Matters Arising: Time-reversal-based quantum metrology with many-body entangled states

TL;DR

This paper critically examines a recent experiment claiming quantum-enhanced measurement precision using entangled atoms, concluding that the claimed surpassing of the standard quantum limit is incorrect and that such a surpass has not been experimentally demonstrated.

Contribution

The paper provides a critical analysis showing that previous claims of surpassing the standard quantum limit with entangled states are invalid due to flawed comparisons.

Findings

The experiment did not surpass the precision bound for 300 independent particles.

The experiment did not surpass the precision bound for a single particle.

Previous claims of surpassing the standard quantum limit with entanglement are based on misleading comparisons.

Abstract

In their paper "Time-reversal-based quantum metrology with many-body entangled states" Nature Physics (2022), Colombo et. al. claim to measure both an unknown phase and an oscillating magnetic field with a precision that cannot be achieved using independent particles - a limit known as the standard quantum limit. By entangling an ensemble of $\sim300$ atoms, Colombo et. al. measure an angle of rotation away from a known initial state and additionally measure a magnetic field oscillating at 290 Hz. The authors report an experimental precision approximately a factor of 4 beyond what is possible with the same number of independent atoms (12.8 dB and 11.8 dB for these tasks respectively). These claims are incorrect. Colombo et. al. do not surpass the precision bound for 300 independent particles, nor do they even surpass the precision bound for a single particle. Colombo et. al. cite several experiments that surpass the standard quantum limit using entanglement. Each and every paper cited performs incorrect, incomplete or misleading comparisons of the type that we highlight here. The consequence being that the single particle precision bound has never been experimentally surpassed with entanglement.