A team of UNSW quantum engineers has demonstrated a world-first: the quantum entanglement of two electrons, each bound to a different atom of phosphorus, placed inside a silicon quantum computer chip.
Entanglement is the most striking of quantum phenomena: two particles can exist in a state of perfect mutual correlation, while having no state of their own. Its consequences have baffled scientists and philosophers for decades.
"But today, entanglement is a resource, the most important one for building powerful quantum computers," says UNSW Professor Andrea Morello, leader of the team that conducted the research, published recently in the journal Nature Communications.
The UNSW team specialises in building quantum computer devices where information is encoded in the magnetic orientation, or 'spin', of individual electrons, bound to atoms of phosphorus that are implanted inside an almost conventional silicon chip. This approach to building quantum computers is very powerful: it combines the large-scale manufacturability of silicon computer chips – a trillion-dollar industry that underpins the totality of our digital world – with the minuscule size and natural quantum behaviour of atoms.
Dr Holly Stemp, the lead author of the paper, explains: "The spin of a phosphorus atom is an excellent quantum bit. But because the atoms are so small, it's not easy to make them 'talk' to each other, let alone create genuine quantum entanglement. This is, in fact, the first time the provable entanglement has been created between two atoms in silicon."
The interaction used to entangle the atoms is itself very 'quantum', she adds.
"Electrons are not just particles but also waves, and when two waves overlap with each other, they give rise to the so-called 'exchange interaction', which is what we used here to entangle the atoms."
From the strength of the interaction, the researchers estimated that the atoms are about 20 nanometres apart, or 1/1000th of the thickness of a human hair.
Because quantum entanglement is so elusive and fragile, demonstrating that it exists is a challenge of its own. The UNSW engineers teamed up with experts at Sandia National Laboratories in the US, to develop and apply sophisticated techniques to quantify the 'fidelity' – that is, the degree of perfection – of the quantum operations used to entangle the atoms.
"This is not the first time that such operations have been attempted," Dr Stemp says, "but it's the first time they have been perfect enough to prove beyond doubt that we have entanglement between the atoms."
The proof relies upon creating 'Bell states' – named after John Bell, who in 1964 explained the deep meaning of quantum entanglement, which challenges our views of location and reality. The breakthrough also required the development of bespoke techniques to implant the atoms into the silicon chips, an operation conducted by the team of Professor David Jamieson at the University of Melbourne.
Entanglement is the key resource for quantum computing
Prof Morello stresses how important this result is for the operation of a quantum computer.
"When trying to explain in simple terms what makes quantum computers powerful, people often quote quantum superposition 'being 0 and 1 at the same time'," he says.
"But the real game-changer is entanglement, because it allows to create digital codewords that really do not exist in a classical computer."
Furthermore, entanglement is the 'quantum link' between different quantum bits, so it's the essential tool for scaling up the quantum computer.
"We want to build quantum computers using atoms implanted in silicon," Dr Stemp says.
"Atoms are small and perfect, and it would be amazing if we could handle them using methods borrowed from the trillion-dollar semiconductor industry that underpins every digital device we use today. Demonstrating quantum entanglement between two atoms unlocks the functionality of the next generation of silicon quantum computer chips."
