In the relentless pursuit of surpassing the fundamental limits of precision in measurement, researchers at the Niels Bohr Institute, University of Copenhagen, have engineered a groundbreaking quantum sensing system that combines large-scale entanglement with advanced noise suppression methods. This innovative device marks a significant leap forward in the quest for enhanced sensitivity across a broad spectrum of sensing technologies, ranging from biomedical diagnostics to the detection of gravitational waves. The findings, recently published in the prestigious journal Nature, introduce a hybrid quantum network that synergistically harnesses multi-photon light states entangled with a large atomic spin ensemble, resulting in unprecedented control over quantum noise in practical, compact setups.
The core challenge in quantum sensing stems from the so-called standard quantum limit, a barrier arising from intrinsic noise introduced by the quantum nature of measurement. This noise, which includes both back-action noise caused by the act of measurement perturbing the system and detection noise inherent to the readout process, places stringent restrictions on the accuracy of even the most sensitive optical sensors. While classical optics and measurement techniques have matured over decades, pushing sensitivity beyond this limit demands the nuanced application of quantum phenomena such as entanglement, squeezed light, and backaction evasion—concepts that were previously confined mostly to microscopic systems.
What sets this new system apart is the unique integration of multi-photon entangled light with a sizable atomic spin ensemble that effectively acts as a negative mass oscillator. Traditionally, entanglement has been confined to tiny systems such as individual photons or atoms. Here, experimentalists have expanded entanglement into a macroscopic regime, enabling frequency-dependent squeezing that dynamically suppresses quantum noise over a wide frequency bandwidth. This sophistication allows the sensor to adapt its noise reduction strategy seamlessly, shifting between attenuating amplitude noise and phase noise at different frequencies—an essential feature for tackling the diverse signal environments encountered in real-world applications.
The engineering of this frequency-dependent squeezing is particularly ingenious. By passing squeezed light through the atomic spin ensemble, the system utilizes the frequency-sensitive rotation of the phase of the squeezed state to tailor the noise characteristics dynamically. The spin ensemble’s capacity to invert noise signs—from positive to negative—is crucial, as it enables destructive interference of noise components when the sensor’s output signal is combined with the spin system’s response. This interplay effectively cancels out substantial portions of both back-action and detection noise, achieving broadband noise suppression that was previously unattainable without colossal, complex apparatuses.
Large installations such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) or European detectors like VIRGO have traditionally relied on extensive optical resonators spanning hundreds of meters to kilometers to accomplish frequency-dependent noise squeezing. The revolutionary aspect of the Niels Bohr Institute’s setup lies in its compactness and scalability; the entire apparatus fits on a tabletop, roughly the size of an ordinary dining table, providing an unprecedented combination of performance with practicality. This miniaturization is a vital step toward deploying quantum-enhanced sensing technologies outside specialized physics laboratories, making them accessible for a range of commercial and scientific applications.
Among these applications, biomedical imaging and diagnostics stand out as particularly promising beneficiaries. Magnetic resonance imaging (MRI), for instance, relies heavily on detecting faint magnetic field variations to generate detailed images. By integrating this quantum noise suppression technique, future MRI machines could achieve dramatically enhanced resolution and sensitivity, enabling earlier and more accurate detection of neurological and other disorders. Furthermore, biosensors tasked with monitoring molecular markers or metabolic changes in real-time could leverage these advancements to deliver faster and more precise results, ultimately revolutionizing patient care.
Beyond medicine, the system’s applicability extends to fundamental physics and environmental science. The enhancement of gravitational wave detectors with this hybrid quantum network could increase their sensitivity to subtle ripples in spacetime caused by cataclysmic astrophysical events, deepening our understanding of black hole mergers, neutron star collisions, and even the early universe’s formation processes. Moreover, the platform could be adapted for the detection of minute changes in magnetic fields, timekeeping accuracy, and acceleration, impacting a broad spectrum of sensing fields from geophysics to navigation systems.
The system’s design also opens new avenues for quantum communication and quantum information processing. Quantum repeaters, which are essential for establishing secure long-distance quantum communication, could benefit from this architecture through noise reduction and enhanced signal fidelity. Likewise, quantum memories employed in quantum networks stand to gain improved storage and retrieval capabilities, leveraging the negative mass spin ensemble’s properties to protect quantum states against decoherence.
Eugene Polzik, a leading visionary behind this work at the Niels Bohr Institute, articulates the essence of the device’s performance succinctly: “The sensor and spin system interact with two entangled beams of light. Following their interaction, simultaneous detection and combination of these beams’ signals enables broadband sensitivity that transcends the standard quantum limit.” This elegant yet powerful interplay between entangled subsystems manifests as a technologically feasible route to surpass constraints once believed to be insurmountable.
Technically, the integration of large atomic spin ensembles acting as negative mass oscillators is a sophisticated feat. In classical mechanics, negative mass is counterintuitive; however, in this quantum context, the atomic spin ensemble’s effective negative mass behavior allows it to mirror quantum fluctuations of the sensor’s measurement process but with inverted phase, facilitating the crucial noise cancellation effect. This contrasts with traditional methods that rely primarily on passive optical components and fixed squeezing profiles, as this dynamic system adjusts noise suppression characteristics by manipulating quantum state phases in real-time via entanglement-assisted feedback.
Another critical advancement is how the hybrid system preserves entanglement over macroscopic scales. Maintaining coherence among a vast number of atoms and photons, while exposed to environmental decoherence and technical noise sources, represents an experimental milestone. The researchers succeeded in mitigating these deleterious effects through precise control of the atomic ensemble’s quantum state and optimized interaction protocols, thereby enabling the practical realization of a hybrid quantum sensor capable of operational stability under laboratory conditions.
The implications of these findings are far-reaching. As quantum technologies continue to advance, the ability to engineer devices that leverage large-scale entanglement and dynamic noise suppression ushers in a new era of sensors that could dramatically outpace classical counterparts in sensitivity, resolution, and operational bandwidth. The tabletop nature of the device hints at future commercialization possibilities, where quantum-enhanced sensors might become standard components in fields as diverse as medical diagnostics, space exploration, precision navigation, and environmental monitoring.
In essence, the Niels Bohr Institute’s novel hybrid quantum network represents a confluence of pioneering quantum optics, atomic physics, and engineering ingenuity. By breaking the standard quantum limit across a broad acoustic frequency range, this work not only pushes the frontier of measurement science but also lays a versatile foundation for diverse quantum technologies poised to transform multiple industries and scientific disciplines.
Subject of Research: Quantum sensing and noise suppression using a hybrid quantum network involving frequency-dependent squeezing and atomic spin ensembles.
Article Title: Hybrid quantum network for sensing in the acoustic frequency range
News Publication Date: 2-Jul-2025
Web References: DOI: 10.1038/s41586-025-09224-3
Image Credits: Ola Jakup Joensen
Keywords
Quantum sensing, entanglement, squeezed light, quantum noise reduction, frequency-dependent squeezing, atomic spin ensemble, negative mass oscillator, gravitational wave detection, biomedical imaging, quantum communication, quantum networks, hybrid quantum systems
