Physicists at Boston University have achieved what amounts to a long-sought benchmark in quantum measurement: detecting the spontaneous magnetic fluctuations of nuclear spins in a macroscopic sample with sensitivity limited only by quantum mechanics itself. The technique, described in a recent study in Nature Physics, works without any external excitation of the sample, instead reading the faint quantum noise that arises naturally from ensembles containing roughly a sextillion nuclear spins. The result bridges a gap between the quantum world and everyday-scale materials that has frustrated experimentalists for decades.
Listening to Quantum Noise in Bulk Materials
Standard nuclear magnetic resonance, or NMR, relies on bombarding a sample with radio-frequency pulses and recording the response. As researcher Mike P. Williamson noted in a 2019 review, conventional NMR methods never observe single spins but instead measure populations of very large numbers of spins that behave in ways expected by classical physics. That classical averaging masks the quantum behavior of individual nuclei. The new method flips the script: rather than driving the spins, it treats their inherent quantum fluctuations as the signal itself.
To pick up those whisper-quiet fluctuations, the Boston University team cooled samples of fluorine-19 in PTFE and hydrogen in nylon inside a dilution refrigerator to temperatures below one kelvin. A superconducting receiver circuit, read out by a SQUID magnetometer, captured the magnetic signals. Because no excitation pulse is applied, the measurement qualifies as perturbation-free, meaning the act of observing does not push the spins away from their natural quantum state.
The team also reports that their setup can be understood as a resonant LC circuit whose inductive element is formed by a superconducting coil closely coupled to the sample. In this configuration, the random orientation changes of nuclear spins produce tiny fluctuations in magnetic flux, which translate into voltage noise in the circuit. The SQUID, operating at the edge of its own quantum sensitivity, converts that flux noise into a measurable electrical signal. Careful shielding and filtering ensure that environmental disturbances, from stray radio waves to mechanical vibrations, remain well below the intrinsic quantum noise of the spins.
From Tens of Spins to a Sextillion
Earlier generations of experiments had demonstrated quantum-limited sensitivity only for very small collections of atoms, typically on the order of tens of spins. Alexander Sushkov of Boston University and coauthors report in the Nature Physics access page that their technique extends that threshold to millimole-scale ensembles. In a seminar at the University of Michigan, Sushkov’s group described precision nuclear magnetic resonance measurements on macroscopic ensembles of about 1021 nuclear spins, with sensitivity limited by quantum spin projection noise rather than by instrument electronics or thermal interference.
That jump, from roughly 10 spins to 1021, is not merely incremental. It means the quantum boundary of measurement precision now applies to samples large enough to hold in your hand. For physicists, reaching the spin-projection-noise floor in a bulk sample confirms that every other source of classical noise (thermal vibrations, amplifier noise, stray electromagnetic fields) has been suppressed below the quantum threshold. The detector, in effect, is limited only by the uncertainty principle.
In practical terms, the experimenters diagnose this limit by analyzing the frequency spectrum of the measured noise. The power spectral density follows a form predicted for uncorrelated nuclear spins in thermal equilibrium, with no excess noise at low frequencies where technical imperfections often lurk. By varying the sample size and geometry, they verify that the noise scales with the square root of the number of spins, as quantum theory prescribes, rather than with linear or more complicated dependences characteristic of classical fluctuations.
Two Decades of Spin Noise Spectroscopy
The intellectual roots of this work stretch back to 2004, when researchers first demonstrated that the random fluctuations of atomic spins could serve as a useful spectroscopic signal. That foundational experiment, published in a Nature article, established spin noise spectroscopy as a perturbation-free approach to magnetic resonance. The concept was striking. Instead of treating noise as something to eliminate, the technique made noise the measurement itself.
In spin noise spectroscopy, a probe (often a laser beam or a sensitive magnetometer) monitors the spontaneous magnetization fluctuations of a sample without applying resonant driving fields. The resulting noise spectrum reveals the same resonance frequencies and relaxation times that conventional NMR or electron spin resonance would extract, but with minimal disturbance to the system. This feature is especially attractive for fragile quantum materials and cold atomic gases, where strong driving can heat or decohere the sample.
Parallel advances in quantum nondemolition, or QND, measurement techniques sharpened the theoretical framework. A key experiment published in Physical Review Letters demonstrated continuous QND measurements in a high-sensitivity atomic magnetometer and analyzed spin-projection noise in detail. That work helped define what “quantum limit” means in practice: the floor set by the unavoidable randomness of quantum spin projections. It also clarified how measurement backaction (the disturbance a detector inflicts on what it measures) can be managed through careful experimental design. The companion preprint of that study contains expanded derivations and apparatus details that informed subsequent experimental designs.
The Boston University experiment can be viewed as a direct descendant of these ideas, transplanted from dilute atomic vapors to dense solid-state samples. Instead of optical rotation, the observable is the inductive coupling between nuclear spins and a superconducting coil, but the underlying philosophy remains the same. Listen to the system’s own quantum noise rather than forcing it to respond to an external drive.
Why Quantum-Limited Detection Matters Beyond the Lab
Much popular coverage of quantum sensing focuses on exotic hardware or distant applications. Yet the practical consequence of reaching the quantum noise floor in a bulk NMR-style measurement is concrete and near-term. If the dominant noise source is quantum spin projection rather than instrument imperfection, then any further improvement in sensitivity requires genuinely quantum strategies, such as spin squeezing, rather than better electronics.
Sushkov and coauthors have already moved in that direction. A preprint describing approaches to spin squeezing of macroscopic nuclear spin ensembles outlines how the same experimental platform could be pushed past the standard quantum limit. The motivations listed in that work include searches for ultralight dark matter, a class of hypothetical particles whose faint signatures could be buried just below the spin-projection-noise floor. If spin squeezing can reduce quantum noise in these large ensembles, the detector becomes sensitive to signals that no classical instrument could reach.
Separate work at Oak Ridge National Laboratory has explored a complementary angle. A team there used single-qubit sensing with a nitrogen-vacancy center in diamond to probe spin textures in quantum materials, effectively using an individual electronic spin as an ultra-local magnetometer. While that experiment operates at the opposite extreme of system size compared with the Boston University work, both efforts share a common theme: pushing magnetic measurements to regimes where quantum noise, not technical noise, defines the ultimate sensitivity.
Looking ahead, quantum-limited detection in bulk nuclear spin ensembles could influence several fields. In materials science, perturbation-free NMR may enable studies of delicate phases (such as spin liquids or unconventional superconductors) that are easily disturbed by strong radio-frequency pulses. In fundamental physics, long-lived nuclear spins in solids offer an appealing platform for detecting tiny, slowly varying fields associated with dark matter or violations of fundamental symmetries. And in metrology, techniques that tame or exploit spin projection noise could feed into next-generation timekeeping and navigation devices.
For now, the Boston University results mark a conceptual turning point. By demonstrating that even a hand-sized chunk of matter can be measured with a sensitivity bounded only by quantum mechanics, the work closes a long-standing gap between tabletop quantum optics experiments and the macroscopic world. The noise that engineers once fought to suppress has become a resource, and the quiet murmur of nuclear spins in a frozen polymer now speaks volumes about how far quantum measurement has come, and how much further it can still go.
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*This article was researched with the help of AI, with human editors creating the final content.
