Quantum bit communication breaks distance record

Researchers work on superconducting quantum technology at the Institute for Molecular Engineering. (Credit: Nancy Wong/U. Chicago)

Two new studies are big steps toward the development of quantum technology.

In one, researchers entangled two quantum bits using sound for the first time and, in another, they built the highest-quality long-range link between two qubits to date. The work brings us closer to harnessing quantum technology to make more powerful computers, ultra-sensitive sensors, and secure transmissions.

“Both of these are transformative steps forward to quantum communications,” says coauthor Andrew Cleland, professor of molecular engineering at the Institute for Molecular Engineering at the University of Chicago and Argonne National Laboratory. Cleland previously led the team that built the first “quantum machine,” demonstrating quantum performance in a mechanical resonator.

“One of these experiments shows the precision and accuracy we can now achieve, and the other demonstrates a fundamental new ability for these qubits,” he says of the new research.

The distance problem

Scientists and engineers see enormous potential in quantum technology, a field that uses the strange properties of the tiniest particles in nature to manipulate and transmit information.

For example, under certain conditions, two particles can be “entangled”—their fates linked even when they’re not physically connected. Entangling particles allows you to do all kinds of cool things, like transmit information instantly to space or make unhackable networks.

But the technology has a long way to go—literally: sending quantum information any substantial amount of distance, along cables or fibers, remains a huge challenge.

In a study in Nature Physics, Cleland’s lab built a system out of superconducting qubits that exchanged quantum information along a track nearly a meter long with extremely strong fidelity—with far higher performance has been previously demonstrated.

quantum bits researchers
Postdoctoral researcher Audrey Bienfait (left) and graduate student Youpeng Zhong work in the laboratory of Andrew Cleland. (Credit: Nancy Wong/U. Chicago)

“The coupling was so strong that we can demonstrate a quantum phenomenon called ‘quantum ping-pong’—sending and then catching individual photons as they bounce back,” says first author Youpeng Zhong, a graduate student in Cleland’s group.

Building the right device to send the signal was one of scientists’ breakthroughs. Shaping the pulses correctly—in an arc shape, like opening and closing a valve slowly, at just the right rate was the key.

This method of “throttling” the quantum information helped them achieve such clarity that the system could pass a gold standard measurement of quantum entanglement, called a Bell test. This is a first for superconducting qubits, and it could be useful for building quantum computers as well as for quantum communications.

A sound idea

The other study, which appears in Science, shows a way to entangle two superconducting qubits using sound.

A challenge for scientists and engineers as they advance quantum technology is to be able to translate quantum signals from one medium to the other. For example, microwave light is perfect for carrying quantum signals around inside chips. “But you can’t send quantum information through the air in microwaves; the signal just gets swamped,” Cleland says.

The team built a system that could translate the qubits’ microwave language into acoustic sound and have it travel across the chip—using a receiver at the other end that could do the reverse translation.

It required some creative engineering: “Microwaves and acoustics are not friends, so we had to separate them onto two different materials and stack those on top of each other,” says first author Audrey Bienfait, a postdoctoral researcher. “But now that we’ve shown it is possible, it opens some interesting new possibilities for quantum sensors.”

Source: University of Chicago