One of the ways to realize the full potential of quantum computers is to base them on both light and matter – this way information can be stored and processed, but also travel at the speed of light.
Scientists have just taken a step closer to this goal, succeeding in producing the largest hybrid particles of light and matter ever created.
These quasiparticles, called Rydberg polaritons, were made using a piece of stone containing cuprous oxide (Cu2O) crystals from an ancient deposit in Namibia, one of the few places in the world where cuprous oxide has been found as a gemstone.
Crystal recovered from the stone was polished and thinned to less than the width of a human hair and sandwiched between two mirrors to trap light, resulting in Rydberg polaritons 100 times larger than those seen previously.
This achievement brings us closer to producing a quantum simulator capable of operating off of these Rydberg polaritons, using quantum bits or qubits to store information in 0s, 1s, and several intermediate values - rather than just the 1s and the 0s of the classic computer bits.
“Making a quantum simulator out of light is the holy grail of science,” says physicist Hamid Ohadi, from the University of St Andrews in the UK.
“We took a big step towards that by creating Rydberg’s polaritons, the key ingredient in this.”
What makes Rydberg polaritons so special is that they continually transition from light to matter and back again. The researchers compare light and matter to two sides of the same coin, and it is on the matter side that polaritons can interact with each other.
This is important because light particles move quickly, but do not interact with each other. Matter is slower, but it is able to interact. Bringing these two capabilities together could help unlock the potential of quantum computers.
This flexibility is crucial in dealing with quantum states that remain undefined until they are observed. A fully functional quantum computer built on this technology is still a long way off, but we are now closer than ever to being able to assemble one.
Rydberg polaritons are formed by coupling excitons and photons. That’s where Namibia’s ancient gemstone comes in: cuprous oxide is a superconductor, a material that allows electrons to flow without resistance – and previous research had shown it to contain giant Rydberg excitons.
Excitons are electrically neutral quasi-particles that can be forced, under the right conditions, to couple with light particles. These large excitons found in cuprous oxide can be coupled to photons in a special configuration known as a Fabry-Pérot microcavity – essentially a mirror sandwich.
This was a key element in being able to create the largest Rydberg polaritons.
“Buying the stone on eBay was easy,” says physicist Sai Kiran Rajendran, from the University of St Andrews. “The challenge was to make Rydberg polaritons that exist in an extremely narrow color gamut.”
Once fully capable quantum computers can be assembled – perhaps using these Rydberg polaritons – the exponential improvements in computing power will allow them to tackle extremely complex calculations beyond the reach of computers whose we have today.
Examples put forward by the researchers include the development of high-temperature superconducting materials and a better understanding of how proteins fold (which could increase our ability to produce drug treatments).
The methods described in the new research will need to be refined for these particles to be used in quantum circuits, but the basics are now there – and the team believe their results can also be improved in the future.
“These results pave the way for the realization of strongly interacting exciton-polaritons and the exploration of strongly correlated phases of matter using light on a chip,” the researchers write in their paper.
The research has been published in Natural materials.