Breakthrough in experimental light-powered quantum computers could mean scaling them up is now far more viable
Researchers have demonstrated a revolutionary method for preventing errors in light-powered quantum computers before they even occur.
This milestone, which was achieved using a new technique called photon distillation, means physicists are moving closer to developing light-based “photonic” quantum computers capable of achieving a quantum advantage over classical supercomputers.
The research addresses what is arguably the biggest obstacle on the path to developing universal fault-tolerant quantum computers, namely the presence of noisy errors that can cause calculations to fail.
Contrary to superconducting quantum computerswhich exploit electronic circuits to create qubits – the quantum equivalent of computer bits – photonic quantum computers are powered by light. Scientists project beams of photons (units of light) through fields of specially designed mirrors and beam splitters. The photons themselves are then manipulated into complex quantum states that allow calculations to be performed.
One of the main advantages of this quantum computing paradigm is that it operates at room temperature. The underlying reason why this is possible also lies behind the biggest problem in photonic quantum computing.: Photonic quantum computers can operate without generating a lot of excess heat because the light is in constant motion. This movement allows calculations to be carried out thanks to the interactions between photons as they move. But it also produces a lot more errors.
The problem of fault tolerance
Superconducting quantum computers must power circuits to create qubits – a process that generates heat. Although photons do not suffer from this problem, there is a tradeoff: photonic quantum computers are very fragile. Photons are, by their very nature, imperfect, which means that there is usually a significant percentage of “bad” photons bouncing around that can ruin a given calculation.
“Because photons move at speed of lightyou have qubits that are constantly moving through the system”, Jelmer Renemachief scientist and co-founder of QuiX Quantum, told Live Science. “And the calculations work because of the interactions between these photons when they meet on the chip.”
“Errors occur when one of the photons is not working well,” Renema said. “Every once in a while, there’s some sort of maverick photon that decides not to play by the rules of the other photons.”
This “rogue” photon will make its way through the system without ever interacting with other photons, producing a distinct error. Because this happens before the photon is even transformed into a qubit for processing, this problem is difficult to solve by conventional methods. quantum error correctionwhich typically involves techniques for resolving qubit errors once they have occurred.
Since qubits can exist in a superposition state, they can be prone to errors.
(Image credit: Jorg Greuel/Getty Images)
The amount of qubits you have to spend to create a single good qubit is so enormous that the cost of the computer skyrockets.
Jelmer Renema, chief scientist and co-founder of QuiX Quantum
Using a technique called quantum photonic distillation, QuiX used error mitigation to tackle the root cause of these errors before they occur.
“You set up the interference in such a way that the probability that your bad photon will make it to the output… is less than the probability that the photons that are playing nice will make it to that output,” Renema said.
This probability is at the heart of photonic quantum computing. As Renema says: “Everything in photonics is probabilistic”. When researchers shine beams of photons through a series of mirrors and beam splitters, there is some probability that each photon will do what it wants, and if nothing is done to mitigate errors, they rely essentially on luck to produce viable calculations.
The chances of success become even worse for each photon as engineers add more quantum computing gates to the system.
Below the threshold
With a superconducting quantum computer, you can add “logical” qubits to perform fault tolerance on the physical qubits to compensate for errors. These are collections of physical qubits that share the same data, so that if one or more qubits fail, the data is available elsewhere in the cluster and calculations are not disrupted. But with quantum computing, adding overhead tends to produce more errors than it fixes.
Photonic distillation also exhibits “subthreshold error attenuation” — a metric the study authors used to indicate that their technique reduces the number of errors that occur as the system scales, instead of adding more, which is normally the case when you scale up a quantum computer, the QuiX scientists wrote in the study.
Similar steps in fault tolerance have been achieved in superconducting and neutral-atom quantum computers. Google managed to fix errors below the threshold in his willow quantum processing unit (QPU) in December 2024, for example. But the new study represents the first time this has been done in light-powered systems.
“The amount of qubits you have to spend to make a single good qubit is so enormous that the cost of the computer explodes enormously,” Renema said. “So there is a compromise.”
Photonic distillation sends imperfect photons through a specialized optical circuit that uses “quantum interference” – a strange feature of quantum mechanics in which the probability amplitudes of quantum states combine – to filter out physical inconsistencies and produce a high-quality single photon. All this happens before the photons are transformed into qubits.
These high quality photons are then sent through the system with a much lower probability of becoming unwanted. This increase in quality provides a net gain in error correction even taking into account all the errors introduced when photons are used as qubits.
Because photonic computers are probabilistic, this experimental work demonstrates a scalable approach to error mitigation that should provide subthreshold performance at scales high enough to produce useful quantum calculations, the study authors said.
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