Rapid and unconditional parametric reset protocol for tunable superconducting qubits

ARTICLE https://doi.org/10.1038/s41467-021-26205-y OPEN Rapid and unconditional parametric reset protocol for tunable superconducting qubits 1234567890():,; Yu Zhou 1,4, Zhenxing Zhang1,4, Zelong Yin1, Sainan Huai1, Xiu Gu1, Xiong Xu1, Jonathan Allcock1, Fuming Liu1, Guanglei Xi1, Qiaonian Yu1, Hualiang Zhang1, Mengyu Zhang1, Hekang Li 2,3, Xiaohui Song2,3, Zhan Wang2,3, Dongning Zheng2,3, Shuoming An 1 ✉, Yarui Zheng1 & Shengyu Zhang1 Qubit initialization is a critical task in quantum computation and communication. Extensive efforts have been made to achieve this with high speed, efficiency and scalability. However, previous approaches have either been measurement-based and required fast feedback, suffered from crosstalk or required sophisticated calibration. Here, we report a fast and highfidelity reset scheme, avoiding the issues above without any additional chip architecture. By modulating the flux through a transmon qubit, we realize a swap between the qubit and its readout resonator that suppresses the excited state population to 0.08% ± 0.08% within 34 ns (284 ns if photon depletion of the resonator is required). Furthermore, our approach (i) can achieve effective second excited state depletion, (ii) has negligible effects on neighboring qubits, and (iii) offers a way to entangle the qubit with an itinerant single photon, useful in quantum communication applications. 1 Tencent Quantum Laboratory, Tencent, Shenzhen, Guangdong 518057, China. 2 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. 3 School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China. 4These authors contributed equally. Yu Zhou and Zhenxing Zhang ✉email: NATURE COMMUNICATIONS | (2021)12:5924 | https://doi.org/10.1038/s41467-021-26205-y | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-26205-y Q ubit initialization is fundamental and crucial for many quantum algorithms and quantum information processing tasks. The ability to quickly reset qubits to the zero state is one of DiVincenzo’s essential criteria for building a quantum computer1 and is critical for quantum error correction2–4, where the reset of syndrome qubits needs to be accomplished with high fidelity in the time scale of a single qubit pulse. Furthermore, significant reduction of state preparation and measurement (SPAM) errors can be achieved by evacuating residual excited state populations with high fidelity5,6. The simplest way to reset qubits is to passively wait for them to de-excite, but as qubit relaxation times increase beyond 100 μs7,8, this method becomes impractically slow. Alternatively, active reset implementations can shorten the wait time between cycles and significantly improve computational efficiency9,10. Various reset protocols for superconducting qubits have been proposed which fall into two main types: measurement- and nonmeasurement-based protocols. In measurement-based schemes, a qubit is measured and either heralded in the ground state11, or else is found to be in the excited state and reset via a conditional π-pulse6,12–15. These protocols depend heavily on measurement fidelity and suffer from measurement-induced state mixing5,16. In addition, the hardware implementation of necessary short-latency feedback loops is also a challenge. In non-measurement based protocols, qubits are coupled to a lossy environment, usually a resonator. While numerous approaches to this have been proposed, they each suffer from their own drawbacks. For instance, in one such approach, flux control17,18 is used to rapidly tune the qubit frequency to that of the resonator. However, this process significantly affects neighboring qubits via crosstalk19,20. Another approach is based on a microwave-induced interaction between 9,21. However, the the qubit and a low-quality factor resonator

involvement of the second excited state f makes these schemes complicated and necessitates sophisticated calibration. Furthermore, intense microwave driving is required to activate the required cavity-assisted Raman processes21–23, affecting adjacent qubits as well. In21, an additional resonator is required to achieve the best performance. In contrast to the above methods, the driven reset scheme proposed in10 is free from flux control and complicated pulses. On the other hand, this protocol requires that the resonator dissipation rate κr be smaller than the dispersive shift χ, imposing a trade-off between readout speed and fidelity. In this work, we demonstrate a rapid and unconditional parametric reset scheme for tunable superconducting qubits. By parametric modulation of the qubit frequency, a controllable interaction is generated between the qubit and a lossy readout resonator. This interaction unconditionally transfers the qubit excitation to the resonator and thus resets the qubit on demand. Using this method, we can suppress the residual excited population to
0.08% ± 0.08% within 34 ns. We also demonstrate  effective
f state depletion in the case when leakage to higher states is non-negligible. Our protocol only involves AC modulation of at most two frequencies and does not need sophisticated calibration. Moreover, it has a negligible effect on subsequent gates and other qubits. It is compatible with circuit quantum electrodynamics systems24–26 and can be applied to all frequencytunable superconducting qubits, requiring no additional hardware or modifications to chip components. The method also imposes no restriction on operation flux position or specific system parameters such as resonator dissipation rate κr or dispersive shift. Results Theory. Our qubit reset protocol is based on a parametric activated interaction between a tunable qubit and a rapidly decaying 2 resonator. Such a parametric modulation induces an effective tunable coupling between the qubit and other quantum systems such as another qubit or resonator27–29 and has been used to implement multi-qubit quantum gates30–35, state transfer36,37, switches for quantum circuits38 and parity measurements39. In our reset protocol, the parametric
modulation induces Rabi oscillations between je; 0i and
g; 1 , where js; li denotes
 the tensor product of the qubit state jsi (the cases jsi ¼
g and jsi ¼ jei correspond to the ground and excited states, respectively) and the resonator Fock state jli. When the qubit is excited, as illustrated in Fig. 1a, the population
can  be transferred from
g; 1 ), which then rapidly the qubit (je; 0i) to the resonator (
 decays to the target state
g; 0 at decay rate κr, which is mainly due to the large photon emission rate of the readout resonator. We consider a qubit-resonator coupled system described by the Jaynes-Cummings model. In the dispersive regime, there is no population exchange due to the large detuning between the qubit and the resonator. The external (...truncated)