Hybrid quantum circuits combine two or more physical systems, with the goal of harnessing the advantages and strengths of the different systems in order to better explore new phenomena and potentially bring about novel quantum technologies. This article presents a brief overview of the progress achieved so far in the field of hybrid circuits involving atoms, spins and solid-state devices (including superconducting and nanomechanical systems). How these circuits combine elements from atomic physics, quantum optics, condensed matter physics, and nanoscience is discussed, and different possible approaches for integrating various systems into a single circuit are presented. In particular, hybrid quantum circuits can be fabricated on a chip, facilitating their future scalability, which is crucial for building future quantum technologies, including quantum detectors, simulators, and computers.

We study effective light-matter interactions in a circuit QED system consisting of a single LC resonator, which is coupled symmetrically to multiple superconducting qubits. Starting from a minimal circuit model, we demonstrate that in addition to the usual collective qubit-photon coupling the resulting Hamiltonian contains direct qubit-qubit interactions, which have a drastic effect on the ground and excited state properties of such circuits in the ultrastrong coupling regime. In contrast to a superradiant phase transition expected from the standard Dicke model, we find an opposite mechanism, which at very strong interactions completely decouples the photon mode and projects the qubits into a highly entangled ground state. These findings resolve previous controversies over the existence of superradiant phases in circuit QED, but they more generally show that the physics of two-or multi-atom cavity QED settings can differ significantly from what is commonly assumed.

We show that NV centers in diamond interfaced with a suspended carbon nanotube carrying a dc current can facilitate a spin-nanomechanical hybrid device. We demonstrate that strong magnetomechanical interactions between a single NV spin and the vibrational mode of the suspended nanotube can be engineered and dynamically tuned by external control over the system parameters. This spin-nanomechanical setup with strong, intrinsic and tunable magnetomechanical couplings allows for the construction of hybrid quantum devices with NV centers and carbon-based nanostructures, as well as phonon-mediated quantum information processing with spin qubits.Carbon-based structures and devices are very commonly used in our everyday life and in state-of-the-art science and technology. In quantum information science, nitrogen-vacancy (NV) centers in diamond are outstanding solid state qubits due to their long coherence times and high controllability [1][2][3][4][5]. In nano-mechanics, mechanical resonators made out of allotropes of carbon (such as nanotubes [6][7][8], diamond [9][10][11], and graphene [12]), are being extensively studied for fundamental research and practical applications [13][14][15][16][17][18][19][20][21][22][23].Recently, much attention has been paid to coupling NV spins in diamond to mechanical resonators, which can be achieved extrinsically [24][25][26][27][28][29][30][31][32][33][34] or intrinsically [35][36][37][38][39]. In the first case, the interaction arises from the relative motion of the NV spin and a source of local magnetic field gradients [24]. In such setups, a magnetic tip mounted on a vibrating cantilever [40] is often used to generate the magnetic coupling between an NV spin and the mechanical motion [24][25][26][27][28][29][30][31][32][33][34]. However, creating very strong, well-controlled, local gradients remains challenging for such setups, in particular when arrays of NV centers are placed in close proximity to the same cantilever. Thus far, experiments with the extrinsic coupling scheme have yet to reach the strong-coupling regime [26][27][28][29]. In the second case, the coupling of a diamond cantilever to the spin of an embedded NV center is induced by crystal strain during mechanical motion [35][36][37][38][39]. Unfortunately, the strain-induced interaction between a single NV spin and the cantilever quantized motion is inherently tiny [37,38], which makes the strong strain coupling at a single quantum level very challenging.In this Letter, we propose that NV centers in diamond interfaced with carbon nanotubes can facilitate a spin-nanomechanical hybrid device. This hybrid structure takes advantage of the unprecedented mechanical and electrical characteristics of carbon nanotubes, as well as the exceptional coherence properties of NV centers in diamond. We demonstrate that the physics of an NV center in diamond placed near a carbon nanotube with a dc current flowing through it can be well mapped to cavity quantum-electrodynamics (QED). In particular, going beyond earlier work in this fie...