The engineers say they have have proven – with the highest score ever obtained – that a quantum version of computer code can be written and manipulated using two quantum bits in a silicon microchip. The advance, “removes lingering doubts that such operations can be made reliably enough to allow powerful quantum computers to become a reality.”
The quantum code written at UNSW is built upon quantum entanglement, which allows for seemingly counterintuitive phenomena such as the measurement of one particle instantly affecting another – even if they are at opposite ends of the universe.
“This effect is famous for puzzling some of the deepest thinkers in the field, including Albert Einstein, who called it ‘spooky action at a distance’,” said Professor Andrea Morello, of the School of Electrical Engineering & Telecommunications at UNSW and Program Manager in the Centre for Quantum Computation & Communication Technology, who led the research. “Einstein was sceptical about entanglement, because it appears to contradict the principles of ‘locality’, which means that objects cannot be instantly influenced from a distance.”
Physicists have since struggled to establish a clear boundary between our everyday world – which is governed by classical physics – and this strangeness of the quantum world. For the past 50 years, the best guide to that boundary has been a theorem called Bell’s Inequality, which states that no local description of the world can reproduce all of the predictions of quantum mechanics.
Bell’s Inequality demands a very stringent test to verify if two particles are actually entangled, known as the ‘Bell test’, named for the British physicist who devised the theorem in 1964. The Australian team asserts, “"We have succeeded in passing the test, and we have done so with the highest ‘score’ ever recorded in an experiment.”
“The key aspect of the Bell test is that it is extremely unforgiving: any imperfection in the preparation, manipulation and read-out protocol will cause the particles to fail the test,” said Dr Juan Pablo Dehollain, a UNSW Research Associate who with Dr Stephanie Simmons was a lead author of the Nature Nanotechnology paper. “Nevertheless, we have succeeded in passing the test, and we have done so with the highest ‘score’ ever recorded in an experiment,” he added.
The apparent contradictions of entanglement are often cited in the context of particles that are physically separated by some – perhaps, considerable – distance. In the UNSW experiment (by contrast), the two quantum particles involved are constituens of a single atom; an electron and the nucleus of a single phosphorus atom, placed inside a silicon microchip. Therefore, there is no complication arising from the spookiness of action at a distance. The graphic shows a (false-colour) electron microscope image of the silicon nanoelectronic device which contains the phosphorus atom used for the demonstration of quantum entanglement.