Practical quantum computers are still years away, but lately the pace of research seems to have picked up. After building the basic blocks of a quantum computer in silicon and storing quantum information for up to 30 seconds, scientists at the University of New South Wales (UNSW) have now violated a principle of classical physics to demo for the first time a pair of entangled, high-fidelity quantum bits (qubits) in silicon. The advance could help unleash the power of a new kind of computation that would affect everything from data cryptography to drug design, overnight deliveries and subatomic particle experiments.
A mathematical relationship known as Bell's inequality places a limit on how strongly two particles can correlate without violating two intuitive principles that govern classical physics – locality, meaning what happens in one place can only be influenced by nearby objects; and realism, meaning physical objects exist whether or not they are observed.
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But when two quantum particles commune, or entangle, their correlation can be strong enough to break this principle, giving rise to what Einstein famously dismissed as spooky action at a distance. While it is possible to achieve entanglement without violating the inequality, in the context of quantum computing a violation is desirable as it means qubit operations are more reliable and have access to more "spooky" – and useful – behavior for faster number-crunching.
Professor Andrea Morello and team have now, for the first time, demonstrated a violation of Bell's inequality in silicon, paving the way for quantum computers that are reliable and highly scalable.
To achieve this, the scientists used the electron and nuclear spins of a phosphorus atom in a silicon-28 (a common isotope of silicon) substrate as qubits. By doing so, they were able to produce with high fidelity all of the four possible entangled states between those two qubits.
Vital to this advance was achieving a very strong correlation between the two quantum bits. This correlation is an important metric, as it directly affects the fidelity of quantum operations. On a scale where (according to Bell's principle) correlation between classical particles can't exceed two and the maximum possible figure for entangled particles is slightly below 2.83, Morello's team managed a record, near-ideal 2.70, which translates to a very high fidelity of 96 to 97 percent.
"When you look at what it takes to prepare and acquire the measurements on entangled states, you find that even the slightest imperfection, at any stage of the experiment, will make the Bell number drop very quickly," Morello told Gizmag. "In a sense, the Bell test is a single-figure qualifier of how perfect the whole experiment is, and it's extremely unforgiving. Here we have shown that we have some of the very best qubits in the world and we are able to handle them with near-perfect accuracy."
This is a promising result especially when combined with the fact that the device was built in silicon, fabricated with standard industry processes that lend themselves to miniaturization and scaling up the number of qubits. (Like similar quantum computers, this device still needs to operate at temperatures near absolute zero and under a strong magnetic field; but since quantum computers are not meant to replace traditional computers altogether, this should not be a big issue.)
Future experiments will explore how nuclear spins can help build logic gates between electron qubits. Then, the focus will shift to building longer-lived memory and creating the right architectures to build error-proof quantum computers that make use of many more entangled qubits.
"This particular advance does not scale very much, because it was done using the electron and the nucleus of the same atom," Morello told us. "That's why our next step is to entangle two different atoms.
"Still, this is important because we have learned a lot about how to make the process of writing quantum information near-perfect. Once you have many atoms, you can entangle them pairwise, or you can entangle them in large bunches. The more spins you entangle all together, the more powerful your computer code becomes."
Part of the team's plans is also to work on demonstrating a new method to entangle two atoms at very large distances of up to 1 cm (0.4 in).
"We have very clear goals to make silicon [quantum computers] with [more than] 10 qubits within 4-5 years," revealed Morello. "Beyond that point, it will depend on how much we can integrate our qubits with standard industry manufacturing processes. Here is where we expect that having done all this work in silicon will give us a huge advantage over the competitors."
The work appears in today's edition of the journal Nature Nanotechnology. The researchers further explain their work in the video below.