Single-shot qubit readout in circuit Quantum Electrodynamics (1005.3436v1)

Abstract: The future development of quantum information using superconducting circuits requires Josephson qubits [1] with long coherence times combined to a high-fidelity readout. Major progress in the control of coherence has recently been achieved using circuit quantum electrodynamics (cQED) architectures [2, 3], where the qubit is embedded in a coplanar waveguide resonator (CPWR) which both provides a well controlled electromagnetic environment and serves as qubit readout. In particular a new qubit design, the transmon, yields reproducibly long coherence times [4, 5]. However, a high-fidelity single-shot readout of the transmon, highly desirable for running simple quantum algorithms or measur- ing quantum correlations in multi-qubit experiments, is still lacking. In this work, we demonstrate a new transmon circuit where the CPWR is turned into a sample-and-hold detector, namely a Josephson Bifurcation Amplifer (JBA) [6, 7], which allows both fast measurement and single-shot discrimination of the qubit states. We report Rabi oscillations with a high visibility of 94% together with dephasing and relaxation times longer than 0:5 \mu\s. By performing two subsequent measurements, we also demonstrate that this new readout does not induce extra qubit relaxation.

• The paper presents a novel circuit design incorporating a Josephson Bifurcation Amplifier to achieve single-shot readout of transmon qubits with 94% visibility.
• It embeds the qubit in a coplanar waveguide resonator, enabling rapid measurements below T1 without introducing significant qubit relaxation.
• The study introduces quantum shelving via the second excited state to enhance readout contrast, paving the way for scalable, frequency-multiplexed quantum processors.

Single-Shot Qubit Readout in Circuit Quantum Electrodynamics

The paper "Single-shot qubit readout in circuit Quantum Electrodynamics" by Mallet et al. tackles an essential challenge in quantum information processing using superconducting circuits: achieving high-fidelity single-shot readout of transmon qubits. The authors present a novel circuit design within the circuit Quantum Electrodynamics (cQED) framework and demonstrate a single-shot readout method utilizing a Josephson Bifurcation Amplifier (JBA), which exhibits both rapid measurement capabilities and minimal added qubit relaxation.

The crux of the proposed method revolves around embedding a transmon qubit within a coplanar waveguide resonator (CPWR) that includes a Josephson junction. The setup functions both as an electromagnetic environment and a readout mechanism. The transmon qubit design, known for long coherence times, is complemented by the JBA functionality, enabling single-shot discrimination between qubit states without inducing significant relaxation. The authors achieve a readout with 94% visibility, while ensuring that dephasing and relaxation times surpass 0.5 microseconds.

A significant result from their paper is the absence of additional qubit relaxation induced by the readout process, a notable improvement attributed to the careful design of the qubit-resonator interaction. This interaction is characterized by the bifurcation between two dynamical states of the non-linear resonator, which can be mapped back to the qubit states with high fidelity. The bifurcation process is activated by a precisely tuned microwave pulse and can achieve measurement times shorter than the qubit's natural relaxation time $T_1$, thus avoiding state transitions that compromise readout accuracy.

The authors also introduce a technique that involves transferring the qubit from its first to second excited state before readout, thereby leveraging quantum shelving to further enhance readout fidelity, reaching up to 92% contrast. This technique capitalizes on the inherently low decay rate from the higher excited state in the transmon architecture.

In the broader context of quantum computing applications, the ability to perform single-shot qubit readout with minimal back-action paves the way for more complex operations and measurements. The low back-action is primarily due to the qubit frequency's dependence on the resonator's slowly varying photon number rather than its intra-resonator current, a notable distinction from previous designs.

The findings presented in this paper have critical implications for scaling up quantum computing systems. The authors suggest potential applications in testing Bell's inequalities and mention the possibility of integrating this approach in a scalable quantum processor, where multiple transmons and their respective JBA elements can be read using frequency multiplexing.

Future developments in the field of quantum computing should consider optimizing such readout techniques further to accommodate broader frequency ranges and enhance compatibility with other quantum information processing protocols. The continued reduction of qubit relaxation due to readout processes will also contribute to overall performance gains in quantum computing architectures. This paper serves as a vital step forward in achieving reliable and efficient quantum information processing, underscoring the synergy between device engineering and quantum information science.