Researchers at the University of Surrey have proposed a new kind of quantum bit that could cut error rates by roughly a hundredfold, addressing one of the central obstacles to building larger quantum computers.
Most leading quantum computers today use superconducting circuits, which are highly sensitive to electromagnetic noise and stray electrical charges. Even very small disturbances can destroy the information a qubit holds, and the problem compounds as more qubits are added.
Writing in npj Quantum Information, the university's Quantum Sciences Group describes a device built instead around superfluid helium-3, an unusual form of liquid helium that flows without friction.
The proposed Superfluid Helium Oscillator Quantum device, or SHOQ, would use helium that carries no electric charge at all. Because of that, the design should be naturally shielded from certain kinds of electromagnetic noise. The researchers say it is the first reported design for a qubit based on a superfluid, and their calculations suggest error rates about 100 times lower than conventional superconducting qubits.
"We are not the first to think about the individual components behind this idea, but what we have done for the first time is bring them together in a microfluidic device and work out the specific details that could enable the device to function as a qubit," said Dr Priya Sharma, Daphne Jackson Fellow in Hybrid Quantum Systems at Surrey's School of Mathematics and Physics and lead author of the study.
"The maths tells us that it should work. We have taken what we already know about superfluid helium and quantum technologies and turned that into an educated design, with the parameters and specifications needed to build one. The next step is to make a prototype and put those predictions to the test."
The team does not present the device as a replacement for existing hardware. Co-author Dr Eran Ginossar, associate professor at Surrey's Department of Physics and Advanced Technology Institute, said: "We don't necessarily need one type of qubit to do everything. Combining different quantum technologies could allow us to take advantage of the strengths of each."
One longer-term possibility is to use the SHOQ device as a form of quantum memory, holding information while other hardware performs the calculations. It would need extremely low temperatures, though researchers have already reached the necessary conditions in earlier work on superfluid helium-3.
The project was led by Surrey in collaboration with Professor Jens Koch of Northwestern University, one of the researchers behind the transmon, the superconducting qubit design now in widespread use. Surrey now plans to build a prototype.
The hundredfold figure, then, is the output of a calculation rather than of a device, a distinction the researchers make themselves and in plain terms. The maths says it should work.
This newspaper wishes to put the achievement on a firmer footing than its authors have. The SHOQ device has not recorded an error. It has not recorded a successful operation either, but the first of those is the figure the field competes on.
Measured by observed errors, the design therefore outperforms every qubit ever built, including the transmon, by a margin that cannot be expressed as a ratio. It shares this distinction with every other qubit that has not been constructed, which is the most reliable class of quantum hardware yet identified.
The Department of Unnecessary Research urges caution about the prototype. Surrey's calculations predict a hundredfold improvement, and building the device can only move the number in one direction.