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arXiv:1909.01050 · Physical review. B./Physical review. B

Continuous microwave hole burning and population oscillations in a diamond spin ensemble

Haitham A. R. El-Ella, Alexander Huck, Ulrik L. Andersen

Published Dec 2, 2019• 11 citations
arXivHTMLPDF

Abstract

Dense nitrogen-vacancy spin ensembles in diamond are exciting systems for exploring novel collective spin states and for advancing quantum-limited metrology and information technology. However, severe inhomogeneous broadening usually accompanies increasing densities, making ensemble characterization and control challenging when using conventional pulsed multiresonance protocols. Here, the authors demonstrate the utility of employing a continuous-wave double-resonance scheme and of generating spin-ensemble population oscillations. Through this approach, some unexpected aspects of the inhomogeneous broadening mechanism are revealed, while coupled nuclear spins are detected at energies that are challenging to measure via conventional pulsed schemes.

DOI
10.1103/physrevb.100.214407

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Dense nitrogen-vacancy spin ensembles in diamond are exciting systems for exploring novel collective spin states and for advancing quantum-limited metrology and information technology. However, severe inhomogeneous broadening usually accompanies increasing densities, making ensemble characterization and control challenging when using conventional pulsed multiresonance protocols. Here, the authors demonstrate the utility of employing a continuous-wave double-resonance scheme and of generating spin-ensemble population oscillations. Through this approach, some unexpected aspects of the inhomogeneous broadening mechanism are revealed, while coupled nuclear spins are detected at energies that are challenging to measure via conventional pulsed schemes.
Negatively charged nitrogen-vacancy (NV${}^{\ensuremath{-}}$) centers in diamond have generated much recent interest for their use in sensing. The sensitivity improves when the NV ground-state microwave transitions are narrow, but these transitions suffer from inhomogeneous broadening, especially in high-density NV ensembles. To better understand and remove the sources of broadening, we demonstrate room-temperature spectral ``hole burning'' of the NV ground-state transitions. We find that hole burning removes the broadening caused by magnetic fields from 13{}^{13}13C nuclei and demonstrate that it can be used for magnetic-field-insensitive thermometry.
Abstract Decoherence of Rabi oscillation in a two-level quantum system consists of two components, a simple exponential decay and a damped oscillation. In dense-ensemble spin systems like negatively charged nitrogen-vacancy (NV−) centers in diamond, fast quantum state decoherence often obscures clear observation of the Rabi nutation. On the other hand, the simple exponential decay (or baseline decay) of the oscillation in such spin systems can be readily detected but has not been thoroughly explored in the past. This study investigates in depth the baseline decay of dense spin ensembles in diamond under continuously driving microwave (MW). It is found that the baseline decay times of NV− spins decrease with the increasing MW field strength and the MW detuning dependence of the decay times shows a Lorentzian-like spectrum. The experimental findings are in good agreement with simulations based on the Bloch formalism for a simple two-level system in the low MW power region after taking into account the effect of inhomogeneous broadening. This combined investigation provides new insight into fundamental spin relaxation processes under continuous driving electromagnetic fields and paves ways to better understanding of this underexplored phenomena using single NV− centers, which have shown promising applications in quantum computing and quantum metrology.
Rabi oscillations with a decay time of 26.7 μs are observed in a system comprising the electron spins in a diamond nitrogen–vacancy centre and a superconducting microwave cavity. Such oscillations are achieved by engineering the spectral hole burning of the spin ensemble. Spectral hole burning, used in inhomogeneously broadened emitters, is a well-established optical1 technique, with applications from spectroscopy to slow light2 and frequency combs3. In microwave photonics4, electron spin ensembles5,6 are candidates for use as quantum memories7 with potentially long storage times8. Here, we demonstrate long-lived collective dark states9 by spectral hole burning in the microwave regime10. The coherence time in our hybrid quantum system (nitrogen–vacancy centres strongly coupled to a superconducting microwave cavity) becomes longer than both the ensemble's free-induction decay and the bare cavity dissipation rate. The hybrid quantum system thus performs better than its individual subcomponents. This opens the way for long-lived quantum multimode memories, solid-state microwave frequency combs, spin squeezed states11, optical-to-microwave quantum transducers12 and novel metamaterials13. Beyond these, new cavity quantum electrodynamics experiments will be possible where spin–spin interactions and many-body phenomena14 are directly accessible.
Defects in solid-state materials provide an ideal, robust platform for quantum sensing. To deliver maximum sensitivity, a large ensemble of noninteracting defects hosting quantum states with long coherence is required. Control of such an ensemble is challenging due to the spatial variation in both the defect energy levels and in any control field across a macroscopic sample. In this work, we experimentally demonstrate that we can overcome these challenges using Floquet theory and optimal control optimization methods to efficiently and coherently control a large defect ensemble, suitable for sensing. We apply our methods experimentally to a spin ensemble of up to $4\ifmmode\times\else\texttimes\fi{}{10}^{9}$ nitrogen-vacancy centers in diamond. By explicitly including the hyperfine interaction to the intrinsic 14N^{14}\mathrm{N}14N nuclear spin in the optimization, we design shaped microwave control pulses that can outperform conventional ($\ensuremath{\pi}$) pulses when applied to sensing schemes, with an improvement in the strength of ensemble response of between 11% and 78%. Through simulation of the ensemble dynamics, we shed light on the bandwidth limitations of large-ensemble reinitialization and propose alternative routes for further improvement.
Quantum relaxometry based on nitrogen-vacancy (NV) centers in diamond is an easy-to-use technique that detects magnetic noise by measuring the longitudinal relaxation of NV centers. It is favored in chemical and biological applications due to the robustness against imperfect spin control. By tuning the energy level, NV relaxometry can detect magnetic noise generated by other spins and obtain magnetic resonance spectra through cross relaxation. However, the inhomogeneous broadening of NV centers greatly limits the spectral resolution of cross-relaxation spectra. Here we demonstrate a hole-burning technique to remove the inhomogeneous broadening. We utilize a weak pump field to deplete specific NV centers during the relaxation measurement, which contribute a reverse signal in the cross-relaxation spectra with a much narrower linewidth. Our method retains the convenience and sensitivity of NV relaxometry, opening up new avenues for high-resolution magnetic resonance spectroscopy in complex scenarios.