Researchers at the University of Oxford have demonstrated a new kind of quantum interaction using a single trapped ion. By carefully generating and controlling increasingly complex forms of “squeezing” — including a fourth-order effect called quadsqueezing — they have made quantum behaviors accessible that had previously been out of reach. The work also introduces a new way to engineer these interactions, with potential uses in quantum simulation, sensing, and computing. The findings were published today (May 1) in Nature Physics.
Many physical systems behave like tiny oscillating objects, similar to springs or pendulums. In quantum physics, these are known as quantum harmonic oscillators. This description applies to a wide range of systems, including light waves, molecular vibrations, and even the motion of a single trapped atom.
Controlling these oscillations is essential for modern quantum technologies. Applications range from extremely precise measurement tools to the development of next-generation quantum computers.
Squeezing and the Limits of Quantum Precision
One of the most common techniques for controlling quantum oscillators is called squeezing. Quantum mechanics places strict limits on how precisely certain pairs of properties, such as position and momentum, can be measured at the same time. Squeezing redistributes this uncertainty by making one property more precise while increasing uncertainty in the other.
This concept is not just theoretical. Squeezed light is already used in gravitational-wave detectors such as LIGO to enhance sensitivity.
Going Beyond Standard Squeezing
Standard squeezing is only one part of a broader set of possible interactions. Physicists have long aimed to create more complex versions, known as trisqueezing and quadsqueezing. These higher-order effects are much harder to achieve because they are naturally very weak and quickly become overwhelmed by noise.
As a result, observing these advanced quantum interactions has remained a major challenge.
A New Method Using Non-Commuting Forces
The Oxford team developed a solution by combining two precisely controlled forces acting on a single trapped ion. This approach builds on a theory proposed in 2021 by Dr. Raghavendra Srinivas and Robert Tyler Sutherland.
Each force on its own produces a simple, predictable effect. When applied together, however, they generate a stronger and more complex interaction. This happens because of non-commutativity, a quantum effect in which the order and combination of actions change the outcome, allowing the forces to amplify each other.
Lead author, Dr. Oana Băzăvan, Department of Physics, 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.”
First-Ever Demonstration of Quadsqueezing
Using the same experimental setup, the researchers were able to switch between different levels of squeezing. They successfully produced standard squeezing, trisqueezing, and, for the first time on any platform, quadsqueezing, a fourth-order interaction.
By adjusting the frequencies, phases, and strengths of the applied forces, they could control which interaction appeared while minimizing unwanted effects.
Dr. Oana 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.”
Confirming the Quantum Effects
To verify their results, the team reconstructed the quantum motion of the trapped ion. The measurements revealed distinct patterns corresponding to second-, third-, and fourth-order squeezing. These patterns provided clear evidence that each type of interaction had been successfully created.
Future Applications in Quantum Technology
The researchers are now extending this method to more complex systems with multiple modes of motion. Because the technique relies on tools already available in many quantum platforms, it could become a widely useful way to explore advanced quantum behavior.
The approach 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 (Department of Physics, University of Oxford), who supervised the work, 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.”