New Theory Lowers The Speed Limit For Information Processing in Quantum Computing

22 APRIL 2015

A new study has narrowed the theoretical speed limit for how quickly quantum computers of the future will be able to transmit and process information.

Quantum computing systems have the potential to perform certain calculations exponentially faster than classical computers. As such, they could offer enormous advantages for solving complex problems, like searching expansive databases, cracking modern encryption, and modelling atomic-scale systems for drug development.


The fundamental building blocks of these computers are quantum bits, or qubits. While several candidate particles exist, most - if not all - qubits are single atoms.

Information is stored on the magnetic spin of these atomic particles, which can point either "up" or "down" - states that are considered equivalent to the 0 and 1 of binary code.

Importantly, qubits can harness a strange quantum phenomenon called superposition, which allows the spin to exist in both states simultaneously. A scalable quantum computer would need thousands of these qubits working in concert across varying distances. These particles would rapidly transmit information to other qubits through another phenomenon known as entanglement.

The question is, just how fast can information move between particles, which are spread out over varying distances, via entanglement?

"Previous results suggested that the time needed for entanglement to spread throughout a system can be very small when interactions between qubits are long-ranged, leaving open the possibility of very fast transfer of information when interactions are long-ranged," Michael Foss-Feig, a physicist at the National Institute of Standards and Technology (NIST) told Jeremy Hsu at IEEE Spectrum.

"Our result places a tighter constraint on how much time you need to distribute information and entanglement across a system of a given size."

The team's result, which was recently published in the journal Physical Review Letters, builds on two papers that have previously explored the theoretical speed limit of quantum computing.

The first paper, published in 1972, discovered a finite speed limit for how quickly qubits could exchange information, if they were only able to do so with the qubit next-door, across relatively short distances. As Hsu points out for IEEE Spectrum, this limit is known as the Lieb-Robinson Bounds.

The second study, published in 2005, was interested in how quickly qubits could communicate with non-neighbouring qubits, across greater distances - an important consideration for quantum systems needing to link up different components. It suggested that interactions over longer ranges might actually result in a faster speed limit.

"Those results implied a quantum computer might be able to operate really fast, much faster than anyone had thought possible," said Foss-Feig in a press release "But over the next decade, no one saw any evidence that the information could actually travel that quickly."

Measuring the speed of quantum interactions is a bit like lining up dominos and timing how long the chain-reaction takes for the last one to fall down. Physicists exploring this aspect of the quantum world often line up several particles and watch how fast changing the spin of the first particle affects the one farthest down the line.

The NIST team analysed years of research to show that the speed limit predicted by the 2005 study was too great, and developed a new mathematical theory, which constrains how fast quantum information can travel via spin-state interactions.

"The tighter a constraint we have, the better, because it means we'll have more realistic expectations of what quantum computers can do," says Foss-Feig.

The team is now working on refining it's speed limit predictions, but as Hsu explains for IEEE Spectrum, there is a caveat to the work: "Their calculations are based on the assumption that long-range entanglement interactions decay at a particular rate. If the entanglement interactions don't decay at all with distance, a qubit could theoretically transfer information instantaneously to another qubit very far away."

If that were the case, quantum computers might still be speed demons yet.

Source: IEEE Spectrum