The concept of measurement is integral to how we understand and navigate the world. Every day, we use our senses, such as sight and hearing, to gather data about our surroundings, constantly inferring the state of the world around us. But what if measuring nothing at all could provide even more valuable information? Surprisingly, it turns out that the absence of something — specifically, the absence of scattered light — can have a significant impact on cooling the vibrations of an object. This discovery challenges conventional wisdom and opens new doors in the world of quantum physics.
A group of researchers from Imperial College London, the University of Oxford, the University of Waterloo, the University of Leeds, and the University of Copenhagen have collaborated on an experiment that demonstrates how measuring the absence of scattered light can cool the motion of a tiny object — a glass bead — to temperatures below room temperature. This finding not only has implications for cooling technologies but also provides insight into the complex relationships between light and sound waves in the quantum realm.
The results of this groundbreaking study were published in two companion papers in the journals Physical Review Letters and Physical Review A, where the researchers also presented a mathematical model that describes the effect they discovered. The experiment’s results are a breakthrough in the realm of quantum measurement and contribute significantly to our understanding of the intricate dynamics between light and sound.
The Glass Bead and Whispering-Gallery-Mode Resonators
To carry out this experiment, the team used a glass microsphere — a tiny object that is only about four times wider than a human hair. This bead was placed in a special experimental setup where it could trap both light and high-frequency sound waves. The unique properties of the glass bead allowed the light and sound waves to be continually reflected around its circumference, much like the phenomenon observed in the Whispering Gallery at St. Paul’s Cathedral in London, where sound waves can travel along the curved walls of the dome.
This setup, called a whispering-gallery-mode resonator, traps light and sound long enough for them to interact and correlate with each other. The sound waves and light waves inside the glass bead become intertwined, meaning that by measuring the light leaving the bead, the researchers could gain insights into the sound wave’s behavior. This interaction forms the basis of the experiment: the relationship between the two types of waves is measured in a novel way, offering new potential for controlling quantum systems.
Zero-Photon Detection: Cooling by Not Observing
At the heart of the team’s work lies the idea of zero-photon detection. In traditional experiments involving lasers and light, the detection of photons — particles of light — plays a key role in determining the properties of the system being studied. However, in this case, the researchers used single-photon detectors to measure whether any photons had been scattered by the sound waves at any given moment.
In some instances, the detectors registered the absence of any photons — a moment when no light had been scattered. By considering only these moments of no photon detection, the researchers were able to observe that the sound waves inside the glass bead were quieter than usual, as though their vibrations had been cooled. Conversely, when a single photon was detected, the sound waves were found to be louder.
This observation, though seemingly counterintuitive, is based on the correlation between light and sound within the system. The idea that measuring nothing — that is, detecting the absence of scattered photons — can cool the vibrations of the sound waves is a surprising yet insightful result. In fact, the cooling effect occurs because the absence of photons conveys useful information about the state of the sound wave, effectively reducing its energy.
As Evan Cryer-Jenkins, co-first author of the study from the Quantum Measurement Lab at Imperial College London, explains, “This result was certainly surprising at first. However, it makes sense, as the light and sound are correlated in our experiment, so the information gained from the measurement enables the state of the sound wave to be further cooled.”
Co-first author Jack Clarke, also from the Quantum Measurement Lab, adds, “While it seems counterintuitive at first, updating our knowledge about the world after noticing something isn’t there is actually something we do every day. Whether it’s checking for rain or realizing you’ve misplaced your keys, noticing absence is often as telling as presence.”
Reaching New Limits in Cooling Quantum Systems
This experiment is not only a novel exploration of quantum measurement but also a significant step forward in the field of quantum cooling. Laser cooling has long been a powerful technique used to cool atoms and ions to incredibly low temperatures. The principle behind laser cooling involves using laser light to slow down the motion of particles, thus cooling them. However, this experiment pushes the boundaries of traditional laser cooling by incorporating quantum measurements into the cooling process, providing a new method of cooling systems beyond what was previously possible.
By coupling quantum measurement with the measurement of the absence of photons, the researchers demonstrated a technique that can help cool quantum systems, such as trapped atoms, ions, and even quantum computers. This approach could play a crucial role in developing more advanced quantum networks and potentially in building more efficient quantum computers.
Arjun Gupta, another co-first author from the Quantum Measurement Lab, emphasizes, “Quantum measurement is a fascinating subject and I’m sure there are further discoveries to be made ahead.”
Implications for Quantum Technology and Fundamental Physics
The implications of this work go beyond cooling techniques and reach into the future of quantum technology. As Kyle Major, co-first author of the study, points out, “Using zero-photon detection to help cool quantum systems into their ground state will help with the development of quantum computers and quantum networks, as well as testing the fundamental laws of physics.”
The results of this study offer a new approach to managing quantum systems, providing insights that could lead to better control over systems that rely on superposition and entanglement — key phenomena in quantum mechanics. The ability to cool these systems more effectively is a critical step toward realizing the potential of quantum computing and quantum communication.
Principal Investigator Michael R. Vanner, who leads the Quantum Measurement Lab at Imperial College London, sums up the importance of the research, saying, “These results are an exciting milestone for our team and provide a powerful new technique to control quantum systems. We’re thrilled to see how zero-photon detection will help the work of our lab and the wider scientific community.”
A New Frontier in Quantum Physics
The discovery of cooling quantum systems by measuring the absence of photons opens up new possibilities for controlling the behavior of quantum systems. This research represents a significant advancement in quantum measurement, where the idea of measuring “nothing” proves to be just as important as detecting “something.”
As the team moves forward, they hope to explore how zero-photon detection can be applied to other areas of quantum science, potentially leading to new technologies that harness the unique properties of quantum mechanics. For now, this research has added a new dimension to the world of quantum measurement and cooling, and the implications for both quantum computing and our fundamental understanding of physics are vast.
This study demonstrates that sometimes the absence of something — in this case, photons — can provide more information than the presence of something. This discovery not only challenges our understanding of how measurements work in quantum systems but also holds promise for advancing future technologies that could shape the next generation of quantum innovations.
References: Evan A. Cryer-Jenkins et al, Enhanced Laser Cooling of a Mechanical Resonator via Zero-Photon Detection, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.073601
Jack Clarke et al, Theoretical framework for enhancing or enabling cooling of a mechanical resonator via the anti-Stokes or Stokes interaction and zero-photon detection, Physical Review A (2025). DOI: 10.1103/PhysRevA.111.023516
Evan A. Cryer-Jenkins et al, Something from Nothing: Enhanced Laser Cooling of a Mechanical Resonator via Zero-Photon Detection, arXiv (2024). DOI: 10.48550/arxiv.2408.01734
Jack Clarke et al, Something from Nothing: A Theoretical Framework for Enhancing or Enabling Cooling of a Mechanical Resonator via the anti-Stokes or Stokes Interaction and Zero-Photon Detection, arXiv (2024). DOI: 10.48550/arxiv.2408.01735