Quantum router could speed up quantum computers

Quantum computers may be able to run useful algorithms more quickly, thanks to a new quantum router that helps data get to the right place faster.

Traditional computers avoid becoming slow when faced with a complicated program in part by using random access memory (RAM) to temporarily store some information. The component key to building RAM’S quantum counterpart, QRAM, is a router. This isn’t the router that directs your internet queries to the right IP address, but rather an internal router that directs informational traffic inside a computer.

Connie Miao at Stanford University in California and her colleagues have now built such a device. “The project was motivated by algorithms that use QRAM. There were a lot of papers basically coming out saying: ‘If we had a QRAM, we could do XYZ,’ but this hadn’t been shown [experimentally],” she says.

The new router consists of qubits, the basic building blocks of all quantum computers and quantum memories, made from tiny superconducting circuits and controlled by electromagnetic pulses. Similar to a traditional router, this quantum one sent quantum information to quantum addresses. What distinguishes the device as being fully quantum is that it allows the address to be encoded not just in one place but in a superposition of two. The team tested this with three qubits and found the routing to have about 95 per cent fidelity.

This means that if it were incorporated into a QRAM, the device could push information into a quantum state where it is impossible to say which of the two locations it is stored in – exactly the kind of phenomenon believed to make quantum computers powerful.

Luming Duan at Tsinghua University in China, whose team previously built a quantum router that only worked during some runs, says the new device is an important step towards building practical QRAMs, which may enable running quantum machine learning algorithms.

Team member David Schuster at Stanford University says there are still many open questions about where exactly quantum routing could make a practical difference, but the options are broad, from well-known algorithms for searching databases to creating quantum IP addresses for future iterations of the internet.

Nonetheless, the current version of the router is still not reliable enough for some of those uses, so it needs to be made to have fewer errors and to contain a larger number of qubits going forward, says Sébastien Léger at Stanford University, who worked on the project.