Coherent manipulation of nuclear spins in the strong driving regime

Abstract

Spin-based quantum information processing makes extensive use of spin state
manipulation. This ranges from dynamical decoupling of nuclear spins in
quantum sensing experiments to applying logical gates on qubits in a quantum
processor. Fast manipulation of spin states is highly desirable for accelerating
experiments, enhancing sensitivity, and applying elaborate pulse sequences. Strong
driving using intense radio-frequency (RF) fields can, therefore, facilitate fast
manipulation and enable broadband excitation of spin species.
In this work, we present an antenna for strong driving in quantum sensing
experiments and theoretically address challenges of the strong driving regime. First,
we designed and implemented a micron-scale planar spiral RF antenna capable of
delivering intense fields to a sample. The planar antenna is tailored for quantum
sensing experiments using the diamond’s nitrogen-vacancy (NV) center and should be
applicable to other solid-state defects. The antenna has a broad bandwidth of 22 MHz,
is compatible with scanning probes, and is suitable for cryogenic and ultrahigh vacuum
conditions. We measure the magnetic field induced by the antenna and estimate a field-to-
current ratio of 113±16 G/A, representing a six-fold increase in efficiency compared
to the state-of-the-art, crucial for cryogenic experiments. We demonstrate the antenna
by driving Rabi oscillations in 1H spins of an organic sample on the diamond surface
and measure 1H Rabi frequencies of over 500 kHz, i.e., π-pulses shorter than 1 μs –
faster than previously reported in NV-based nuclear magnetic resonance (NMR).
Finally, we discuss the implications of driving spins with a field tilted from the
transverse plane in a regime where the driving amplitude is comparable to the spin state
splitting, such that the rotating wave approximation does not describe the
dynamics well. We present a simple recipe to optimize pulse fidelity in this regime
based on a phase and offset-shifted sine drive, which may be optimized in situ without
numerical optimization procedures or precise modeling of the experiment. We consider
this approach in a range of driving amplitudes and show that it is particularly efficient
in the case of a tilted driving field.
The results presented here constitute a foundation for implementing fast nuclear
spin control in various systems.