Engineers have successfully realized a famous quantum thought experiment in a tangible way. Their results offer a novel and more resilient approach for conducting quantum computations, which significantly impacts error correction—a major hurdle on the way to achieving a functional quantum computer.
Engineers from UNSW have successfully brought a well-known quantum thought experiment to life. Their results present a new and robust method for carrying out quantum computations, with vital implications for error correction—one of the key challenges in creating a practical quantum computer.
For over a century, quantum mechanics has intrigued scientists and philosophers alike. One of the most famous thought experiments in this field is “Schrödinger’s cat,” which involves a cat whose existence is tied to the decay of a radioactive atom.
According to quantum mechanics, without direct observation, the atom exists in a state of superposition—meaning it is simultaneously ‘decayed’ and ‘not decayed.’ This introduces the perplexing notion that the cat exists in a superposition of being both dead and alive.
“While no one has observed an actual cat being both dead and alive simultaneously, the Schrödinger’s cat analogy helps illustrate a superposition of quantum states that differ significantly,” notes Professor Andrea Morello from UNSW, who is the principal investigator of this research published in the journal Nature Physics.
Atomic cat
In this research, Professor Morello’s team used an antimony atom, which is more intricate than typical quantum bits, or ‘qubits.’
“In our study, the ‘cat’ is represented by an atom of antimony,” explains lead author Xi Yu.
“Antimony is a more massive atom with a significant nuclear spin, which results in a large magnetic dipole. The antimony spin can orient in eight different directions rather than just two. Though this may seem trivial, it profoundly alters the dynamics of the system. A superposition of the antimony spin pointing in opposing directions isn’t merely a superposition of ‘up’ and ‘down’; it’s enriched by numerous quantum states living between the two extremes.”
This complexity has major implications for scientists striving to develop quantum computers that utilize nuclear spin as their foundational building block.
“Typically, researchers use a quantum bit, or ‘qubit,’ defined by only two quantum states, as the basic unit of quantum information,” co-author Benjamin Wilhelm explains.
“When a qubit is considered as a spin, we refer to ‘spin down’ as the ‘0’ state and ‘spin up’ as the ‘1’ state. However, if the spin’s direction shifts suddenly, it can instantly create a logical error: the ‘0’ flips to a ‘1,’ or the other way around, in a heartbeat. This vulnerability is why quantum information is particularly delicate.”
However, with the antimony atom’s eight different spin orientations, if the ‘0’ state is interpreted as a ‘dead cat’ and the ‘1’ state as an ‘alive cat,’ a single mistake is insufficient to disrupt the quantum information.
“As the saying goes, a cat has nine lives. A minor scratch can’t eliminate it. Our metaphorical ‘cat’ has seven lives: it would require seven consecutive errors to change ‘0’ into ‘1’! This illustrates how the superposition of antimony spin states in opposing directions is ‘macroscopic’—as it operates on a larger scale, realizing a Schrödinger cat,” Yu elaborates.
Scalable technology
The antimony cat is housed within a silicon quantum chip, resembling those in our computers and smartphones but modified to access the quantum state of an individual atom. The chip was created by UNSW’s Dr. Danielle Holmes, while colleagues from the University of Melbourne integrated the antimony atom into the chip.
“Housing the atomic ‘Schrödinger cat’ in a silicon chip allows us exceptional control over its quantum state—essentially its life and death,” says Dr. Holmes.
“Additionally, integrating the ‘cat’ within silicon means this technology could be scaled up using methods similar to those used in current computer chip manufacturing.”
This breakthrough is significant as it paves the way for a new approach to quantum computations. While information continues to be encoded in binary ‘0’ or ‘1,’ there is greater ‘error tolerance’ within the logical codes.
“Single or even multiple errors won’t immediately alter the information,” states Professor Morello.
“If an error happens, we can detect it right away and correct it before more errors compile. Continuing with the ‘Schrödinger cat’ analogy, it’s like seeing our cat return home with a noticeable scratch. Although it’s not dead, we recognize it got into a scuffle; we can intervene and identify the source of the problem before any further damage occurs to our cat.”
Demonstrating quantum error detection and correction—a ‘Holy Grail’ in quantum computing—is the next goal the team aims to tackle.
This project emerged from extensive international collaborations. Several contributors from UNSW Sydney, along with colleagues from the University of Melbourne, played a role in fabricating and operating the quantum devices. Theoretical collaborators from the USA, including Sandia National Laboratories and NASA Ames, as well as the University of Calgary in Canada, supplied valuable insights on creating the cat and evaluating its intricate quantum state.
“This endeavor exemplifies remarkable cross-border collaboration among world-class teams with complementary skills,” adds Professor Morello.
