Quantum computing: cutting out the noise If electron spins in solid materials are to be exploited in quantum computers, there is a need to minimize the quantum 'noise' that inevitably accompanies the interaction of the spins with their environment. Experiments in single crystals of malonic acid undergoing pulsed electron paramagnetic resonance now show how this can be achieved. The use of external pulses to induce the electron spins to 'flip' an optimal number of times causes their noisy interaction with the environment to be averaged towards zero, and the timescale over which spin coherence is maintained is markedly increased — from 0.04 to 30 microseconds in the system studied.
We demonstrate significant improvements of the spin coherence time of a dense ensemble of nitrogen-vacancy (NV) centers in diamond through optimized dynamical decoupling (DD). Cooling the sample down to 77 K suppresses longitudinal spin relaxation
T1 effects and DD microwave pulses are used to increase the transverse coherence time
T2 from $\ensuremath{\sim}0.7\phantom{\rule{0.28em}{0ex}}\mathrm{ms}$ up to $\ensuremath{\sim}30\phantom{\rule{0.28em}{0ex}}\mathrm{ms}$. We extend previous work of single-axis (Carr-Purcell-Meiboom-Gill) DD towards the preservation of arbitrary spin states. Following a theoretical and experimental characterization of pulse and detuning errors, we compare the performance of various DD protocols. We identify that the optimal control scheme for preserving an arbitrary spin state is a recursive protocol, the concatenated version of the XY8 pulse sequence. The improved spin coherence might have an immediate impact on improvements of the sensitivities of ac magnetometry. Moreover, the protocol can be used on denser diamond samples to increase coherence times up to NV-NV interaction time scales, a major step towards the creation of quantum collective NV spin states.