Oxford Physicists Pull Off the First-Ever 'Quadsqueezing' Quantum Interaction in a Single Trapped Ion, Producing a Fourth-Order Effect 100 Times Faster Than Anything Built Before

Physicists at the University of Oxford have demonstrated a quantum interaction that has eluded experimentalists for two decades, using a single laser-trapped ion to produce a fourth-order quantum 'squeezing' effect known as quadsqueezing — a long-theorized phenomenon that until now existed only in the chalkboard mathematics of nonlinear optics. The team reported the result on May 1 in Nature Physics.

Quantum squeezing is one of the most useful tricks in the modern physics toolkit. By trading certainty in one variable of a quantum system — say the position of a particle — for greater certainty in its conjugate partner, such as momentum, scientists can build sensors that beat the standard quantum limit and detectors that, in the case of the LIGO gravitational-wave observatories, can hear two black holes colliding hundreds of millions of light years away. Standard squeezing is a second-order effect. A third-order variant called trisqueezing was demonstrated in solid-state systems in the past few years. But the fourth-order interaction — generating quadsqueezing on demand and switching between orders in a single experiment — had never been observed in a single, controllable platform.

The Oxford team, led by experimental physicists at the Department of Physics' ion-trap quantum computing laboratory, achieved the feat by combining two non-commuting forces acting on a single calcium ion held in an electromagnetic Paul trap and cooled to near absolute zero. When the forces are applied in sequence rather than simultaneously, the underlying Lie algebra of the trapped ion's motion produces emergent nonlinear terms that the researchers can dial in or out at will. The trick allowed them to generate standard squeezing, trisqueezing and quadsqueezing using the same physical apparatus and to switch between them in microseconds — and to do so with interactions more than 100 times faster than conventional approaches that rely on weak optical nonlinearities.

The implications cut across quantum technology. In quantum metrology, where squeezed states have become a standard tool, higher-order squeezing can in principle suppress noise in regimes inaccessible to conventional squeezing — opening the door to gravimeters, magnetometers and atomic clocks with sharper resolution. In quantum simulation, the ability to engineer arbitrary nonlinear bosonic interactions makes it possible to model exotic quantum field theories that have so far existed only in equations. And in quantum computing, controllable nonlinear interactions of this kind are the missing ingredient for bosonic quantum error correction and for so-called 'cat-state' encoding schemes that may prove more fault-tolerant than today's qubit-based architectures.

The Oxford result is the latest in a wave of UK-led milestones in trapped-ion quantum computing. Just last month an Oxford team demonstrated record-low two-qubit gate errors of one in 6.7 million operations, and a separate Imperial College London group reported the first long-baseline quantum sensing demonstration across the Cambridge–London corridor. Industry partners including Quantinuum, IonQ and Oxford Ionics have already shown interest in the quadsqueezing protocol because it does not require any new hardware — only a different sequence of laser pulses applied to existing trapped-ion processors. Lead author David Lucas, a longtime ion-trap pioneer, told reporters at a briefing that the team is now working to extend the technique to entangled multi-ion systems, which would open the path to fully programmable quantum simulators of nonlinear field theories.