Ultralong Dephasing Times in Solid-State Spin Ensembles via Quantum Control

Abstract

Quantum spin dephasing is caused by inhomogeneous coupling to the environment, with resulting limits to the measurement time and precision of spin-based sensors. The effects of spin dephasing can be especially pernicious for dense ensembles of electronic spins in the solid-state, such as nitrogenvacancy (NV) color centers in diamond. We report the use of two complementary techniques, spin bath driving, and double quantum coherence magnetometry, to enhance the inhomogeneous spin dephasing time (T * 2 ) for NV ensembles by more than an order of magnitude. In combination, these quantum control techniques (i) eliminate the effects of the dominant NV spin ensemble dephasing mechanisms, including crystal strain gradients and dipolar interactions with paramagnetic bath spins, and (ii) increase the effective NV gyromagnetic ratio by a factor of two. Applied independently, spin bath driving and double quantum coherence magnetometry elucidate the sources of spin ensemble dephasing over a wide range of NV and bath spin concentrations. These results demonstrate the longest reported T * 2 in a solid-state electronic spin ensemble at room temperature, and outline a path towards NV-diamond DC magnetometers with broadband femtotesla sensitivity.

Hamiltonian engineering with constrained optimization for quantum sensing and control

“…Transitions between 0 and±1 are driven directly at frequencies D±γB z , whereas transitions between±1 states require composite microwave pulses. Current experimental setups include multiple microwave drive channels, each with independent phase and amplitude control, and well-tuned π/2 and π pulses suitable for SQ and DQ magnetometry [35,64].…”

“…In the definitions of {V i , W j } in (46), with the substitution V W VW i j i j k  ( ) . While we consider only Clifford rotations here given the prevalence of pulsed quantum control of NV ensembles [35], this restriction is not fundamental. Quantum control techniques that are robust against noise, such as fast holonomic gates [65], which have recently been demonstrated in NV systems [66], present an interesting avenue for future work.…”

“…where T 2 *{·} denotes the dephasing time limit due to a specific mechanism [35]. The P1 spin bath, strain inhomogeneity, and temperature fluctuations likely dominate NV − dephasing in high nitrogen density ensembles.…”

“…The optimal time for a Ramsey magnetometry measurement is on the order of the inhomogeneous spin dephasing time T 2 *. P1 spin bath driving combined with DQ magnetometry has resulted in improving native T 2 2 * » μs to T 30 2,DQ Drive » + μs [35], which we use to estimate the power required for fast pulses. A Rabi frequency of 2 7.7 MHz R p W =´has been obtained using a loop gap resonator with incident microwave power P 16 W » [68], with Ω R 10 MHz commonly achieved using a wire loop antenna applying a homogeneous field over the sample detection volume [59].…”

“…Various broadening mechanisms from paramagnetic defects can be mitigated by driving the environment spins [31,32] while inhomogeneous broadening mechanisms like strain variation and temperature fluctuation can be mitigated by double quantum [33,34]. Used independently, these techniques help to identify sources of dephasing; in combination, they can result in over an order of magnitude improvement in T 2 * [35]. NV − -NV − dipolar interactions will likely ultimately limit NV − coherence time [36].…”