Solid-state spin ensembles are a promising platform for quantum sensors, but their readout has long been limited by photon shot noise, which drowned out the spins' intrinsic quantum noise. Unlike atomic ensembles, where projection noise has already been overcome, solid-state systems remained at the photon limit. This work joins a line of research on diamond NV centers and quantum information, with neighboring studies exploring quantum memory, spin-spin coupling, and sensitivity to electric fields.
The experiment was performed on a home-built confocal microscope at room temperature. A diamond plate with a thin layer of NV centers was placed in a superconducting magnet with a field of 2.7 T. Microwave control of electron spins at 73 GHz was achieved via a resonator, while radio-frequency control of nuclear spins used a copper wire. Readout relied on repeated mapping of the nuclear spin onto the electron spin, followed by optical readout and reinitialization. The key technique was repeating the readout cycle multiple times to accumulate signal and suppress photon noise.
Noise reduction of 3.8 dB below the thermal projection noise level was achieved. This allowed direct observation of fluctuations in an ensemble of 31(3) NV centers within the confocal spot. A transition from the photon limit to the projection limit was demonstrated as the number of repeated readouts increased. The method enabled tracking of the spin distribution width during Rabi oscillations and distinguishing correlated from uncorrelated noise sources. In relaxometry and dynamical decoupling experiments, signatures of spatial correlations induced by a common noise source were observed.
This work is the first to demonstrate readout of a solid-state spin ensemble at the projection noise limit, previously accessible only in atomic systems. It bridges a fundamental gap between atomic and solid-state quantum sensors. Unlike neighboring works, this study used repeated nuclear-assisted readout on a mesoscopic ensemble under a high magnetic field, overcoming the photon limit and directly measuring quantum fluctuations.
Future developments may include applying spin squeezing to reduce projection noise below the standard quantum limit. A wide-field microscope could be built where each pixel records the spin distribution, enabling correlation maps. The method also holds promise for quantum simulations, extracting higher-order cumulants and observing non-Gaussian many-body states.
The results will improve the sensitivity of NV-center quantum sensors by several orders of magnitude, accelerating NMR, MRI, magnetometry, and relaxometry. The method is also applicable to microscopy of correlated signals in magnetic materials, phase transitions, and diagnostics of electronic chips.
Implementing spin squeezing in mesoscopic solid-state ensembles to surpass the standard quantum limit. Developing wide-field microscopy with pixel-by-pixel analysis of spin distributions.
A key open question in quantum sensing is achieving and exploiting entangled states in solid-state ensembles at room temperature. This work provides a tool for directly observing collective quantum effects, which is essential for testing many-body physics theories.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study demonstrates non-demolition readout for a mesoscopic ensemble of NV centers in diamond at room temperature. The repeated nuclear-assisted readout method reduced noise by 3.8 dB below the thermal projection noise level, granting access to internal spin correlations.
Where its strength is
The key achievement is overcoming photon shot noise and reaching the projection limit in a solid-state system, which was previously only possible with atomic ensembles.
Where it could go next
Worth trying
Why this may matter
Achieving the projection limit in a solid-state ensemble paves the way for quantum-enhanced metrology and direct observation of many-body correlations, which could significantly improve the precision of quantum sensors.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.55, against a usual 0.64. It looks like few have drilled nearby.
The number of NV centers in the confocal spot (31) was determined from the projection noise level, marking the first direct count of active centers without destructive probing.