Mesoscopic shelving readout of superconducting qubits in circuit quantum electrodynamics

Abstract

We present a method for measuring the internal state of a superconducting qubit inside an on-chip microwave resonator. We show that one qubit state can be associated with the generation of an increasingly large cavity coherent field, while the other remains associated with the vacuum. By measuring the outgoing resonator field with conventional devices, an efficient single-shot QND-like qubit readout can be achieved, enabling a high-fidelity measurement in the spirit of the electronshelving technique for trapped ions. We expect that the proposed ideas can be adapted to different superconducting qubit designs and contribute to the further improvement of qubit readout fidelity.PACS numbers: 03.65. Yz, 03.67.Lx, 03.65.Wj, 42.50.Lc Superconducting nanocircuits [1,2] are considered promising candidates for diverse implementations of quantum information tasks [3]. In this context, circuit quantum electrodynamics (QED) [4,5], which studies superconducting qubits [1,6] coupled to on-chip microwave resonators, occupies a central role. To achieve the desired goals, it is important to implement high-fidelity two-qubit gates [7] and efficient schemes to read out the qubit state [8]. In both cases, trapped-ion systems represent the state-of-the-art for qubit realizations [9]. In particular, electron-shelving qubit readout has produced fidelity benchmarks of approximately 99.99% [10]. These astonishing achievements suggest the potential impact of transferring key ideas from quantum optics to circuit QED. Unfortunately, electron shelving relies strongly on the use of single-photon detectors [9], which are unavailable in microwave technology in the range 1−10 GHz [11]. Nevertheless, in this manuscript we show that a singleshot QND-like fast qubit readout can be designed by exploiting the electron-shelving concept in circuit QED.We first present the physics of electron shelving in trapped ions. In Fig. 1, we show a three-level atom where an unknown qubit state |ψ = α|g + β|e is encoded in states |g and |e . Via a laser beam, the ground state |g is coupled to a third level |u , which can decay producing a continuous cyclic transition. In this case, the qubit is projected onto state |g and many photons are emitted in free space, one at each cycle. In contrast, when the qubit is projected onto state |e , no photons are emitted. A lens is used to collect the photons more efficiently by improving the solid angle. Although the photodetector has a low efficiency η d , the qubit readout fidelity can be very high. Typically, it is estimated through F = 1 − e −η d N , which rapidly approaches unity for η d N ≫ 1, N being the number of detected photons. Key elements for electron cyclic transition Photodetector lens FIG. 1: (Color online) Sketch of electron shelving in trapped ions. The |g ↔ |u transition is driven with a laser beam, performing a cyclic transition and emitting many photons when |g is projected. No photons are detected when |e is measured. Undesired transitions are inhibited via selection rules.shelving are the use of three...

Optical driving of macroscopic mechanical motion by a single two-level system

“…Levels |g , |m and |e of our quantum emitter (cf. Fig.1) can be identified with the first three lowest levels of the qubit arranged in a so-called λ-shape configuration [20][21][22]. As shown in Fig.4, we find that p = 15 is reached within the rise time T r = 105ns.…”

Ultrafast quantum nondemolition measurements based on a diamond-shaped artificial atom

“…Consequently, further improvements are necessary in order to reach very high fidelity measurements in a few ten's of nanoseconds. New quantum measurement protocols inspired by ion traps and quantum optics were recently proposed with this purpose [18,19].…”

“…Consequently, further improvements are necessary in order to reach very high fidelity measurements in a few ten's of nanoseconds. New quantum measurement protocols inspired by ion traps and quantum optics were recently proposed with this purpose [18,19].Here we propose an original method to realize ultrafast QND measurements of a qubit with large resonator linewidth and measurement bandwidth, while preserving high fidelity. Our system is a resonator coupled to a four level diamond-shape atom, which can be seen as two qubits coupled by crossed Kerr interaction.…”

Dynamical Autler-Townes control of a phase qubit

“…With improved dissipation and decoherence, it could be possible to use electromagnetic-induced transparency effects to build similar devices with superconducting quantum circuits 9 . Other applications employing multilevel systems - for example state preparation, emulation of large quantum-number spins 10 , single-shot fast quantum nondemolition readout techniques 11 , implementation of controlled Z- π gates in certain quantum computing architectures 12 , single-qubit microwave amplifiers 13 , etc. - can be envisioned.…”