An international team of researchers has developed a theory of multi-level quantum coherence, a significant step beyond the theory which allows the famous Schrödinger’s cat to be in a quantum superposition of being dead and alive at the same time.
The team, involving the University of Nottingham, has demonstrated the use of its theory through an experiment on a four-level quantum system.
Gerardo Adesso, Professor of Mathematical Physics in the School of Mathematical Sciences at the University of Nottingham, said: “This work provides a comprehensive tool set for characterising, detecting and quantifying the structure of coherence in multilevel systems in any experiment.
The research - Certification and Quantification of Multilevel Quantum Coherence – has been published in the academic journal Physical Review X. It was led by Dr Martin Ringbauer at the University of Queensland. Dr Ringbauer said: “Quantum coherence is at the heart of all quantum phenomena and technology. Unlike the classical physics that you experience every day – where a ball must be either ‘here’ or ‘there’ – quantum systems are almost always in some form of superposition of ‘here&there’ at the same time. Yet, the quantum systems used in today’s quantum technology don’t stop at ‘here’ and ‘there’, but usually have multiple distinct states.”
Dr Marco Piani from the University of Strathclyde said that such complex superpositions presented a great opportunity, but also a big challenge for the researchers.
“Imagine a very active Schrödinger’s cat not only wanting to eat and play at the same time, but also wanting to climb a pole and poke a toy,” he said.
Beyond the increase in the number of possible ways to create superpositions, the team showed that the properties of such systems were fundamentally different to those of their two-state counterpart.
“Understanding coherence in multi-level systems, is much more complicated than distinguishing presence and absence of coherence, and requires a detailed study of the type and degree of coherence present in the system,” Dr Piani said.
The researchers developed the tools to tell the difference between a cat that is truly doing all four things at once, and one that does `only’ two at a time, while some other times choosing the other options. They experimentally demonstrated their results on a single particle of light, which in their experiment can be in any superposition of four distinct states.
Professor Adesso said: “We show that being in a multilevel superposition provides advantages in a practical task, which could roughly be described as faithfully reading time off an effective quantum clock.”
The study could help researchers understand and explore the coherent properties of quantum systems, at a time when governments and leading companies are making significant investments in quantum technologies.
The researchers hope the results and tools explained in their work could have far-reaching applications from the foundations of physics to new fields such as quantum biology.
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