Reducing noise below the sound of silence

Researchers from Australia, Singapore and China have developed a technology that manipulates quantum states of light at noise levels quieter than the sound of silence.

The breakthrough, reported in the journal Nature Photonics, is, they say, an important next step in the development of quantum computers.

Quantum mechanics predicts the presence of “vacuum noise” – a surprising phenomenon where silence does not equate to the complete absence of noise.

The new device, developed by a team drawn from the Australian National University (ANU), Nanyang Technological University, National University of Singapore (NUS) and Shanxi University, allows information to be encoded and manipulated within a quantum state to a resolution that is typically drowned out by vacuum noise.

“Even in an empty, dark room, there is still noise in the form of energy that permeates all space,” says ANU’s Ping Koy Lam.

“While imperceptible at everyday scales, the fluctuation of this energy can cause problems, by distorting the signal or information encoded in situations where extreme precision is required.”

One way to reduce the effect of this vacuum noise is to use a light source known as “squeezed light”, which is quieter than emptiness, Lam says.

“Our team has been working on squeezed light for more than two decades. We have used this technique to increase sensitivity of optical measurements, such as in kilometre-long optical interferometers for detecting ripples in space and time known as gravitational waves.

“Our focus in recent years has been to use this technique for information processing in quantum computing and encryption.”

Jayne Thompson from NUS says the level of fine control the team has achieved has many technological benefits.

“From certain viewpoints, such states also appear to have temperatures below that of absolute zero and, as such, they can also act like a power resource for information processing at the quantum scale,” she says.

The experiment does not guarantee success every time, cautions ANU’s Sophie Zhao, the paper’s lead author.  But when it works, “it can efficiently squeeze many quantum states to a level that was once thought to be unachievable”.

ANU’s Syed Assad notes that conventional approaches for an in-line squeezing gate always require a lot of non-classical resources, and the heavy reliance on resources ultimately limits their performance.