupling between computational states and higher-energy states. While this interaction is usually viewed as a nuisance, here we experimentally demonstrate that it can be exploited to produce a universal set of fast single- and two-qubit entangling gates in a coupled transmon qubit system. To implement arbitrary single-qubit rotations, we design a new protocol called the two-axis gate that is based on a three-part composite pulse. It rotates a single qubit independently of the state of the other qubit despite the strong ZZ-coupling. We achieve single-qubit gate fidelities as high as 99.1% from randomized benchmarking measurements. We then demonstrate both a CZ gate and a CNOT gate. Because the system has a strong ZZ-interaction, a CZ gate can be achieved by letting the system freely evolve for a gate time $t_g=53.8$ ns. To design the CNOT gate, we utilize an analytical microwave pulse shape based on the SWIPHT protocol for realizing fast, low-leakage gates. We obtain fidelities of 94.6% and 97.8% for the CNOT and CZ gates respectively from quantum progress tomography.

arXiv:2103.12305Mar 23, 20217 citations

State-conditional coherent charge qubit oscillations in a Si/SiGe quadruple quantum dot

Daniel R. Ward, Dohun Kim, D. E. Savage +5 more

Abstract Universal quantum computation requires high-fidelity single-qubit rotations and controlled two-qubit gates. Along with high-fidelity single-qubit gates, strong efforts have been made in developing robust two-qubit logic gates in electrically gated quantum dot systems to realise a compact and nanofabrication-compatible architecture. Here we perform measurements of state-conditional coherent oscillations of a charge qubit. Using a quadruple quantum dot formed in a Si/SiGe heterostructure, we show the first demonstration of coherent two-axis control of a double quantum dot charge qubit in undoped Si/SiGe, performing Larmor and Ramsey oscillation measurements. We extract the strength of the capacitive coupling between a pair of double quantum dots by measuring the detuning energy shift (≈75 μeV) of one double dot depending on the excess charge configuration of the other double dot. We further demonstrate that the strong capacitive coupling allows fast, state-conditional Landau–Zener–Stückelberg oscillations with a conditional π phase flip time of about 80 ps, showing a promising pathway towards multi-qubit entanglement and control in semiconductor quantum dots.Abstract Universal quantum computation requires high-fidelity single-qubit rotations and controlled two-qubit gates. Along with high-fidelity single-qubit gates, strong efforts have been made in developing robust two-qubit logic gates in electrically gated quantum dot systems to realise a compact and nanofabrication-compatible architecture. Here we perform measurements of state-conditional coherent oscillations of a charge qubit. Using a quadruple quantum dot formed in a Si/SiGe heterostructure, we show the first demonstration of coherent two-axis control of a double quantum dot charge qubit in undoped Si/SiGe, performing Larmor and Ramsey oscillation measurements. We extract the strength of the capacitive coupling between a pair of double quantum dots by measuring the detuning energy shift (≈75 μeV) of one double dot depending on the excess charge configuration of the other double dot. We further demonstrate that the strong capacitive coupling allows fast, state-conditional Landau–Zener–Stückelberg oscillations with a conditional π phase flip time of about 80 ps, showing a promising pathway towards multi-qubit entanglement and control in semiconductor quantum dots.

arXiv:1604.07956npj Quantum InformationOct 18, 201648 citations

Fast, high-fidelity two-qubit entangling gates in superconducting multiqubit systems

Hsiang‐Sheng Ku, Junling Long, Russell E. Lake +3 more

Bulletin of the American Physical SocietyMar 7, 20180 citations

Fast High-Fidelity Gates for Galvanically-Coupled Fluxonium Qubits Using Strong Flux Modulation

Daniel Weiss, Helin Zhang, Chunyang Ding +3 more

Long coherence times, large anharmonicity, and robust charge-noise insensitivity render fluxonium qubits an interesting alternative to transmons.Recent experiments have demonstrated record coherence times for low-frequency fluxonium qubits.Here, we propose a galvanic coupling scheme with flux-tunable XX coupling.To implement a high-fidelity entangling √ iSWAP gate, we modulate the strength of this coupling and devise variable-time identity gates to synchronize required single-qubit operations.Both types of gates are implemented using strong ac flux drives, lasting for only a few drive periods.We employ a theoretical framework capable of capturing qubit dynamics beyond the rotating-wave approximation as required for such strong drives.We predict an open-system fidelity of F > 0.999 for the √ iSWAP gate under realistic conditions.Long coherence times, large anharmonicity, and robust charge-noise insensitivity render fluxonium qubits an interesting alternative to transmons.Recent experiments have demonstrated record coherence times for low-frequency fluxonium qubits.Here, we propose a galvanic coupling scheme with flux-tunable XX coupling.To implement a high-fidelity entangling √ iSWAP gate, we modulate the strength of this coupling and devise variable-time identity gates to synchronize required single-qubit operations.Both types of gates are implemented using strong ac flux drives, lasting for only a few drive periods.We employ a theoretical framework capable of capturing qubit dynamics beyond the rotating-wave approximation as required for such strong drives.We predict an open-system fidelity of F > 0.999 for the √ iSWAP gate under realistic conditions.

arXiv:2207.03971PRX QuantumDec 27, 202227 citations

A High-Fidelity Gateset for Exchange-Coupled Singlet-Triplet Qubits

René Otten, Pascal Cerfontaine, M. A. Wolfe +1 more

Bulletin of the American Physical SocietyMar 5, 20190 citations

t_g=53.8

ns. To design the CNOT gate, we utilize an analytical microwave pulse shape based on the SWIPHT protocol for realizing fast, low-leakage gates. We obtain fidelities of 94.6% and 97.8% for the CNOT and CZ gates respectively from quantum progress tomography.