For the first time, Australian engineers have demonstrated that they can write and manipulate the quantum version of computer code on a silicon microchip. This was done by entangling two quantum bits with the highest accuracy ever recorded, and it means that we can now start to program for the super-powerful quantum computers of the future.
Engineers code regular computers using traditional bits, which can be in one of two states: 1 or 0. Together, two bits create code words that can be used to program complex instructions. But in quantum computing language there's also the possibility for bits to be in superposition, which means they can be 1 and 0 at the same time. This opens up a vastly more powerful programming language, but until now researchers haven't been able to figure out how to write it.
Now engineers from the University of New South Wales (UNSW) in Australia have demonstrated that not only can they do this, but they can do it on silicon microchips very similar to the ones that make up today's computers, which means the technology will be easy and quick to scale up.
So how exactly do you write quantum code? It all comes down to a phenomenon known as quantum entanglement. When two particles are entangled, it basically means that the measurement of one of them will instantly affect the state of its entangled particle, even if it's thousands of kilometres away.
"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 lead researcher Andrea Morello, from the Centre for Quantum Computation and Communication Technology at UNSW. "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."
But entanglement has been demonstrated time and time again through something by something known as Bell's test, which requires engineers to violate Bell's Inequality Principle. Basically, Bell's Inequality Principle sets a limit for the amount of correlation there can be between two classical bits – anything above that must be quantum entangled.
"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 one of the researchers, Juan Pablo Dehollain. "Nevertheless, we have succeeded in passing the test, and we have done so with the highest 'score' ever recorded in an experiment."
In their experiment, the two entangled particles in question were the electron and the nucleus of a single phosphorous atom, which was placed inside a silicon microchip. By entangling the two particles, they made it so that the state of the electron was entirely dependent on the state of the nucleus.
This meant that they expanded on the four possible digital codes that can be made with two traditional bits (00, 01, 10, or 11) to being able to create a much wider set of code words with two entangled bits, such as 00+11, 00-11, 01+10 or 01-10.
"This is, in some sense, the reason why quantum computers can be so much more powerful," said team member Stephanie Simmons. "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."
The next step is to entangle more particles and create more complex quantum code words, so that the team can begin to program an entire quantum computer. All the other pieces are already in place, in large part thanks to another UNSW team, which just last month built the first logic gate in silicon. The material is important, because it's something we're already incredibly familiar with building computers out of.
"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," said Morello. "It's a real triumph of electrical engineering."
The research has been published in Nature Nanotechnology.
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