The path to quantum supremacy is complicated by a fairy tale challenge: how do you carry a cloud without changing its shape?
The possible solution sounds almost as fantastic as the problem. You could make the cloud dance as it travels, to the beat of a unique material known as a time crystal.
Krzysztof Giergiel and Krzysztof Sacha of the Jagiellonian University in Poland and Peter Hannaford of the Swinburne University of Technology in Australia propose that a new kind of “time” circuit could be tasked with preserving the fuzzy states of qubits as they are swept through storms of quantum logic.
Unlike descriptions of objects with clearly defined locations and motions, a quantum perspective of the same particle describes features such as its position, momentum, and rotation as a fuzzy set of probabilities.
This ‘cloud’ of possibilities is best understood in isolation. As the particle interacts with its environment, the distribution of possibilities changes, like the probability that a runner will win the 100-meter sprint at the Olympics, until eventually only one outcome is observed.
Just as a classical computer can use the binary states of particles as “on-off” switches in logic gates, quantum computers can theoretically exploit the propagation of uncertainties in a particle to quickly solve their own kinds of algorithms, many of which would be impractical are. or even impossible to solve the old-fashioned way.
The challenge is to maintain the coherence of that quantum cloud of possibilities – called a qubit – for as long as possible. With every bump, every electromagnetic breeze, comes an increased risk of errors that ruin the computational process.
Practical quantum computers require hundreds if not thousands of qubits to remain intact for long periods of time, making building a full system a huge challenge.
Researchers have looked at various ways to make quantum computing more robust, by trapping individual qubits to protect them from decoherence, or by building safety nets around them.
Now physicists Giergiel, Sacha and Hannaford have described a new approach that turns quantum computers into a qubit symphony, controlled by a very strange kind of conductor.
Time crystals are materials that transform into repeating patterns over time. Considered curiosities just over a decade ago, versions of these ‘ticking’ systems have now been developed using the gentle push of a laser and ultra-cold clusters of atoms, where flashes of light send particles into periodic oscillations that change the timing of the laser defy.
In a paper available on the pre-peer review server arXiv, the trio of physicists propose using the unique periodicity of a time crystal as the basis for a new kind of “time-tronics” circuit. This periodicity, used to guide the delicate waves of large numbers of information-laden qubits, could help reduce the accidental collisions responsible for many errors.
Such a temporal circuit of constantly drifting qubits would make it easier to direct virtually any computing particle into another’s path, entangling their quantum capabilities in useful rather than error-ridden ways.
While the proposal remains purely theoretical, the team showed how the physics of groups of potassium ions, cooled to near-absolute temperatures and directed by a laser pulse, could form an “orchestra” for qubits to waltz to.
Translating the idea into a practical, large-scale quantum computer would take years of innovation and experimentation, if it worked at all.
But now that we know that at least some types of time crystals exist that can be used for practical purposes, the challenge of carrying a cloud may not be such a fairytale quest after all.
This study is available on the pre-peer review server arXiv.