Oxford Physicists Achieve First-Ever 'Quadsqueezing' Quantum Effect, Opening New Path for Quantum Sensors and Computers

Physicists at the University of Oxford have demonstrated for the first time a fourth-order quantum interaction known as "quadsqueezing," a technical achievement that quantum-information researchers say has the potential to dramatically expand the toolkit available for quantum sensors, simulators, and the next generation of trapped-ion computers. The result, reported Friday in the journal Nature Physics, was generated using a single trapped ion and two precisely controlled forces that are individually weak but together produce an interaction more than 100 times stronger than any previous fourth-order squeezing scheme.

Squeezing is a quantum optical technique in which uncertainty is redistributed between two complementary properties of a particle — for example, position and momentum — so that one is known with extraordinary precision at the cost of greater uncertainty in the other. Standard "two-photon" squeezing has been used for decades to push gravitational-wave detectors like LIGO past the standard quantum limit. Higher-order squeezing — trisqueezing (third-order) and now quadsqueezing (fourth-order) — promises even sharper measurement and richer simulation, but until now the natural strength of fourth-order interactions in any laboratory system has been so weak that observing them was effectively impossible.

The Oxford team, led by lead author Dr. Oana Băzăvan and co-author Dr. Raghavendra Srinivas of the university's Department of Physics, exploited a quirk of quantum mechanics called non-commutativity, in which the order in which two operations are applied changes the outcome. By alternating two non-commuting forces on a single calcium ion confined between electrodes, the researchers generated a fourth-order term that grew exponentially with each cycle. "In the lab, non-commuting interactions are often seen as a nuisance because they introduce unwanted dynamics," Băzăvan said in a statement released by Oxford. "Here, we took the opposite approach — we engineered them to do something useful." The team verified the squeezed states using mid-circuit measurements of the ion's spin and used the same approach to simulate a lattice gauge theory, a model that underpins parts of high-energy physics.

The practical implications could be substantial. Quantum sensors that exploit higher-order squeezed states can in principle achieve precision that scales more favorably with photon number than conventional designs, which would benefit applications from inertial navigation in GPS-denied environments to dark-matter searches. In quantum computing, trisqueezed and quadsqueezed states are a known route to fault-tolerant operations on bosonic qubits — encodings that store quantum information in the vibrational modes of trapped ions or microwave cavities. The Oxford method works with hardware already in use at Oxford, IonQ, Quantinuum, and other industrial trapped-ion labs, raising the prospect that the technique could be ported quickly into commercial systems.

Outside experts said the demonstration is significant. "What's striking is the speed," Klaus Mølmer, a theoretical physicist at the Niels Bohr Institute in Copenhagen who was not involved, told Physics World. "A 100-fold speedup over the natural process turns a curiosity into something you can actually use in a calculation or in a measurement." Băzăvan said her team is now extending the protocol to multiple modes of motion of the same ion, which would unlock simulations of more complex gauge theories and create the building blocks for higher-dimensional bosonic codes. The research was funded by the U.K. Engineering and Physical Sciences Research Council and the U.S. Office of Naval Research.