Nitrogen-vacancy center is a deep defect in diamond. Its energy level structure helps to initialise its pure spin state in room temperature through optical pumping and rotate its state on a Bloch sphere through application of controlled microwave pulses [1]. Lifetime of a superposition state is limited by pure dephasing caused by interactions with a bath of a few hundred randomly distributed C-13 nuclear spins, naturally present in diamond and coupled to the central qubit with dipolar interaction. The shape of decoherence as a function of total evolution time strongly depends on distribution of those nuclei in the lattice. A number of effects, observed in these systems, can be attributed either to spins closest to the NV center or the remainder of weakly coupled bath spins treated as a source of noise. In room temperature, the state of nuclear bath is completely mixed. However, recently a number of experimental results showed successful polarisation of few spins closest to the NV center. Such a partially polarised state can be investigated in a variety of fundamentally interesting directions around the physics of decoherence, such as qubit-environment entanglement [2] or quantum Darwinism observed through formation of Spectrum Broadcast Structures [3].

References:

[1] Doherty, M. W., Manson, N. B., Delaney, P., Jelezko, F., Wrachtrup, J., and Hollenberg, L. C. L., The nitrogen-vacancy colour centre in diamond. Physics Reports, 528, 1–45 (2013)

[2] Roszak, K., Kwiatkowski, D., and Cywiński, Ł., How to detect qubit-environment entanglement generated during qubit dephasing. Physical Review A, 100, 022318 (2019)

[3] Kwiatkowski, D., Cywiński, Ł., Korbicz, J., Emergence of classicality for NV centers interacting with dynamically polarised nuclear environment, to appear on arXiv in July 2020.

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