Quantum physics has a habit of hiding its most interesting effects in the noise. Researchers at the University of Oxford say they have now pulled one of those effects into view, with the first demonstration of quadsqueezing.

The team used a single trapped ion to create and control increasingly complex forms of squeezing, including standard squeezing, trisqueezing and quadsqueezing, a fourth-order interaction. The findings were published on May 1 in Nature Physics.

Many physical systems can be described as quantum harmonic oscillators, including light waves, molecular vibrations and the motion of a single trapped atom. Controlling those oscillations is used in quantum technology, including precision measurement tools and quantum computers.

Squeezing is a common way to control quantum oscillators. It redistributes uncertainty between paired properties, such as position and momentum, making one more precise while increasing uncertainty in the other. Squeezed light is already used in gravitational-wave detectors such as LIGO to improve sensitivity.

Physicists have long sought more complex versions of squeezing, including trisqueezing and quadsqueezing. The University of Oxford said those higher-order effects are much harder to produce because they are naturally very weak and are quickly overwhelmed by noise.

The Oxford team tackled that by combining two precisely controlled forces on a single trapped ion. The method builds on a theory proposed in 2021 by Dr. Raghavendra Srinivas and Robert Tyler Sutherland.

Each force by itself produces a simple effect. Applied together, they create a stronger and more complex interaction through non-commutativity, a quantum effect in which the order and combination of actions changes the outcome.

Lead author Dr. Oana Băzăvan, from the Department of Physics at the University of Oxford, said: “In the lab, non-commuting interactions are often seen as a nuisance because they introduce unwanted dynamics. Here, we took the opposite approach and used that feature to generate stronger quantum interactions.”

Using the same experimental setup, the researchers said they could switch between different levels of squeezing by adjusting the frequencies, phases and strengths of the applied forces while minimising unwanted effects.

Dr. Băzăvan said: “The result is more than the creation of a new quantum state. It is a demonstration of a new method for engineering interactions that were previously out of reach. The fourth-order quadsqueezing interaction was generated more than 100 times faster than expected using conventional approaches. This makes effects that were previously out of reach accessible in practice.”

To check the results, the team reconstructed the quantum motion of the trapped ion. They said the measurements showed distinct patterns for second-, third- and fourth-order squeezing, providing evidence that each interaction had been created.

The researchers are now extending the method to more complex systems with multiple modes of motion. The University of Oxford said the approach relies on tools already available in many quantum platforms, and it has already been combined with mid-circuit measurements of the ion’s spin to generate flexible combinations of squeezed states and to simulate a lattice gauge theory.

Study co-author Dr. Raghavendra Srinivas, from the Department of Physics at the University of Oxford, said: “Fundamentally, we have demonstrated a new type of interaction that lets us explore quantum physics in uncharted territory, and we are genuinely excited for the discoveries to come.”

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