Physicists create tunable system for enhanced quantum sensing

Researchers at the Niels Bohr Institute, University of Copenhagen, have developed a tunable system that paves the way for more accurate sensing in a variety of technologies, including biomedical diagnostics. The result is published in Nature.

The potential range of technologies is large, stretching from the largest to smallest scales, from detecting gravitational waves in space to sensing the tiny fluctuations in our own bodies.

Optical sensing technologies are already part of everyday life. In recent years, advances in quantum optics have pushed the sensitivity of these devices closer to the so-called standard quantum limit—a practical boundary that arises from the inevitable noise arising from measuring on the smallest scales.

Surpassing this limit requires the use of advanced quantum techniques to cancel or at least reduce the noise. Concepts such as squeezed light, back-action evasion and entanglement are among the tools in the toolchest.

Back-action noise arises when the act of measurement disturbs the system being measured, while detection noise refers to the intrinsic uncertainty in reading out the measurement signal itself.

Entanglements (quantum correlations), which are a key feature that separates classical physics from modern quantum physics, allow for sensing beyond the standard quantum limit.

Entanglement has historically been first observed in microscopic systems, such as individual atoms and photons. The new system developed at NBI uses a large-scale entanglement, which—for the first time—involves a multi-photon light state and a large atomic spin ensemble.

This is an unusual combination of techniques that enables frequency-dependent squeezing, a technique that dynamically reduces quantum noise across a wide frequency band. This is crucial for applications such as gravitational wave detection and for many other sensing technologies.

Suppression of quantum noise with squeezed light and a ‘negative mass’ spin system

Squeezed light is characterized by quantum noise reduced or “squeezed” beyond the standard quantum limit. Usually either the noise of the amplitude or of the phase of a light wave can be reduced by squeezing. However, reduction of quantum noise across a broad frequency range requires squeezing to vary from squeezed amplitude noise to squeezed phase noise at different frequencies.

This variation is achieved by sending squeezed light through an atomic spin ensemble, which rotates the phase of squeezed light depending on its frequency.

Another critical feature of the spin ensemble is its ability to invert the sign of noise from positive to negative. This feature leads to quantum noise suppression when the signal from a sensor is combined with the signal from the spin ensemble.

Thus, frequency-dependent squeezing and the negative mass spin system enable simultaneous suppression of back-action noise and detection noise of the sensor in a broad frequency range. Professor Eugene Polzik, at the Niels Bohr Institute, explains, “The sensor and the spin system interact with two entangled beams of light. After the interaction, the two beams are detected and the detected signals are combined. The result is broadband signal detection beyond the standard quantum limit of sensitivity.”

Size matters—and compactness is a key advantage in practical applicability

Conventional approaches to frequency-dependent squeezing and broadband quantum noise reduction often require large, complex optical setups. For example, systems used in gravitational wave detectors (GWDs), such as LIGO in the US and VIRGO in Italy, employ 300-meter-long optical resonators to achieve frequency-dependent squeezing of quantum noise.

Future GWDs, such as the Einstein Telescope planned for construction in Europe, will require kilometer-long resonators for the same purpose. The new method paves the way towards similar performance using a tabletop device.

Applicability of this system shows great potential

This hybrid quantum network realized by the NBI researchers has broad potential in sensing applications. It could be used in advanced sensors for detecting tiny changes in magnetic fields, time, or acceleration.

In biomedical applications, such sensors could enhance the resolution of magnetic resonance imaging (MRI), enable earlier detection of neurological disorders, or improve the sensitivity of biosensors used in diagnostics and monitoring.

As an example of broader applicability, the researchers also analyzed how their system could enhance the sensitivity of gravitational wave detectors, allowing us to detect faint ripples in spacetime—the signal of violent events in the universe, such as black hole mergers and collisions of neutron stars.

A deeper insight into gravitational waves will also help to understand the processes happening during the formation of the universe.

Beyond sensing, the system’s architecture also opens new possibilities in quantum communication and computation.

It could be adapted for use in quantum repeaters, enhancing the signals for secure long-distance communication and in quantum memories in quantum networks—in other words, the system shows great versatility across multiple domains of quantum technology.