Physicists Built Quantum States So Strange They Only Existed In Theory, Until Now

For decades, the idea of a cat that is simultaneously alive and dead has been the go-to way to explain one of quantum physics’ strangest features: that tiny particles can exist in multiple states at the same time. That concept, called superposition, is the engine behind quantum computers, ultra-precise clocks, and precision sensors. Now, physicists at the University of Oxford have pushed one corner of that idea into new experimental territory, engineering exotic quantum states that had never been experimentally created, states so unusual they were previously explored only on paper.

The team used a single electrically trapped ion, an atom with a missing electron, to build some of the most intricate quantum superpositions ever assembled. By firing precisely controlled laser pulses at the ion and measuring its internal state mid-process, they mixed and matched the building blocks of quantum states with a level of control that let them tune nearly every ingredient of the final result. The work, published in Physical Review X, points toward potential advances in quantum computing, sensing, and error correction.

Most quantum superposition experiments work with systems that have just two possible states to exploit. A quantum harmonic oscillator, the quantum version of a vibrating spring, has an infinite number, meaning the information it can carry is vastly greater. Previous experiments were limited to relatively basic ingredients. The Oxford team went well past those boundaries.

Trisqueezed and Quadsqueezed States Created for the First Time

In quantum physics, a squeezed state is one where the fuzziness inherent to all quantum objects has been pushed more into one direction than another, like squeezing a balloon so it bulges out on the sides. This makes squeezed states useful for precision measurement, because uncertainty can be reduced right where it matters most.

The Oxford researchers also generated what the paper calls trisqueezed and quadsqueezed states, higher-order versions that produce more elaborate, rotationally symmetric shapes and carry stronger quantum properties. These types of states had been predicted in theory but never produced in a real experiment until now. Then the team built superpositions of these exotic states, quantum combinations where the oscillator exists in multiple configurations simultaneously.

The experiment used a single strontium ion held in place by electric fields inside a device called a Paul trap. That ion has two useful properties: its internal electronic state, which acts like a tiny compass needle that can point up, down, or both at once, and its back-and-forth vibration inside the trap, which acts as the quantum harmonic oscillator.

By applying carefully designed laser pulses, the researchers linked the ion’s internal state to its motion. Then came a key step: a measurement of the internal state taken in the middle of the experiment. This midcircuit measurement broke that link in a controlled way, leaving the vibrational state in the desired quantum superposition. Because the measurement was designed so that no light scattered when the desired outcome occurred, the fragile vibrational state was left undisturbed. Repeating this procedure let the team stack layers of complexity onto the quantum state.

Wigner Negativity Confirms These Quantum States Are Real

To verify that the states were genuinely quantum, the researchers reconstructed a Wigner function, a kind of quantum map describing the state in terms of position-like and momentum-like properties. In an ordinary probability map, values cannot go below zero. A Wigner function can dip into negative values, a telltale sign that the state cannot be described classically. All of the newly created states showed those negative regions.

In quantum computing, that negativity matters. States that show it are harder for ordinary computers to mimic, which is part of what makes them useful for achieving a quantum advantage. In the paper’s comparison, ideal versions of these superpositions scored higher on that measure than simpler quantum states at the same energy level, though the paper notes the relationship between that score and real-world computational difficulty is still being worked out.

Squeezed Superposition States Could Improve Quantum Error Correction

The rotational symmetries of the new states make them attractive for quantum error correction. In a standard two-component cat-state setup, a single lost vibration quantum produces an unrecoverable bit-flip error. In an encoding based on squeezed superpositions, that same loss event kicks the system outside the protected space in a detectable way, making the error potentially correctable. These states are also suited for detecting tiny displacements and, in the case of trapped ions, extremely small electric fields.

Because the techniques rely only on coupling between a vibrating system and a two-level quantum system, the approach isn’t limited to trapped ions. The paper identifies superconducting circuits, nanoparticles, atoms coupled to optical cavities, and atoms in optical tweezers as other systems where the same methods could apply. For heavier oscillators, the authors suggest these superpositions could even probe how quantum mechanics interacts with gravity.

The gap between what quantum theory says is possible and what physicists can actually build in the lab just got a little smaller.

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