We study a hybrid quantum system consisting of spin ensembles and superconducting flux qubits, where each spin ensemble is realized using the NV centers in a diamond crystal and thenearestneighbor spin ensembles are effectively coupled via a flux qubit. We show that the coupling strengths between flux qubits and spin ensembles can reach the strong and even ultrastrong coupling regimes by either engineering the hybrid structure in advance or tuning the excitation frequencies of spin ensembles via external magnetic fields. When extending the hybrid structure to an array with equal coupling strengths, we find that in the strong coupling regime, the hybrid array is reduced to a tight-binding model of a 1D bosonic lattice. In the ultrastrong coupling regime, it exhibits quasi-particle excitations separated from the ground state by an energy gap. Moreover, these quasiparticle excitations and the ground state are stable under a certain condition which is tunable via the external magnetic field. This may provide an experimentally accessible method to probe the instability of the system.
We study the microwave absorption of a driven three-level quantum system, which is realized by a superconducting flux quantum circuit (SFQC), with a magnetic driving field applied tothe two upper levels. The interaction between the three-level system and its environment is studied within the Born-Markov approximations, and we take into account the effects of the driving field on the damping rates of the three-level system. We study the linear response of the driven three-level SFQC to a weak probe field. The susceptibility of the probe field can be changed by both the driving field and the bias magnetic flux. When the bias magnetic flux is at the optimal point,the transition from the ground state to the second excited state is forbidden and the three-level system has a ladder-type transition. Thus, the SFQC responds to the probe field like natural atomic systems with ladder-type transitions. However, when the bias magnetic flux is away from the optimal point, the three-level SFQC has Δ-type transition, thus it responds to the probe field like a combination of natural atoms with ladder-type transitions and natural atoms with Λ-type transitions. In particular, we give detailed discussions on the conditions for realizing electromagnetically induced transparency and Autler-Townes splitting in three-level SFQCs.
A phase-slip flux qubit, exactly dual to a charge qubit, is composed of a superconducting loop interrupted by a phase-slip junction. Here we propose a tunable phase-slip flux qubitby replacing the phase-slip junction with a charge-related superconducting quantum interference device (SQUID) consisting of two phase-slip junctions connected in series with a superconducting island. This charge-SQUID acts as an effective phase-slip junction controlled by the applied gate voltage and can be used to tune the energy-level splitting of the qubit. Also, we show that a large inductance inserted in the loop can reduce the inductance energy and consequently suppress the dominating flux noise of the phase-slip flux qubit. This enhanced phase-slip flux qubit is exactly dual to a transmon qubit.
We propose how to realize high-fidelity quantum storage using a hybrid
quantum architecture including two coupled flux qubits and a nitrogen-vacancy
center ensemble (NVE). One of theflux qubits is considered as the quantum
computing processor and the NVE serves as the quantum memory. By separating the
computing and memory units, the influence of the quantum computing process on
the quantum memory can be effectively eliminated, and hence the quantum storage
of an arbitrary quantum state of the computing qubit could be achieved with
high fidelity. Furthermore the present proposal is robust with respect to
fluctuations of the system parameters, and it is experimentally feasibile with
currently available technology.
We propose an experimentally realizable hybrid quantum circuit for achieving
a strong coupling between a spin ensemble and a transmission-line resonator via
a superconducting flux qubitused as a data bus. The resulting coupling can be
used to transfer quantum information between the spin ensemble and the
resonator. More importantly, in contrast to the direct coupling without a data
bus, our approach requires far less spins to achieve a strong coupling between
the spin ensemble and the resonator (e.g., 3 to 4 orders of magnitude less).
This drastic reduction of the number of spins in the ensemble can greatly
improve the quantum coherence of the spin ensemble. This proposed hybrid
quantum circuit could enable a long-time quantum memory when storing
information in the spin ensemble.
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 phenomenaand 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). We discuss how
these circuits combine elements from atomic physics, quantum optics, condensed
matter physics, and nanoscience, and we present different possible approaches
for integrating various systems into a single circuit. 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.