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Laser Physics Letters

Entangling two spin ensembles in diamond nanostructures via a superconducting flux qubit

Yonghong Ma, Xuefeng Zhang

Published Jun 10, 2015• 1 citation
Publisher

Abstract

Entanglement between macroscopic systems is a basic resource for many quantum information processing, and so it is crucially important to investigate the possibility of achieving entanglement between macroscopic systems. In this work, we propose a novel mechanism for achieving entanglement between two spin ensembles in diamond nanostructures. The interaction between two spin ensembles, associated with the nitrogen-vacancy centers in diamond, are mediated by their coupling with a flux qubit. Using numerical methods, we show that two spin ensembles are entangled with the time evolution under realistic experimental conditions. This macroscopic entanglement between two spin ensembles has important implications for ongoing efforts to realize quantum computation and communication.

DOI
10.1088/1612-2011/12/7/075201

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Entanglement between macroscopic systems is a basic resource for many quantum information processing, and so it is crucially important to investigate the possibility of achieving entanglement between macroscopic systems. In this work, we propose a novel mechanism for achieving entanglement between two spin ensembles in diamond nanostructures. The interaction between two spin ensembles, associated with the nitrogen-vacancy centers in diamond, are mediated by their coupling with a flux qubit. Using numerical methods, we show that two spin ensembles are entangled with the time evolution under realistic experimental conditions. This macroscopic entanglement between two spin ensembles has important implications for ongoing efforts to realize quantum computation and communication.
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.
As an essential quantum resource in quantum information processing, entanglement between macroscopic systems has been applied to both quantum technologies and foundational studies of the boundary between the quantum and classical worlds. Inspired by the proposal by Li et al. [Phys. Rev. Lett. 117, 015502 (2016)], we propose a scheme that is composed of two separated nitrogen-vacancy centers coupled to the vibration mode of a nanotube for creating entanglement between the two ensembles. An approximate analytical solution related to the calculation of macroscopic entanglement for this two-spin-ensemble model is derived. This kind of analytical solution will be crucial for directly testing the contribution of the experimental parameters, especially for the calculation of macroscopic entanglement in follow-up work. Including the dephasing and mechanical dissipation, we show that two spin ensembles are entangled with time evolution under realistic experimental conditions. This macroscopic entanglement is applicable for applied ideas related to quantum computation and communication, as well as the studies of the boundary between quantum and classical worlds. Moreover, we describe how to realize this practical model and demonstrate this macroscopic entanglement in experiments.
Abstract We propose a potentially practical scheme for creating macroscopic entangled coherent state between two separate nitrogen-vacancy center spin ensembles placed near a superconducting flux qubit. Through the collective magnetic coupling and the in situ tunability of the flux qubit, the arbitrary entangled coherent states of spin ensembles can be achieved with high success possibilities under the influence from decoherence of the flux qubit and spin ensembles. The experimental feasibility and challenge are justified using currently available technology.
Quantum entanglement is a kernel of quantum computation and quantum communication. We introduce a theoretical scheme to achieve the entanglement between two separated quantum nodes in a hybrid system. The proposed hybrid system based on diamond nitrogen-vacancy (NV) center spin ensemble is coherently coupled to a superconducting quantum circuit consisting of two quantum nodes and a quantum channel. Each node in our setup is composed of an NV center spin ensemble magnetically coupled to a superconducting coplanar resonator. The NV center spin ensemble composed of N identical and non-interacting NV spins, is placed in the magnetic field antinode of the superconducting coplanar resonator where the coupling is maximized. An array of superconducting quantum interference devices (SQUIDs) is inserted in the central conductor of resonator to make its frequency tunable with the magnetic flux threading through the SQUID loops. This flux is generated by passing current through an on-chip wire, so that the resonator can be brought in resonance with the NV center spins without changing their Zeeman splitting. Quantum qubits encoded into two separate nodes are connected by a vacuum superconducting coplanar resonator that is used as a quantum channel. This setup can potentially take the best elements of each individual system:NV center spin ensemble with longer coherence time capable of preparing, storing and releasing photonic quantum information, and the superconducting quantum circuits are easy to manipulate externally and can perform quantum logic gates to control quantum information rapidly. In order to realize the entanglement between two separated quantum nodes, firstly, we make a canonical transformation and obtain the Hamiltonian of the system that is reduced to two NV center spin ensembles resonantly coupled to a single mode of the superconducting coplanar resonator. Then we put forward the hybrid NV center spin-photon qubit encoding. In this hybrid encoding, the NV center spin and photon degrees of freedom enter on an equal footing into the definition of the qubit, in which case, quantum channel will switch on when three superconducting coplanar resonators are in resonance with each other, and all the manipulations can perform simply by tuning the frequencies of the superconducting coplanar resonators. Under the precise control of the evolution time, high fidelity entanglement between two separated quantum nodes is achieved. We show that this proposal can provide high fidelity quantum entanglement under realistic conditions, both in the resonant and the dispersive interaction cases. This hybrid quantum system will exhibit long coherence time and possess features like easy fabrication, integratability, and potential scalability. Furthermore, the quantum node composed of an NV center spin ensemble magnetically coupled to a superconducting coplanar resonator can be respectively integrated, which has practical applications in the realization of quantum information transmission and quantum entanglement among multiple quantum nodes.
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.
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.