Researchers at the University of Oxford have demonstrated a new class of quantum interaction using a single trapped ion, achieving for the first time a fourth-order effect called quadsqueezing. The results, published May 1 in Nature Physics, open a route to quantum effects that were previously too weak to observe in any experimental platform.

The work builds on a well-established quantum technique called squeezing. In quantum mechanics, the precision with which pairs of properties -- such as position and momentum -- can be simultaneously known is constrained by Heisenberg's uncertainty principle. Squeezing redistributes that uncertainty: one property is measured more sharply while the other grows less defined. Squeezed light is already used operationally to boost sensitivity at gravitational-wave detectors, including LIGO.

Ordinary squeezing, however, is just the lowest rung of a broader family of interactions. Physicists have long sought to generate higher-order versions -- trisqueezing and quadsqueezing -- but these effects are naturally very weak, and their strength falls rapidly with increasing order, making them effectively unobservable before noise overwhelms the signal.

The Oxford team resolved this by combining two carefully controlled forces on a single trapped ion rather than attempting to drive a higher-order interaction directly. The approach follows a theoretical proposal by Dr Raghavendra Srinivas and Robert Tyler Sutherland published in 2021. Each force alone produces a simple linear effect, but applied together they exploit non-commutativity -- the property by which two forces alter each other's action -- to generate a much stronger composite interaction in the ion's motion.

"In the lab, non-commuting interactions are often seen as a nuisance because they introduce unwanted dynamics," said lead author Dr Oana Bazavan of Oxford's Department of Physics. "Here, we took the opposite approach and used that feature to generate stronger quantum interactions."

Using the same experimental setup, the team generated squeezing, trisqueezing, and quadsqueezing by adjusting the frequencies, phases, and strengths of the applied forces. Each configuration selectively produced the target interaction while suppressing unwanted higher- or lower-order terms.

The fourth-order quadsqueezing interaction was generated more than 100 times faster than conventional approaches would predict, Dr Bazavan noted -- a speed advantage that puts effects once considered practically unreachable within experimental grasp.

The team verified each interaction by reconstructing the quantum states of motion of the trapped ion. The measurements revealed the distinctive phase-space shapes associated with second-, third-, and fourth-order squeezing, providing a direct experimental signature of each interaction type.

The method is already being extended to more complex multi-mode systems. In combination with mid-circuit measurements of the ion's spin state, it has been used to generate arbitrary superpositions of squeezed states and to simulate a lattice gauge theory -- a class of model central to high-energy physics and condensed matter research.

Because the technique relies on experimental ingredients available across a range of quantum hardware platforms, the Oxford group argues it could serve as a general-purpose route to new forms of quantum simulation, sensing, and computation.

"Fundamentally, we have demonstrated a new type of interaction that lets us explore quantum physics in uncharted territory," said co-author and supervisor Dr Raghavendra Srinivas of the Department of Physics.

Research Report: Squeezing, trisqueezing and quadsqueezing in a hybrid oscillator-spin system