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In this article we present a simple repeater scheme based on the negatively-charged nitrogen vacancy centre in diamond (NV-). Each repeater node is built from simple modules comprising an optical cavity containing a single NV-, with one nuclear spin from 15N as quantum memory. The operation in the module only uses deterministic processes and interactions and achieves high fidelity (>99%) operation, and modules are connected by optical fiber. In the repeater node architecture, the processes between modules by photons can be in principle deterministic, however current limitations on optical components lead to the processes to be probabilistic but heralded. The most resource modest repeater architecture contains at least two modules at each node, and the repeater nodes are than connected by telecom wavelength entangled photon pairs. We discuss the performance of quantum repeaters starting from the minimum-resource strategy with several modules (~10) and then incorporating more resource-intense strategies step by step. Our architecture enables large-scale quantum information networks with existing technology.
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Nano-NMR spectroscopy with nitrogen-vacancy centers holds the potential to provide high resolution spectra of minute samples. This is likely to have important implications for chemistry, medicine and pharmaceutical engineering. One of the main hurdles facing the technology is that diffusion of unpolarized liquid samples broadens the spectral lines thus limiting resolution. Experiments in the field are therefore impeded by the efforts involved in achieving high polarization of the sample which is a challenging endeavor. Here we examine a scenario where the liquid is confined to a small volume. We show that the confinement counteracts the effect of diffusion, thus overcoming a major obstacle to the resolving abilities of the NV-NMR spectrometer.
Physics and information are intimately connected, and the ultimate information processing devices will be those that harness the principles of quantum mechanics. Many physical systems have been identified as candidates for quantum information processing, but none of them are immune from errors. The challenge remains to find a path from the experiments of today to a reliable and scalable quantum computer. Here, we develop an architecture based on a simple module comprising an optical cavity containing a single negatively-charged nitrogen vacancy centre in diamond. Modules are connected by photons propagating in a fiber-optical network and collectively used to generate a topological cluster state, a robust substrate for quantum information processing. In principle, all processes in the architecture can be deterministic, but current limitations lead to processes that are probabilistic but heralded. We find that the architecture enables large-scale quantum information processing with existing technology.
Initializing quantum registers with high fidelity is a fundamental precondition for many applications like quantum information processing and sensing. The electronic and nuclear spins of a Nitrogen-Vacancy (NV) center in diamond form an interesting hybrid quantum register that can be initialized by a combination of laser, microwave, and radio-frequency pulses. However, the laser illumination, which is necessary for achieving electron spin polarization, also has the unwanted side-effect of depolarizing the nuclear spin. Here, we study how the depolarization dynamics of the $^{14}$N nuclear spin depends on the laser wavelength. We show experimentally that excitation with an orange laser (594 nm) causes significantly less nuclear spin depolarization compared to the green laser (532 nm) typically used for excitation and hence leads to higher nuclear spin polarization. This could be because orange light excitation inhibits ionization of NV$^{0}$ into NV$^{-}$ and therefore suppresses one source of noise acting on the nuclear spin.
We investigated the depth dependence of coherence times of nitrogen-vacancy (NV) centers through precisely depth controlling by a moderately oxidative at 580{deg}C in air. By successive nanoscale etching, NV centers could be brought close to the diamond surface step by step, which enable us to trace the evolution of the number of NV centers remained in the chip and to study the depth dependence of coherence times of NV centers with the diamond etching. Our results showed that the coherence times of NV centers declined rapidly with the depth reduction in their last about 22 nm before they finally disappeared, revealing a critical depth for the influence of rapid fluctuating surface spin bath. By monitoring the coherence time variation with depth, we could make a shallow NV center with long coherence time for detecting external spins with high sensitivity.
We present a scheme to generate entangled photons using the NV centers in diamond. We show how the long-lived nuclear spin in diamond can mediate entanglement between multiple photons thereby increasing the length of entangled photon string. With the proposed scheme one could generate both n-photon GHZ and cluster states. We present an experimental scheme realizing the same and estimating the rate of entanglement generation both in the presence and absence of a cavity.
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