Thermal preparation of an entangled steady state of distant nitrogen-vacancy-center ensembles
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THE EUROPEAN PHYSICAL JOURNAL B
Regular Article
Thermal preparation of an entangled steady state of distant nitrogen-vacancy-center ensembles Natalia Teper a Ulyanovsk State Pedagogical University, Ulyanovsk 432071, Russia Received 18 May 2020 / Received in final form 5 August 2020 / Accepted 20 August 2020 Published online 5 October 2020 c EDP Sciences / Societ`
a Italiana di Fisica / Springer-Verlag GmbH Germany, part of Springer Nature, 2020 Abstract. Steady state entanglement is studied in the continuous-variable system of two distant nitrogenvacancy-center ensembles (NVEs) driven by squeezed microwave field. Transmission line resonators and a current-biased Josephson junction are used for transfer of excitations between two NVEs. The circuit is enhanced by Josephson parametric amplifiers (JPAs), which produce squeezed microwave field. Squeezed fields of JPAs are driving force, which provides generation of an entangled steady state of two bosonic modes of NVEs for a wide range of system parameters.
1 Introduction Numerous investigations provided large progress in understanding and manipulating the nitrogen vacancy (NV) centers in diamond, opening promising possibilities for quantum information applications. In experimental setups, optical and microwave excitation is used for preparation and control of spin states of NV centers [1–3]. The long coherence time of the NV centers [4–7] allows measurements and readout of a single electronic spin [8,9]. A single excitation of an individual NV center√within a superconducting circuit is enhanced by factor N for ensemble of N ∼ 1012 spins. Experimental results confirm the strong coupling between NVEs and a superconducting resonator [10], and show that microwave field can be stored into the collective spin states of ensemble [11]. It was shown that collective spin variables of an NVE behave as a harmonic oscillator in the limit of low excitation energy. These properties found application in the various circuits, which are based on the NV centers coupled to superconducting qubit and resonator [12–17]. The superconducting resonator, the qubit and the NVE were integrated into experimental setup for implementation of spin-based quantum memory. In this scheme the resonator was used as a quantum bus between the qubit and the spins [12]. Recently, Astner et al. reported coherent coupling between two macroscopically separated nitrogen-vacancy electron spin ensembles in transmission line resonator [13]. The coherent interaction between the distant ensembles is directly detected in the cavity transmission spectrum by observing bright and dark collective multiensemble states and an increase of the coupling strength to the cavity mode. a
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Collective variables of NVE are proposed as a spinbased quantum memory for storing the state of a currentbiased Josephson junction (CBJJ) phase qubit fabricated in a transmission line resonator (TLR) and used as a quantum computing processor [14]. It was predicted that an NVE and a CBJJ can be coupled via the microwave
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