Physicists at the University of Oxford have demonstrated a fundamentally new family of quantum cat states — quantum superpositions whose individual components are not the smooth, near-classical wave packets physicists have used for decades, but deeply exotic quantum states in their own right. The experiment, published June 3 in Physical Review X, used a single strontium-88 ion held in a Paul trap to produce squeezed, trisqueezed, and quadsqueezed motional superpositions whose measured quantum character exceeded that of conventional cat states at the same energy — exactly the class of states that squeezed-cat bosonic error correction theory predicts should correct both dephasing and photon-loss errors simultaneously, a capability standard cat states lack.

The result has drawn attention from ScienceDaily, Gizmodo, and other outlets this week, recognizing a paper that may redirect how engineers design error-correction codes for future quantum computers.

What a Quantum Cat State Actually Is

Quantum mechanics allows objects to exist in multiple states simultaneously — a phenomenon with no equivalent in everyday physics. In the laboratory, physicists create real analogues of this idea using atoms, light, and the controlled motion of trapped particles. The most familiar version places a quantum oscillator in a superposition of two wave packets called coherent states: wave packets that behave as classically as quantum mechanics allows, displaced in opposite directions in phase space. That two-component superposition is the standard Schrödinger's cat state, and it has been the workbench for decades of quantum optics and quantum computing research.

What the Oxford team changed is what goes inside. Rather than assembling the superposition from coherent-state wave packets — familiar building blocks — the researchers built it from components that are themselves already strongly nonclassical: squeezed states, in which quantum uncertainty is redistributed unevenly across conjugate observables rather than shared equally, and trisqueezed and quadsqueezed states, in which that redistribution takes on three- and fourfold rotational symmetries in phase space. A superposition of two trisqueezed states, for example, displays the sixfold rotational symmetry visible in the reconstructed Wigner function the team published — a geometric interference pattern with no counterpart in conventional cat state physics. Notably, the quadsqueezed interaction used in this work marked the first realization of a fourth-order quadsqueezing interaction on any platform when it was first demonstrated by the same Oxford group.

The Mechanism: Sculpting Superpositions in a Strontium Ion

The experiment exploited a specific feature of trapped-ion systems: a single ion gives experimenters two quantum systems simultaneously. The strontium-88 ion's internal electronic state behaves as a qubit — the quantum equivalent of a binary bit. Its axial motion inside the Paul trap behaves as a quantum harmonic oscillator, capable of occupying many different energy levels rather than just the two a qubit offers. That richer landscape is what makes the motional degree of freedom a powerful medium for engineering complex quantum states.

To generate the new superpositions, the Oxford team first applied engineered, spin-dependent interactions that entangled the ion's internal qubit state with several distinct possible motional states. They then performed a mid-circuit quantum measurement of the internal state. When the measurement yielded the target outcome, the ion's motion was projected — heralded — into the desired superposition of nonclassical components, with the qubit and oscillator disentangled. Because the underlying interactions are unitary, the procedure can be applied repeatedly within a single experimental sequence without destroying intermediate states, giving the researchers programmatic control over the final shape of the superposition.

"This approach gave us a tool to sculpt quantum superpositions into almost any shape," said Dr. Sebastian Saner, lead author and researcher at Oxford's Department of Physics. "The states we produced exhibit rotational symmetries and form striking geometric interference patterns."

By adjusting the experimental parameters, the team could change the relative size, phase, and orientation of the superposition's components, or switch between entirely different types of nonclassical component in a single run — combining, for instance, a squeezed state with a trisqueezed state in the same superposition. The oscillator's axial motional frequency was 1.2 MHz throughout.

Why Squeezed-Cat States Beat Standard Cat States at Error Correction

To confirm that the states were genuinely nonclassical, the Oxford team used quantum state tomography — a process that reconstructs the full quantum state from a series of measurements — and looked for Wigner negativity. The Wigner function is a mathematical representation of a quantum state in phase space; its negative regions can arise only in genuine quantum superpositions and cannot be reproduced by any classical description. In particular, negative Wigner function values are a necessary condition for quantum computational advantage in continuous-variable systems.

The new superpositions, particularly the "odd" variants, displayed large Wigner negativity — and critically, outperformed both conventional Fock states and standard coherent-state cat states when compared at the same average energy. That last comparison matters enormously for practical quantum computing: it means that for a given physical resource budget, the new states carry more quantum character than their predecessors.

The relevance to error correction runs deeper than improved quantum character alone. A 2022 theoretical framework by Schlegel, Minganti, and Savona showed that cat states built from squeezed components — squeezed-cat bosonic codes — can correct for both single-photon loss and dephasing errors simultaneously. Standard cat codes protect against only one of these error channels; to correct the other, they must be concatenated with a second code, adding hardware overhead. The squeezed-cat code sidesteps that concatenation requirement in the large-squeezing limit. The Oxford experiment provides the first fine-grained experimental demonstration of programmable access to exactly this class of states.

"We believe we're still scratching the surface of what's possible, both for practical applications and for understanding these states at a more fundamental level," said supervisor Dr. Raghavendra Srinivas, also at Oxford's Department of Physics, who was awarded the Optica Hänsch Prize in Quantum Optics in 2024 for work in this area. The team is now working with theorists to quantify more precisely just how quantum the new states are — a collaboration that will help establish whether they clear the resource thresholds required for practical bosonic codes.

Continuous-Variable Computing and What Changes Now

The Oxford result fits inside a broader shift in quantum hardware strategy. Most current quantum computers encode information in two-level qubits — devices that can be 0, 1, or a superposition of both. Continuous-variable quantum systems take a different approach: they encode information in quantum harmonic oscillators, which can occupy many energy levels and support far more complex quantum states in a single physical degree of freedom.

The appeal of continuous-variable systems is hardware efficiency: a single bosonic oscillator mode can, in principle, encode a logical qubit with built-in error protection, rather than requiring many physical qubits to implement one logical qubit. Amazon's Ocelot quantum chip, announced in February 2025, is one example of this approach, using cat qubits for bosonic quantum error correction. A separate April 2026 paper introduced the "walking-cat" trapped-ion architecture, which incorporates bosonic error correction with low-density parity-check codes and projects the ability to execute roughly one million T gates per day using 2,514 physical qubits encoding 110 logical qubits.

What the Oxford result adds to this landscape is programmable access to a richer set of quantum states in the motional degree of freedom of a trapped ion — including precisely the squeezed-component superpositions that squeezed-cat code theory identified as candidates for superior error protection.

Part of a Broader 2026 Superposition Push

The Oxford work lands during a surge of activity on multiple frontiers of macroscopic quantum superposition. In January 2026, researchers publishing in Nature demonstrated quantum interference in sodium nanoparticles containing more than 7,000 atoms — particles with masses exceeding 170,000 atomic mass units, a scale record for matter-wave superposition. In May 2026, a separate team publishing in Nature Physics used ultracold atoms in optical lattices to demonstrate massive Schrödinger's cat states through quantum tunneling, with composite objects of up to 608 atomic mass units behaving as a single quantum entity — a result aimed at probing the relationship between quantum mechanics and gravity.

The Oxford approach differs from both: rather than scaling superpositions up in physical size or mass, it deepens their quantum character from the inside — replacing familiar, near-classical components with intrinsically nonclassical ones and demonstrating programmable control over the resulting state. Whether larger-scale and more intrinsically nonclassical superpositions eventually converge into a single platform remains one of the most open questions in modern quantum physics.

Frequently Asked Questions

What is a quantum cat state, and how does the Oxford experiment change the standard recipe?

A quantum cat state is a quantum superposition of two or more macroscopically distinct states — a laboratory realization of Schrödinger's famous thought experiment. The standard recipe uses coherent states, wave packets that behave as classically as quantum mechanics allows, as the superposition's components. The Oxford experiment replaces those with squeezed, trisqueezed, and quadsqueezed states — motional quantum states in which uncertainty is redistributed in geometrically complex ways rather than shared equally — producing a new family of cat-like superpositions whose measured quantum character exceeds the standard version at the same energy.

What is Wigner negativity and why does it matter for quantum computing?

The Wigner function is a mathematical representation of a quantum state in phase space. For classical states, it is always non-negative, like an ordinary probability distribution. For quantum states whose uncertainty cannot be described classically, it develops negative regions — Wigner negativity. Those negative values are a necessary condition for quantum computational advantage in continuous-variable systems, and they served as the benchmark Oxford used to confirm and compare the new states. The new superpositions' "odd" variants displayed Wigner negativity larger than conventional cat states at equal energy, meaning they are more useful quantum resources for the same physical cost.

How do the new states connect to quantum error correction for quantum computers?

Bosonic quantum error correction encodes quantum information in harmonic oscillator states rather than two-level qubits, using the oscillator's larger Hilbert space to build in redundancy. A 2022 theoretical framework showed that cat states built from squeezed components — squeezed-cat bosonic codes — can correct both photon-loss and dephasing errors simultaneously, where standard cat codes handle only one. The Oxford experiment is the first to demonstrate programmable experimental generation of precisely this class of states, supplying the laboratory platform that squeezed-cat code implementations will need.

What is the difference between a trapped-ion quantum computer and a superconducting quantum computer?

Both are leading hardware platforms for quantum computing. Superconducting systems use circuits cooled near absolute zero, in which microwave photons serve as the quantum information carrier. Trapped-ion systems use individual charged atoms — typically strontium or ytterbium — held in electromagnetic traps and manipulated by lasers. Trapped-ion systems generally achieve higher gate fidelity and longer coherence times per qubit, while superconducting systems currently achieve faster gate speeds and larger qubit counts. For continuous-variable applications like the Oxford cat-state work, the motional degree of freedom of the trapped ion provides a natural quantum harmonic oscillator that superconducting architectures must replicate with a separate resonator element.

The paper, "Generating Arbitrary Superpositions of Nonclassical Quantum Harmonic Oscillator States," by S. Saner, O. Băzăvan, D.J. Webb, G. Araneda, D.M. Lucas, C.J. Ballance, and R. Srinivas, was published in Physical Review X, Volume 16, Article 021049 (2026). DOI: 10.1103/k1xk-yt42