Spurious harmonic response of multipulse quantum sensing sequences

TL;DR

This paper investigates how real-world imperfections in multipulse quantum sensing sequences cause harmonic responses, leading to potential ambiguities in identifying nuclear spins in diamond-based magnetometry.

Contribution

It reveals that finite pulse durations and detunings induce harmonics in the filter function, affecting signal interpretation in quantum sensing.

Findings

Harmonics appear at 2fac, 4fac, 8fac and subharmonics due to non-idealities.

XY sequences generate multiple harmonic signals, complicating spectral analysis.

Harmonic responses can cause misidentification of nuclear spin species.

Abstract

Multipulse sequences based on Carr-Purcell decoupling are frequently used for narrow-band signal detection in single spin magnetometry. We have analyzed the behavior of multipulse sensing sequences under real-world conditions, including finite pulse durations and the presence of detunings. We find that these non-idealities introduce harmonics to the filter function, allowing additional frequencies to pass the filter. In particular, we find that the XY family of sequences can generate signals at the 2fac, 4fac and 8fac harmonics and their odd subharmonics, where fac is the ac signal frequency. Consideration of the harmonic response is especially important for diamond-based nuclear spin sensing where the NMR frequency is used to identify the nuclear spin species, as it leads to ambiguities when several isotopes are present.