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Physical Review A

Controllable coupling between a charge qubit and a spin ensemble

Xiao-Ling He, Chui‐Ping Yang, JunYan Luo, Yu Chen Shen, Weiqing Pan, Sheng Li

Published Jun 4, 2014• 1 citation
Publisher

Abstract

We propose a scheme to control the coupling between the charge qubit and the nitrogen-vacancy center ensemble (NVCE). The coupling is mediated by a large Josephson junction (JJ) and a flux qubit. The large JJ can be treated classically and behaves like an effective inductance to switch the coupling between the charge and flux qubits. The flux qubit serves as a bridge to couple the NVCE and the charge qubit, and quantum state transfer can be implemented, with the NVCE acting as a memory unit. The hybrid circuit can reinforce various advantages: controllability of the charge qubit and robust spin coherence of the NVCE.

DOI
10.1103/physreva.89.062306

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We propose a scheme to control the coupling between the charge qubit and the nitrogen-vacancy center ensemble (NVCE). The coupling is mediated by a large Josephson junction (JJ) and a flux qubit. The large JJ can be treated classically and behaves like an effective inductance to switch the coupling between the charge and flux qubits. The flux qubit serves as a bridge to couple the NVCE and the charge qubit, and quantum state transfer can be implemented, with the NVCE acting as a memory unit. The hybrid circuit can reinforce various advantages: controllability of the charge qubit and robust spin coherence of the NVCE.
We propose a method to achieve coherent coupling between nitrogen-vacancy (NV) centers in diamond and superconducting (SC) flux qubits. The resulting coupling can be used to create a coherent interaction between the spin states of distant NV centers mediated by the flux qubit. Furthermore, the magnetic coupling can be used to achieve a coherent transfer of quantum information between the flux qubit and an ensemble of NV centers. This enables a long-term memory for a SC quantum processor and possibly an interface between SC qubits and light.
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 the flux 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 feasible with currently available technology.
We investigate the dissipative dynamics of a hybrid system consisting of two inductively coupled superconducting flux qubits (FQs), each of which couples magnetically with a nitrogen-vacancy-center spin ensemble (NVE). The system displays a series of damped oscillations under various experimental situations, where we obtain analytically the time-dependent populations associated with arbitrary values of the FQ-FQ inductively coupling strength ggg and the FQ-NVE magnetically coupling strength GGG in the one-excitation case. Our results show that a reliable high-fidelity quantum state transfer between the two distant NVEs can be realized by modulating the parameters ggg and G.G.G. Furthermore, we also present a potentially practical idea to entangle the two separate NVEs, which is within reach due to the currently available technology.
We present a theoretical proposal to couple a single nitrogen-vacancy (NV) center to a superconducting flux qubit in the regime where both systems are off resonance. The coupling between both quantum devices is achieved through the strong driving of the flux qubit by a classical microwave field that creates dressed states with an experimentally controlled characteristic frequency. We discuss several applications such as controlling the NV center's state by manipulation of the flux qubit, performing the NV center full tomography and using the NV center as a quantum memory. The effect of decoherence and its consequences to the proposed applications are also analyzed. Our results provide a theoretical framework describing a promising hybrid system for quantum information processing, which combines the advantages of fast manipulation and long coherence times.
We propose a hybrid quantum circuit with both charge and flux qubits connected to a large Josephson junction that gives rise to an effective interqubit coupling controlled by the external magnetic flux. This switchable interqubit coupling can be used to transfer back and forth an arbitrary superposition state between the charge qubit and the flux qubit working at the optimal point. The proposed hybrid circuit provides a promising quantum memory because the flux qubit at the optimal point can store the tranferred quantum state for a relatively long time.
We have built a hybrid system composed of a superconducting flux qubit (the processor) and an ensemble of nitrogen-vacancy centers in diamond (the memory) that can be directly coupled to one another, and demonstrated how information can be transferred from the flux qubit to the memory, stored, and subsequently retrieved. We have established the coherence properties of the memory and succeeded in creating an entangled state between the processor and memory, demonstrating how the entangled state's coherence is preserved. Our results are a significant step towards using an electron spin ensemble as a quantum memory for superconducting qubits.