Separated Nitrogen-vacancy Centers Research Articles

Quantum Repeaters with Encoding on Nitrogen-Vacancy-Center Platforms

We investigate quantum repeater protocols that rely on three-qubit repetition codes using nitrogen-vacancy (NV) centers in diamond as quantum memories. NV centers offer a two-qubit register, corresponding to their electron and nuclear spins, which makes it possible to perform deterministic two-qubit operations within one NV center. For quantum repeater applications, we, however, need to do joint operations on two separate NV centers. Here, we study two NV-center based repeater structures that enable such deterministic joint operations. One structure offers less consumption of classical communication, at the cost of more computation overhead, whereas the other one relies on a fewer number of physical resources and operations. We assess and compare their performance for the task of secret key generation under the influence of noise and decoherence with current and near-term experimental parameters. We quantify the regimes of operation, where one structure outperforms the other, and find the regions where encoded quantum repeaters offer practical advantages over their non-encoded counterparts.

Low-Charge-Noise Nitrogen-Vacancy Centers in Diamond Created Using Laser Writing with a Solid-Immersion Lens

We report on pulsed-laser-induced generation of nitrogen-vacancy (NV) centers in diamond facilitated by a solid-immersion lens (SIL). The SIL enables laser writing at energies as low as 5.8 nJ per pulse and allows vacancies to be formed close to a diamond surface without inducing surface graphitization. We operate in the previously unexplored regime, where lattice vacancies are created following tunneling breakdown rather than multiphoton ionization. We present three samples in which NV center arrays were laser-written at distances between ∼1 and 40 μm from a diamond surface, all presenting narrow distributions of optical linewidths with means between 62.1 and 74.5 MHz. The linewidths include the effect of long-term spectral diffusion induced by a 532 nm repump laser for charge-state stabilization, thereby emphasizing the particularly low-charge-noise environment of the created color centers. Such high-quality NV centers are excellent candidates for practical applications employing two-photon quantum interference with separate NV centers. Finally, we propose a model for disentangling power broadening from inhomogeneous broadening in the NV center optical linewidth.

Generation of distributed steady entangled state between two solid-state spins

The generation of distributed entangled states among solid-state spins is key to the development of large-scale quantum networks and quantum computation. We propose a dissipative scheme for generating stable entanglement between the electron-spin states of two separated nitrogen-vacancy centers, each coupled to a microtoroidal resonator and separated in space. An optical fiber-taper waveguide links the two microtoroidal resonators. Numerical simulations show that spontaneous emission from the NV centers and the collective decay of delocalized field modes can act as effective resources to generate stationary singlet-like states without the need for initialization and precise control of the evolution of the system over time. Results indicate that the proposed scheme can reach high-fidelity and purity of states, and is resilient against small parameter fluctuations. We also discuss how the pure spin dephasing that arises from longitudinal magnetic-near-field noise affects the fidelity of the target state.

Entanglement purification and concentration based on hybrid spin entangled states of separate nitrogen-vacancy centers

We present a hybrid entanglement purification and concentration protocol for nuclear spin entangled states of nitrogen-vacancy (N-V) centers resorting to the ancillary photons and electrons. In the entanglement purification protocol, the mixed entangled states of the N-V centers can be purified with high fidelity using two pairs of entangled photons to interact with each electron in N-V centers. By performing local measurement on the electrons, the original entangled state can be obtained between the two remote parities. In the entanglement concentration protocol, the partially entangled states of the nuclear spin in N-V centers can be concentrated to the maximally entangled state by using an ancillary photon on one local user. The fidelity of the entangled state with different excited state decay rate and the relation between the yield and different decay rate are numerically simulated.

Dynamics of quantum correlation between separated nitrogen-vacancy centers embedded in plasmonic waveguide

We investigate the dynamics of quantum correlation between two separated nitrogen vacancy centers (NVCs) placed near a one-dimensional plasmonic waveguide. As a common medium of the radiation field of NVCs propagating, the plasmonic waveguide can dynamically induce quantum correlation between the two NVCs. It is interesting to find that such dynamically induced quantum correlation can be preserved in the long-time steady state by locally applying individual driving on the two NVCs. In particular, we also show that a large degree of quantum correlation can be established by this scheme even when the distance between the NVCs is much larger than their operating wavelength. This feature may open new perspectives for devising active decoherence-immune solid-state optical devices and long-distance NVC-based quantum networks in the context of plasmonic quantum electrodynamics.

Efficient multipartite entanglement concentration protocol for nitrogen-vacancy center and microresonator coupled systems

Here in this study we propose an efficient entanglement concentration protocol (ECP) for separate nitrogen-vacancy (NV) centers, resorting to the single-photon input---output process of the NV center and microtoroidal resonator coupled system. In the proposed ECP, one ancillary single-photon is prepared and passed through a hybrid quantum circuit. By measuring the photon under the suitable polarization basis, maximally entangled state between the separate NV centers can be obtained with a certain success probability. The solid entanglement will be preserved during the process, which can be iterated several rounds to obtain an optimal total success probability. We also discuss the experimental feasibility of the protocol by considering current technologies, and we believe that the protocol is useful in the future applications of long-distance quantum communication and distributed quantum computation.

Method for Generating W States Basedon Nitrogen-Vacancy Centers and Whispering-Gallery Mode Cavity

We propose a scheme to generate W states of multiple separated nitrogen-vacancy centers coupled resonantly to a whispering-gallery mode cavity. This scheme is faster and simpler than the previous proposals and is reachable by the near future cavity QED techniques. The practical feasibility of this method is also discussed. Our scheme provides a building block based on solid-state technology.

Virtual-photon-induced entanglement with two nitrogen-vacancy centers coupled to a high-Q silica microsphere cavity

We propose a potentially practical scheme for efficient generation of entanglement with two nitrogen-vacancy centers (NVC) coupled to a whispering-gallery mode cavity. By virtue of the virtual-photon-excitation, the entanglement with two separate NVC can be produced in a deterministic way. The required operations are very close to the capabilities of current experimental techniques. The effects of decoherence induced by the cavity decay and the atomic spontaneous decay are also investigated.

Efficient Entanglement Concentration Schemes for Separated Nitrogen-Vacancy Centers Coupled to Low-Q Microresonators

We present several efficient entanglement concentration protocols (ECPs) with the nitrogen-vacancy (N-V) centers coupled to low-Q microresonators. Based on the input-output process of ancillary coherent light pulse in low-Q microresonators, we can obtain the maximally entangled states among remote participants via local operations and classical communication. Our protocols use a conventional photon detector to discriminate the two coherent states |α〉 and |−α〉, which is more convenient than homodyne measurement. We discuss the feasibility of our protocols, and they may be beneficial for quantum repeaters and quantum information processing.

Preservation of quantum correlation between separated nitrogen-vacancy centers embedded in photonic-crystal cavities

We investigate the non-Markovian dynamics of quantum correlation between two initially entangled nitrogen-vacancy centers (NVC) embedded in photonic crystal cavities (PCC). We find that a finite quantum correlation is preserved even asymptotically when the transition frequency of the NVC is within the band gap of the PCC, which is quantitatively different from the result of approaching zero under the Born-Markovian approximation. In addition, once the transition frequency of NVC is far beyond the band gap of the PCC, the quantum correlation initially prepared in NVC will be fully transferred to the reservoirs in the long-time limit. Our result reveals that the interplay between the non-Markovian effect of the structured reservoirs and the existence of emitter-field bound state plays an essential role in such quantum correlation preservation. This feature may open new perspectives for devising active decoherence-immune solid-state optical devices.

Quantum gate operations in decoherence-free fashion with separate nitrogen-vacancy centers coupled to a whispering-gallery mode resonator

We propose a scheme to achieve quantum gate operations within decoherence-free subspace (DFS) using nitrogen-vacancy centers in separate diamond nanocrystals coupled to a whispering-gallery mode resonator. By virtue of Raman transitions in the dispersive regime, we present two different methods for realizing quantum gating within DFS in a deterministic way. The quantum gating is seriously treated by considering all kinds of decoherence mechanisms, and the experimental feasibility is achieved using currently available technology.

Quantum Interference of Single Photons from Remote Nitrogen-Vacancy Centers in Diamond

We demonstrate quantum interference between indistinguishable photons emitted by two nitrogen-vacancy centers in distinct diamond samples separated by two meters. Macroscopic solid immersion lenses are used to enhance photon collection efficiency. Quantum interference is verified by measuring a value of the second-order cross-correlation function g((2))(0)=0.35±0.04<0.5. In addition, optical transition frequencies of two separated nitrogen-vacancy centers are tuned into resonance with each other by applying external electric fields. An extension of the present approach to generate entanglement of remote solid-state qubits is discussed.

Two-Photon Quantum Interference from Separate Nitrogen Vacancy Centers in Diamond

We report on the observation of quantum interference of the emission from two separate nitrogen vacancy (NV) centers in diamond. Taking advantage of optically induced spin polarization in combination with polarization filtering, we isolate a single transition within the zero-phonon line of the nonresonantly excited NV centers. The time-resolved two-photon interference contrast of this filtered emission reaches 66%. Furthermore, we observe quantum interference from dissimilar NV centers tuned into resonance through the dc Stark effect. These results pave the way towards measurement-based entanglement between remote NV centers and the realization of quantum networks with solid-state spins.

Quantum dynamics and quantum state transfer between separated nitrogen-vacancy centers embedded in photonic crystal cavities

We investigate dynamics of a laser-driven and dissipative system consisting of two nitrogen-vacancy (N-$V$) centers embedded in two spatially separated single-mode nanocavities in a planar photonic crystal (PC). Spontaneous emission from the excited states of the N-$V$ centers can be effectively suppressed by virtue of the Raman transition in the dispersive regime. The system displays a series of damped oscillations under various experimental situations, where we solve the time-dependent Schr\"odinger equation analytically for arbitrary values of the hopping and PC--N-$V$ coupling strengths. In particular, our results indicate that some special values should be taken for the hopping strength if we hope to have high-fidelity quantum state transfer between the two distant N-$V$ centers. We have also analyzed the relevant entanglement dynamics in the presence of decoherence. The experimental feasibility and challenge are justified using currently available technology.

Electron-Mediated Nuclear-Spin Interactions between Distant Nitrogen-Vacancy Centers

We propose a scheme enabling controlled quantum coherent interactions between separated nitrogen-vacancy centers in diamond in the presence of strong magnetic fluctuations. The proposed scheme couples nuclear qubits employing the magnetic dipole-dipole interaction between the electron spins and, crucially, benefits from the suppression of the effect of environmental magnetic field fluctuations thanks to a strong microwave driving. This scheme provides a basic building block for a full-scale quantum-information processor or quantum simulator based on solid-state technology.

Entangling separate nitrogen-vacancy centers in a scalable fashion via coupling to microtoroidal resonators

We propose a potentially practical scheme to entangle negatively charged nitrogen-vacancy (N-V) centers in distant diamonds. Each diamond is supposed to be fixed on the exterior surface of a microtoroidal resonator, and the single-photon input-output process-a currently available technique-could entangle separate N-V centers in a scalable fashion. The feasibility of our scheme and the experimental challenge are discussed by considering currently available techniques for qualified N-V centers and cavities.

Entanglement of separate nitrogen-vacancy centers coupled to a whispering-gallery mode cavity

We present a quantum electrodynamical model involving nitrogen-vacancy centers coupled to a whispering-gallery mode cavity. We consider two schemes to create the W state and Bell state, respectively. One scheme makes use of Raman transition with the cavity field virtually excited, and the other enables Bell state preparation and quantum information transfer by virtue of dark state evolution and adiabatic passage, which is tolerant to ambient noise and experimental parameter fluctuations. We justify our schemes by considering their experimental feasibility and challenge, using the currently available technology.