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AUSTRALIAN engineers have proven that a quantum version of computer code can be written and manipulated using two quantum bits in a silicon microchip, allowing silicon-based computers to also be quantum computing devices.
UNSW researchers Professor Andrea Morello, Dr Stephanie Simmons and Dr Juan Pablo Dehollain presented their finding in the international journal, Nature Nanotechnology.
The quantum code written at UNSW is built upon a class of phenomena called quantum entanglement, where the measurement of one particle instantly affects another, entangled particle, regardless of the position of the two in physical space.
To verify if two particles are actually entangled, scientists apply Bell’s Inequality theorem, which demands a very stringent test.
“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,” explained Dr Juan Pablo Dehollain, a UNSW Research Associate who with Dr Stephanie Simmons was a lead author of the Nature Nanotechnology paper.
Despite this strictness, the Australian team passed the test with the highest ‘score’ ever recorded in an experiment.
In the UNSW experiment, the two quantum particles involved are an electron and the nucleus of a single phosphorus atom, placed inside a silicon microchip. The electron orbits around the nucleus, so there is no complication arising from the spookiness of action at a distance.
However, the significance of the UNSW experiment is that creating these two-particle entangled states is tantamount to writing a type of computer code that does not exist in everyday computers. It therefore demonstrates the ability to write a purely quantum version of computer code, using two quantum bits in a silicon microchip.
“Passing the Bell test with such a high score is the strongest possible proof that we have the operation of a quantum computer entirely under control,” said Professor Morello. “In particular, we can access the purely-quantum type of code that requires the use of the delicate quantum entanglement between two particles.”
Unlike normal binary computers, where two bits can be either 00, 01, 10, or 11, a quantum computer can write and use superpositions. Qubits could, for example, express (01 + 10) or (00 + 11). To do this, two particles must be in quantum entanglement.
“These codes are perfectly legitimate in a quantum computer, but don’t exist in a classical one,” said UNSW Research Fellow Stephanie Simmons, the paper’s co-author. “This is, in some sense, the reason why quantum computers can be so much more powerful: with the same number of bits, they allow us to write a computer code that contains many more words, and we can use those extra words to run a different algorithm that reaches the result in a smaller number of steps.”
According to Professor Morello, the experiment has shown beyond any doubt that it is possible to write quantum code inside a device that resembles the silicon microchips currently used in laptops or mobile phones: a real triumph of electrical engineering.