Under extreme cold, familiar physics breaks down just as it does on very small scales, allowing exotic states of matter. However, for all the strangeness we have observed under these conditions, theory has often run far ahead of practice. States of matter known as quantum phases have been predicted long before they are achieved. Two teams have independently reported a previously unseen form of quantum entanglement, both published in the same edition of the journal Science, decades after theoreticians predicted its existence. The work was made possible through advances in quantum information programing and could open the door to making quantum computers more reliable and practical.
Quantum computers potentially hold powers far beyond those of conventional computers, able to perform search problems and cryptography millions of times faster. Although progress in quantum computing is now announced on an almost weekly basis, key problems remain. In particular, quantum computing is particularly susceptible to mistakes, making error correction essential. One way to detect errors is through time-reversal symmetry, defined as behaving the same way if time ran backward.
A team led by Google Quantum AI's Dr Kevin Satzinger used Google's Sycamore quantum processor – which in 2019 was claimed to be the first quantum device capable of outdoing the most powerful supercomputers – to make a two-dimensional lattice suited to quantum error-correction.
Satzinger and co-authors' work relies on quantum entanglement, where subatomic particles' physical properties become so interrelated they can't be described independently. Although entanglement was one of the great shocks of 20th Century physics, famously rejected and mocked by Einstein, increasingly advanced entangled states have been created, involving more particles or greater distances.
In a perspective accompanying Satzinger's paper Professor Stephen Bartlett of the University of Sydney describes the class of quantum phases known as “topologically ordered”. These, Bartlett explains have long-range entanglement between their components that are; “Unchanged under continuous local deformations.” Not being susceptible to local effects should make such phases robust against errors, but until now, exploring topologically ordered phases' properties has only been possible in non-time-symmetric phases.
Satzinger and co-authors report Sycamore running a quantum program that was both protected from errors and can be read again, showing it is replicable and time-symmetric.
In the same edition, Dr Giulia Semeghini of Harvard University describes a different path to a similar result, arranging 219 rubidium atoms in a two-dimensional lattice using “optical tweezers” (lasers that prod atoms into position) to make a quantum spin liquid. The first quantum spin liquid, a phase of matter within a magnetic material with interacting quantum spins and no ordinary magnetic orders, was created five years ago but these exotic states of matter have resisted our efforts to explore their properties. Semeghin called the version she and her co-authors created; “A dream in quantum computation” in a statement.
“You can really touch, poke, and prod at this exotic state and manipulate it to understand its properties," said senior author Professor Mikhail Lukin. "It’s a new state of matter that people have never been able to observe.”
Bartlett notes; “Neither experiment was achieved by using new materials, as is usually the case. Instead, the achievement was realized virtually with quantum processors.” These processors measure the way components of the structures entangle with elements that are not next to them, creating topological order.
Although the error correction in both experiments fell far short of what is necessary for practical use of quantum computers, Bartlett notes, each represents a substantial step towards that goal.