A simple mirror trick can help scientists get rid of quantum noise

Ask quantum physicists what keeps them up at night, and chances are they’ll say it’s noise. Not the kind you hear, but the type that creeps into quantum experiments, messes up results, and places limits on how precisely one can measure a quantum system.

Quantum noise refers to the disturbances in a quantum system that pop up when scientists attempt to measure subatomic particles. However, this measurement brings changes in the state of particles and alters the system.

The noise can occur even when scientists try to just observe subatomic particles using light, but then light (photons) itself ends up agitating the particles. “This happens because photons, particles of light, used for measurement ‘kick’ the tiny particles they hit, an effect known as ‘backaction’,” researchers from Swansea University in Wales note. Backaction has been a major obstacle to building useful quantum computers and sensors.  

However, the Swansea University team has figured out an interesting way to avoid this trap. In their new study, researchers propose using a curved mirror and some well-placed light. With this, they were able to create a setup where backaction and the resulting noise completely disappear.

A simple trick to cancel backaction

The current study is based on the principle of levitated optomechanics. This area of physics studies how tiny particles (like nanoparticles) can be trapped and controlled using laser light, often by levitating them in a vacuum using an optical trap. These levitated particles can act like highly sensitive detectors for tiny forces if one measures them accurately. However, quantum backaction makes this measurement extremely difficult.

The researchers tackled this problem using a novel idea. They placed the particle in front of a hemispherical mirror and trapped it with laser light in a standing wave, a stable pattern of light that acts like an optical cage. The mirror reflects the laser light in a way that cancels out some of the random fluctuations in the light’s momentum, which is the very source of quantum backaction. 

“We found that under specific conditions, the particle becomes identical to its mirror image. When this happens, you can’t extract position information from the scattered light, and at the same time, the quantum backaction vanishes,” said Rafal Gajewski, lead study author and a PhD student at Swansea, in a statement released by the university.

They didn’t stop at just setting up the mirror and trap. To show how much the noise could be reduced, the research team used a mathematical tool called Fisher Information Flow, which measures how much information a detector can extract without disturbing the system. The results showed that their configuration suppresses backaction beyond what conventional setups can do. 

“We compute the corresponding measurement imprecision using the Fisher information flow. Our results show that the standing-wave trapping field is necessary for backaction suppression in three dimensions, and they satisfy the Heisenberg limit of detection,” explained the study authors.

Why reduce noise if you can’t make measurements?

Gajewski observed that in “conditions where measurement becomes impossible, the disturbance (noise) disappears too.” However, the goal of the Swansea team wasn’t to stop measurements altogether.

Instead, the study sheds light on the link between measurement and noise. It’s not about avoiding measurements forever, but about learning how to design systems that reduce disturbance when you do measure.

Insights from the study could allow physicists to build ultra-sensitive quantum sensors that work closer to the limits set by nature. They can also apply symmetry tricks (like the particle-mirror match) to create low-noise zones in quantum devices. 

There could be many other implications of the current research. “This work reveals something fundamental about the relationship between information and disturbance in quantum mechanics,” said James Bateman, supervisor of the study authors and a lecturer at Swansea. “This opens up new possibilities for quantum experiments and potentially more sensitive measurements,” he added.

The study has been published in the journal Physical Review Research.

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