Rapid High-Fidelity Single-Shot Dispersive Readout of Superconducting Qubits

Abstract

The speed of quantum gates and measurements is a decisive factor for the overall fidelity of quantum protocols when performed on physical qubits with finite coherence time. Reducing the time required to distinguish qubit states with high fidelity is therefore a critical goal in quantum information science. The state-of-the-art readout of superconducting qubits is based on the dispersive interaction with a readout resonator. Here, we bring this technique to its current limit and demonstrate how the careful design of system parameters leads to fast and high-fidelity measurements without affecting qubit coherence. We achieve this result by increasing the dispersive interaction strength, by choosing an optimal linewidth of the readout resonator, by employing a Purcell filter, and by utilizing phase-sensitive parametric amplification. In our experiment, we measure 98.25% readout fidelity in only 48 ns, when minimizing read-out time, and 99.2% in 88 ns, when maximizing the fidelity, limited predominantly by the qubit lifetime of 7.6 µs. The presented scheme is also expected to be suitable for integration into a multiplexed readout architecture.

Single-Shot Quantum Nondemolition Detection of Individual Itinerant Microwave Photons

“…To realize this preselection measurement, we perform single-shot readout with the methods described in Ref. [30]. We obtain an integrated quadrature amplitude q for each realization, which we compare to a conservatively chosen threshold value to herald the qubit ground state.…”

“…As a performance metric, we define the detection fidelity F ¼ 1 − Pðgj1Þ − Pðej0Þ ¼ Pðej1Þ − Pðej0Þ ¼ 59.9% as the difference between detection efficiency and dark count probability. This definition of fidelity is the logical equivalent to the one used to characterize single-shot readout [30]. To gain insight into the sources of errors in the protocol, we extract the detection efficiency and dark count probability vs the duration of detection time window, T w [ Fig.…”

“…By contrast, with the transmon in jei [panel (d)], photons of the same frequency are reflected without interacting with the cavity and thus acquire no phase (φ e ¼ 0). The readout resonator, at ω ro =ð2πÞ ¼ 4800 MHz, with effective linewidth κ ro =ð2πÞ ¼ 17 MHz is used to perform high-fidelity, dispersive single-shot readout of the qubit state [30], at a qubit-induced dispersive shift of χ ro =ð2πÞ ¼ 17 MHz.…”

Efficient protocol for qubit initialization with a tunable environment

“…In recent experiments, 19,20 the discrimination of the qubit state is predominantly limited by T 1 , with the current benchmark for the average assignment fidelity being 99.8% in 700-ns read-out time. 19 However, an increase of the fidelity by each order of magnitude requires yet another reduction of the read-out time or an increase of T 1 by an order of magnitude, which still remains an experimental challenge.…”

“…Qubit initialization can also be achieved with feedback-based schemes, 11,[19][20][21][22] which rely on a single-shot measurement of the qubit state. In recent experiments, 19,20 the discrimination of the qubit state is predominantly limited by T 1 , with the current benchmark for the average assignment fidelity being 99.8% in 700-ns read-out time.…”