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arXiv:1211.0491 · Physical Review Letters

Demonstrating a Driven Reset Protocol for a Superconducting Qubit

K. Geerlings, Zaki Leghtas, Ioan M. Pop, Shyam Shankar, Luigi Frunzio, Robert J. Schoelkopf, Mazyar Mirrahimi, Michel Devoret

Published Mar 20, 2013• 218 citations
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Abstract

Qubit reset is crucial at the start of and during quantum information algorithms. We present the experimental demonstration of a practical method to force qubits into their ground state, based on driving appropriate qubit and cavity transitions. Our protocol, called the double drive reset of population, is tested on a superconducting transmon qubit in a three-dimensional cavity. Using a new method for measuring population, we show that we can prepare the ground state with a fidelity of at least 99.5% in less than 3 μs; faster times and higher fidelity are predicted upon parameter optimization.

DOI
10.1103/physrevlett.110.120501

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Qubit reset is crucial at the start of and during quantum information algorithms. We present the experimental demonstration of a practical method to force qubits into their ground state, based on driving appropriate qubit and cavity transitions. Our protocol, called the double drive reset of population, is tested on a superconducting transmon qubit in a three-dimensional cavity. Using a new method for measuring population, we show that we can prepare the ground state with a fidelity of at least 99.5% in less than 3 μs; faster times and higher fidelity are predicted upon parameter optimization.
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.
We demonstrate an active reset protocol in a superconducting quantum circuit. The thermal population on the excited state of a transmon qubit is reduced through driving the transitions between the qubit and an ancillary qubit. Furthermore, we investigate the efficiency of this approach at different temperatures. The result shows that population in the first excited state can be dropped from 7% to 2.55% in 27 ns at 30 mK. The efficiency improves as the temperature increases. Compared to other schemes, our proposal alleviates the requirements for measurement procedure and equipment. With the increase of qubit integration, the fast reset technique holds the promise of improving the fidelity of quantum control.
Active qubit reset is a key operation in many quantum algorithms, and particularly in quantum error correction. Here, we experimentally demonstrate a reset scheme for a three-level transmon artificial atom coupled to a large bandwidth resonator. The reset protocol uses a microwave-induced interaction between the |f,0⟩ and |g,1⟩ states of the coupled transmon-resonator system, with |g⟩ and |f⟩ denoting the ground and second excited states of the transmon, and |0⟩ and |1⟩ the photon Fock states of the resonator. We characterize the reset process and demonstrate reinitialization of the transmon-resonator system to its ground state in less than 500 ns and with 0.2% residual excitation. Our protocol is of practical interest as it has no additional architectural requirements beyond those needed for fast and efficient single-shot readout of transmons, and does not require feedback.
We theoretically propose and experimentally implement a method of measuring a qubit by driving it close to the frequency of a dispersively coupled bosonic mode. The separation of the bosonic states corresponding to different qubit states begins essentially immediately at maximum rate, leading to a speedup in the measurement protocol. Also the bosonic mode can be simultaneously driven to optimize measurement speed and fidelity. We experimentally test this measurement protocol using a superconducting qubit coupled to a resonator mode. For a certain measurement time, we observe that the conventional dispersive readout yields close to 100% higher average measurement error than our protocol. Finally, we use an additional resonator drive to leave the resonator state to vacuum if the qubit is in the ground state during the measurement protocol. This suggests that the proposed measurement technique may become useful in unconditionally resetting the resonator to a vacuum state after the measurement pulse.
Fast, high-fidelity qubit initialization is a key primitive for scalable quantum information processing, but conventional reset protocols either require long relaxation times or discard information contained in continuous measurement records. We demonstrate an adaptive measurement-based reset protocol for superconducting qubits that uses Bayesian inference to update the qubit-state estimate after each readout and dynamically adjust the feedback threshold. Unlike fixed-threshold or repeat-until-success (RUS) protocols, the method utilizes the full analog measurement history, enabling the reset decision to become progressively more conservative as confidence in ground-state preparation increases. Implemented with real-time FPGA-based feedback on a dispersively readout transmon qubit, the protocol achieves a ground-state initialization fidelity of 99.44±0.04%99.44\pm0.04\%99.44±0.04% after a small number of reset rounds. We further compare adaptive reset with RUS strategies in multi-qubit experiments and show that adaptive thresholding provides deterministic reset duration while maintaining a five-qubit simultaneous-reset fidelity of 98.78±0.19%98.78 \pm 0.19\%98.78±0.19%. These results establish Bayesian adaptive thresholding as a practical and scalable route to fast qubit reset in superconducting quantum processors.
Active reset of qubits is a crucial step in achieving quantum algorithms, particularly in fault-tolerant quantum computing, as it allows qubits to be quickly initialized to the ground state. In this paper, we demonstrate active reset of superconducting qubits using the feedback electronics based on RF switches. We are able to reset the qubit to the ground state in 11.412 µs with a success rate of 99.82%, which is better than the natural relaxation method in terms of both time consumption (180 µs) and success rate (99.61%). The demonstrated method facilitates fast and high-fidelity qubit initialization, holding practical significance for large-scale quantum computation.