Quantum technologies with hybrid systems

Abstract

An extensively pursued current direction of research in physics aims at the development of practical technologies that exploit the effects of quantum mechanics. As part of this ongoing effort, devices for quantum information processing, secure communication, and high-precision sensing are being implemented with diverse systems, ranging from photons, atoms, and spins to mesoscopic superconducting and nanomechanical structures. Their physical properties make some of these systems better suited than others for specific tasks; thus, photons are well suited for transmitting quantum information, weakly interacting spins can serve as long-lived quantum memories, and superconducting elements can rapidly process information encoded in their quantum states. A central goal of the envisaged quantum technologies is to develop devices that can simultaneously perform several of these tasks, namely, reliably store, process, and transmit quantum information. Hybrid quantum systems composed of different physical components with complementary functionalities may provide precisely such multitasking capabilities. This article reviews some of the driving theoretical ideas and first experimental realizations of hybrid quantum systems and the opportunities and challenges they present and offers a glance at the near- and long-term perspectives of this fascinating and rapidly expanding field.

Keywords: hybrid quantum systems; quantum information; quantum technologies.

Fig. 1.

HQS overview. The diagram shows a selection of physical systems that represent components of HQS with different functionalities. The individual systems are positioned in the diagram according to their characteristic excitation frequencies (vertical axis) and coherence times (horizontal axis). The arrows indicate possible coupling mechanisms and the corresponding coupling strengths $g_{\text{eff}}$ that can be realistically achieved. The red and the blue arrows represent the coupling between single systems and the coupling to and between ensembles, respectively. Couplings represented by dashed lines are assisted by additional classical laser or microwave fields to bridge the apparent mismatch of the excitation energies. See text for more detail.

Fig. 2.

Spin-ensemble quantum memory. (A) Photograph (Upper) and schematic drawing (Lower) of the hybrid quantum circuit realized in ref. . A transmon qubit (red) is coupled to an ensemble of NV-center electron spins (pink) via a frequency-tunable quantum bus resonator (orange). (B) Swap oscillations of a coherent superposition of quantum states, (|g〉 +|e〉)/√2 initially prepared in the qubit, showing cycles of storage (dashed arrows) in and retrieval (solid arrows) from the spin ensemble.

Fig. 3.

Mechanical quantum transducers. (A) A magnetized mechanical resonator is coupled to a localized electronic spin qubit and converts small spin-induced displacements into electric signals. Thereby spin qubits can be “wired up” electrically or coupled to other charged quantum systems. (B) Illustration of an OM interface between a superconducting qubit and optical (flying) photons. Here the mechanical system, represented by a semitransparent membrane, simultaneously acts as a capacitor and an optical reflector. (C) Experimental setup used in ref. to implement an optomechanical microwave-to-optics interface via simultaneous coupling of a partially metallized membrane to an optical cavity and an LC circuit. (D) A signal photon of frequency $\sim {\omega }_c^{op}\hspace{0.17em}$enters the optical cavity and is down-converted via the driven, parametric OM interaction into a phonon of frequency ${\omega }_m$. Then, via an equivalent process, this mechanical excitation is up-converted again into a microwave photon of frequency $\sim {\omega }_c^{mw}\hspace{0.17em}$in the LC circuit. Via this mechanism and its reverse, quantum information encoded in microwave excitations of a superconducting qubit or LC resonator can be coherently transferred into optical signals for long-distance quantum communication.

Fig. 4.

Potential applications that can be performed by a HQS with achievable infidelity 1-F.