Posted: Nov 16, 2015 |
Quantum computer coding in silicon now possible
(Nanowerk News) A team of Australian engineers has 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.
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The result, obtained by a team at Australia's University of New South Wales (UNSW) in Sydney, appears today in the international journal, Nature Nanotechnology ("Bell's inequality violation with spins in silicon").
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The quantum code written at UNSW is built upon a class of phenomena called 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.
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Project leader Andrea Morello (left) with lead authors Stephanie Simmons (middle) and Juan Pablo Dehollain (right) in the UNSW laboratory where the experiments were performed.
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"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."
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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.
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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.
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"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.
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"Nevertheless, we have succeeded in passing the test, and we have done so with the highest 'score' ever recorded in an experiment," he added.
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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. These particles are, literally, on top of each other -- the electron orbits around the nucleus. Therefore, there is no complication arising from the spookiness of action at a distance.
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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 -- a key plank in the quest super-powerful quantum computers of the future.
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"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 Morello. "In particular, we can access the purely-quantum type of code that requires the use of the delicate quantum entanglement between two particles."
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In a normal computer, using two bits one, could write four possible code words: 00, 01, 10 and 11. In a quantum computer, instead, one can also write and use 'superpositions' of the classical code words, such as (01 + 10), or (00 + 11). This requires the creation of quantum entanglement between two particles.
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"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."
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Morello highlighted the importance of achieving the breakthrough using a silicon chip: "What I find mesmerising about this experiment is that this seemingly innocuous 'quantum computer code' - (01 + 10) and (00 + 11) - has puzzled, confused and infuriated generations of physicists over the past 80 years.
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"Now, we have shown beyond any doubt that we can write this code inside a device that resembles the silicon microchips you have on your laptop or your mobile phone. It's a real triumph of electrical engineering," he added.
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