We report the experimental realization of a hybrid quantum circuit combining a superconducting qubit and an ensemble of electronic spins. The qubit, of the transmon type, is coherently coupled to the spin ensemble consisting of nitrogen-vacancy centers in a diamond crystal via a frequency-tunable superconducting resonator acting as a quantum bus. Using this circuit, we prepare a superposition of the qubit states that we store into collective excitations of the spin ensemble and retrieve back into the qubit later on. These results constitute a proof of concept of spin-ensemble based quantum memory for superconducting qubits.
We demonstrate a scheme for realizing quantum-information storage (retrieval) into (from) a quantum memory using a hybrid quantum system, which consists of
N nitrogen-vacancy (N-
V) centers (as memory),
N transmission line resonators (TLRs) (as data bus), and
N current-biased Josephson-junction (CBJJ) superconducting phase qubits (as heads of read and write). By virtually exciting the vibrational modes of the TLRs, this can produce an effective coupling between the N-
V centers and the CBJJs in the system. Because the operation time ($\ensuremath{\sim}{10}^{\ensuremath{-}8}$ s) is much shorter than the decoherence time of the CBJJ ($\ensuremath{\sim}{10}^{\ensuremath{-}5}$ s), the quantum information can be transferred between the N-
V centers and the CBJJs easily. Owing to the strong-coupling condition $[\ensuremath{\lambda}/2\ensuremath{\pi}\ensuremath{\gg}({\ensuremath{\gamma}}_{eg},\ensuremath{\Gamma},{\ensuremath{\gamma}}_{\ensuremath{\phi}})]$ and the long coherence time ($\ensuremath{\sim}{10}^{\ensuremath{-}3}$ s) of the N-
V centers, the quantum information can be stored in the memory steadily, and it can be retrieved with a high fidelity (
>96). This scheme provides a scalable way of exchanging quantum information between different quantum interfaces, and has the experimental feasibility with currently available experimental technology.
Efficient quantum state transfer between superconducting circuits and solid-state spins would unlock high-coherence quantum memories for superconducting quantum processors. We demonstrate dynamically controlled strong coupling between a Josephson circuit and a rare-earth spin ensemble. Using a parametric pump, we realize on-demand coupling of several MHz, which will enable faithful state transfer between quantum circuits and spins. Our architecture enables quantum control of spin ensembles, and paves the way for hybrid memories with coherence far beyond those of superconducting circuits alone.
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 the nearestneighbor 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 a hybrid quantum system consisting of spin ensembles and superconducting flux qubits, where each spin ensemble is realized using the nitrogen-vacancy centers in a diamond crystal and the nearest-neighbor 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 one-dimensional bosonic lattice. In the ultrastrong-coupling regime, it exhibits quasiparticle excitations separated from the ground state by an energy gap. Moreover, these quasiparticle excitations and the ground state are stable under a certain condition that is tunable via the external magnetic field. This may provide an experimentally accessible method to probe the instability of the system.