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.
鈥淏ut today, entanglement is a resource, the most important one for building powerful quantum computers,鈥 says UNSW Professor Andrea Morello, leader of the team that conducted the research, published recently in the journal .
The UNSW team specialises in building quantum computer devices where information is encoded in the magnetic orientation, or 鈥榮pin鈥, 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 鈥 a trillion-dollar industry that underpins the totality of our digital world 鈥 with the minuscule size and natural quantum behaviour of atoms.
Dr Holly Stemp, the lead author of the paper, explains: 鈥淭he spin of a phosphorus atom is an excellent quantum bit. But because the atoms are so small, it鈥檚 not easy to make them 鈥榯alk鈥 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.鈥
The interaction used to entangle the atoms is itself very 鈥榪uantum鈥, she adds.
鈥淓lectrons are not just particles but also waves, and when two waves overlap with each other, they give rise to the so-called 鈥榚xchange interaction鈥, which is what we used here to entangle the atoms.鈥
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 鈥榝idelity鈥 鈥 that is, the degree of perfection 鈥 of the quantum operations used to entangle the atoms.
鈥淭his is not the first time that such operations have been attempted,鈥 Dr Stemp says, 鈥渂ut it鈥檚 the first time they have been perfect enough to prove beyond doubt that we have entanglement between the atoms.鈥
The proof relies upon creating 鈥楤ell states鈥 鈥 named after John Bell, who in 1964 explained the deep meaning of quantum entanglement, which challenges our views of locality 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.
鈥淲hen trying to explain in simple terms what makes quantum computers powerful, people often quote quantum superposition 鈥榖eing 0 and 1 at the same time鈥,鈥 he says.
鈥淏ut the real game-changer is entanglement, because it allows to create digital codewords that really do not exist in a classical computer.鈥
Furthermore, entanglement is the 鈥榪uantum link鈥 between different quantum bits, so it鈥檚 the essential tool for scaling up the quantum computer.
鈥淲e want to build quantum computers using atoms implanted in silicon,鈥 Dr Stemp says.
鈥淎toms 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.鈥
Related stories
-
UNSW engineers get quantum computing processors working at 20X warmer temperatures
-
Quadruple quantum: engineers perform four quantum control methods in just one atom
-
Jellybeans 鈥 a sweet solution for overcrowded circuitry in quantum computer chips
-
Quantum engineers have designed a new tool to probe nature with extreme sensitivity