Researchers from the University of Oxford have demonstrated a new family of quantum superpositions – Schrödinger’s cat- like quantum states – from highly nonclassical building blocks. The work opens a new route towards quantum computing with non-binary systems, sensing and understanding quantum physics at a more fundamental level.
Quantum mechanics, unlike classical physics, allows objects to exist in more than one state at the same time. This idea is often illustrated by Schrödinger’s cat, imagined as being both alive and dead until it is observed. In the laboratory, physicists can create less dramatic but very real versions of this effect by placing atoms, light, or motion into two distinct quantum states at once. Creating and controlling these superpositions is essential for applications ranging from quantum computing to precision timekeeping.
A simple example is a quantum bit, or qubit, in a superposition of both 0 and 1. But quantum systems are not limited to just two states. In a quantum harmonic oscillator, which can occupy many different energy levels, there is a much richer set of possibilities. Quantum harmonic oscillators describe many physical systems, including light, vibrations and the motion of trapped particles, and have been used to create a wide variety of quantum superpositions. One well-known example is a “cat state”, in which an oscillator is placed in a superposition of two wave packets displaced in opposite directions. These wave packets, known as coherent states, resemble classical motion as closely as quantum mechanics allows.
Researchers at the University of Oxford have now demonstrated a new family of quantum superpositions . Instead of building cat-like states from coherent-state wave packets, they developed a method for creating superpositions from a broad range of components that are themselves highly nonclassical. In examples such as squeezed-state superpositions, quantum uncertainty is redistributed differently in each part of the state.
The experiment used the motion of a single trapped ion. A trapped ion combines two different kinds of quantum system: its internal state acts like a qubit, while its motion behaves like a quantum harmonic oscillator capable of occupying many different motional states. This makes it a powerful platform for engineering quantum states that go beyond ordinary qubits.
To create these states, the team first used engineered interactions to entangle the ion’s internal state with different possible states of motion. A mid-circuit quantum measurement of the internal state then projected the ion’s motion into the chosen superposition of nonclassical components.
“This approach gave us a tool to sculpt the quantum superposition into almost any shape,” explains lead author Dr Sebastian Saner (Department of Physics, University of Oxford).
The method gave the researchers programmable control over the states they produced. By changing the experimental settings, they could tune the relative size, rotation and separation of the components, allowing a wide range of exotic motional superpositions to be generated within the same trapped-ion system.
The team directly reconstructed the quantum states they created. The reconstructions revealed interference patterns and regions of Wigner negativity - signatures that the states could not be described as ordinary classical mixtures. These features confirmed that the experiment had produced true quantum superpositions of genuinely nonclassical motional states.
The researchers are now collaborating with theorists to determine more precisely how “quantum” these states are.
“We were really encouraged by our colleagues’ reaction when we showed them what we had made. 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,” says Dr Raghavendra Srinivas (Department of Physics, University of Oxford), who supervised the work.
This work opens a route towards quantum technologies that use quantum oscillators rather than only simple quantum bits. One promising direction is in quantum computing; such states can be more resilient to errors while enabling simpler and more robust error correction protocols. These systems also provide a new platform for exploring the boundary between classical and quantum behaviour.
Notes for editors:
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The paper ‘Generating arbitrary superpositions of nonclassical quantum harmonic oscillator states ’ has been published in Physical Review X : https://journals.aps.org/prx/abstract/10.1103/k1xk-yt42
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Physical Review X
Generating Arbitrary Superpositions of Nonclassical Quantum Harmonic Oscillator States