We propose a scheme for generating and controlling entangled coherent states (ECS) of magnons, i.e. the quanta of the collective spin excitations in magnetic systems, or phonons inmechanical resonators. The proposed hybrid circuit architecture comprises a superconducting transmon qubit coupled to a pair of magnonic Yttrium Iron Garnet (YIG) spherical resonators or mechanical beam resonators via flux-mediated interactions. Specifically, the coupling results from the magnetic/mechanical quantum fluctuations modulating the qubit inductor, formed by a superconducting quantum interference device (SQUID). We show that the resulting radiation-pressure interaction of the qubit with each mode, can be employed to generate maximally-entangled states of magnons or phonons. In addition, we numerically demonstrate a protocol for the preparation of magnonic and mechanical Bell states with high fidelity including realistic dissipation mechanisms. Furthermore, we have devised a scheme for reading out the prepared states using standard qubit control and resonator field displacements. Our work demonstrates an alternative platform for quantum information using ECS in hybrid magnonic and mechanical quantum networks.
We propose to directly and quantum-coherently couple a superconducting transmon qubit to magnons – the quanta of the collective spin excitations, in a nearby magnetic particle.The magnet’s stray field couples to the qubit via a superconducting interference device (SQUID). We predict a resonant qubit-magnon exchange and a nonlinear radiation-pressure interaction that are both stronger than dissipation rates and tunable by an external flux bias. We additionally demonstrate a quantum control scheme that generates qubit-magnon entanglement and magnonic Schrödinger cat states with high fidelity.
We propose a scheme for controlling a radio-frequency mechanical resonator at the quantum level using a superconducting qubit. The mechanical part of the circuit consists of a suspendedmicrometer-long beam that is embedded in the loop of a superconducting quantum interference device (SQUID) and is connected in parallel to a transmon qubit. Using realistic parameters from recent experiments with similar devices, we show that this configuration can enable a tuneable optomechanical interaction in the single-photon ultrastrong-coupling regime, where the radiation-pressure coupling strength is larger than both the transmon decay rate and the mechanical frequency. We investigate the dynamics of the driven system for a range of coupling strengths and find an optimum regime for ground-state cooling, consistent with previous theoretical investigations considering linear cavities. Furthermore, we numerically demonstrate a protocol for generating hybrid discrete- and continuous-variable entanglement as well as mechanical Schrödinger cat states, which can be realised within the current state of the art. Our results demonstrate the possibility of controlling the mechanical motion of massive objects using superconducting qubits at the single-photon level and could enable applications in hybrid quantum technologies as well as fundamental tests of quantum mechanics.
We propose a scheme for generating arbitrary quantum states in a mechanical resonator using tuneable three-body interactions with two superconducting qubits. The coupling relies onembedding a suspended nanobeam in one of the arms of a superconducting quantum interference device that galvanically connects two transmon qubits, in combination with an in-plane magnetic field. Using state-of-the-art parameters and single-qubit operations, we demonstrate the possibility of ground-state cooling as well as high-fidelity preparation of arbitrary mechanical states and qubit-phonon entanglement, significantly extending the quantum control toolbox of radio-frequency mechanical oscillators.
Detecting weak radio-frequency electromagnetic fields plays a crucial role in wide range of fields, from radio astronomy to nuclear magnetic resonance imaging. In quantum mechanics,the ultimate limit of a weak field is a single-photon. Detecting and manipulating single-photons at megahertz frequencies presents a challenge as, even at cryogenic temperatures, thermal fluctuations are significant. Here, we use a gigahertz superconducting qubit to directly observe the quantization of a megahertz radio-frequency electromagnetic field. Using the qubit, we achieve quantum control over thermal photons, cooling to the ground-state and stabilizing photonic Fock states. Releasing the resonator from our control, we directly observe its re-thermalization dynamics with the bath with nanosecond resolution. Extending circuit quantum electrodynamics to a new regime, we enable the exploration of thermodynamics at the quantum scale and allow interfacing quantum circuits with megahertz systems such as spin systems or macroscopic mechanical oscillators.