Researchers at the University of Oxford have created a new type of quantum superposition, a phenomenon often associated with the famous Schrodinger’s cat thought experiment. Unlike previous versions, these newly demonstrated states are constructed from highly non-classical quantum components. The results could help advance quantum computing beyond traditional binary systems, improve sensing technologies and provide new insights into the fundamentals of quantum physics.
One of the most surprising features of quantum mechanics is that objects can exist in multiple states at the same time. This concept is commonly illustrated by Schrödinger’s cat, a hypothetical cat that is considered both alive and dead until observed.
Although this thought experiment is fictional, scientists routinely create real quantum superpositions in the laboratory. Atoms, light, and even motion can be in multiple quantum states at once. The ability to generate and control these states is important for technologies such as quantum computers and ultra-high precision clocks.
A familiar example is a quantum bit (qubit), which can be a combination of both 0 and 1 at the same time. However, quantum systems can operate in more than two states.
Quantum harmonic oscillators, which can occupy many energy levels, offer a much richer set of possibilities. These oscillators describe a wide range of physical systems, including light, vibrations, and the movement of trapped particles. Scientists have used them to create different types of quantum superpositions. One well-known example is the “cat state” where the oscillator exists as a superposition of two wave packets moving in opposite directions. These wave packets, called coherent states, are the closest quantum equivalent of classical motion.
Construct quantum states from non-classical components
An Oxford team has demonstrated a completely new kind of quantum superposition.
Rather than building a cat-like state from a wave packet of coherent states, the researchers developed a technique that combines a wide range of quantum components that are already highly non-classical. For example, in a superposition of squeezed states, the quantum uncertainty is distributed differently in each part of the state.
This experiment relied on the movement of a single trapped ion. Trapped ions combine two different quantum systems into one platform. Its internal states behave like qubits, and its motion acts as a quantum harmonic oscillator that can occupy many different states of motion. This combination makes the trapped ions particularly useful for creating quantum states beyond traditional qubits.
To generate the new state, the researchers first engineered interactions that intertwined the ion’s internal state with various possible motion states. We then performed circuit-intermediate quantum measurements of the internal states, collapsing the ion’s motion to give rise to the desired superposition of the non-classical components.
“This approach gives us the tools to sculpt quantum superpositions into almost any shape,” explains lead author Dr. Sebastian Sanner (Department of Physics, University of Oxford).
Programmable control of exotic quantum states
The new technique gives the team a high degree of control over the quantum states they generate.
By adjusting experimental parameters, we can change the relative size, orientation, and separation of components within the superposition. This flexibility allowed us to create a variety of unusual kinetic quantum states using the same trapped ionic system.
The researchers then directly reconstructed the quantum state. Their measurements revealed interference patterns and areas of Wigner negativity. This is a clear sign that these states cannot be described as normal classical mixtures. These observations confirm that the experiment was successful in producing a genuine quantum superposition consisting of truly non-classical states of motion.
The research team is now working with theorists to better understand how “quantum” these newly created states are.
“We were really encouraged by the reactions of our colleagues when we showed them what we had created. We believe we are still just scratching the surface of the possibilities, both in terms of practical applications and in understanding these states at a more fundamental level,” said Dr Raghavendra Srinivas (Department of Physics, University of Oxford), who supervised the research.
Potential impact on quantum computing
This research suggests future quantum technologies that rely on quantum oscillators rather than just simple qubits.
One particularly promising application is quantum computing. These types of conditions are likely to be more resilient to errors while supporting simpler and more effective error correction strategies. Beyond computing, they provide a new experimental platform to investigate one of the biggest questions in physics: is there a boundary between the classical world we experience and the underlying quantum reality that governs it?
