Qubits are a basic building block for quantum computers, but they’re also notoriously fragile – tricky to observe without erasing their information in the process. Now, new research from the University of Colorado Boulder and the National Institute of Standards and Technology (NIST) could be a leap forward for handling qubits with a light touch. In the study, a team of physicists demonstrated that it could read out the signals from a type of qubit called a super­conducting qubit using laser light, and without destroying the qubit at the same time.

The group’s results could be a major step toward building a quantum internet, the researchers say. Such a network would link up dozens or even hundreds of quantum chips, allowing engineers to solve problems that are beyond the reach of even the fastest supercomputers around today. They could also, theo­retically, use a similar set of tools to send unbreakable codes over long distances. “Currently, there’s no way to send quantum signals between distant super­conducting processors like we send signals between two classical computers,” said Robert Delaney.

Delaney explained that the traditional bits that run your laptop are pretty limited: They can only take on a value of zero or one, the numbers that underly most computer programming to date. Qubits, in contrast, can be zeros, ones or, through super­position, exist as zeros and ones at the same time. But working with qubits is also a bit like trying to catch a snowflake in your warm hand. Even the tiniest disturbance can collapse that super­position, causing them to look like normal bits.

In the new study, Delaney and his colleagues showed that they could get around that fragility. The team uses a wafer-thin piece of silicon and nitrogen to transform the signal coming out of a super­conducting qubit into visible light – the same sort of light that already carries digital signals from city to city through fiberoptic cables. “Researchers have done experiments to extract optical light from a qubit, but not disrupting the qubit in the process is a challenge,” said Cindy Regal, associate professor of physics at CU Boulder. There are a lot of different ways to make a qubit, she added. 

Some scientists have assembled qubits by trapping an atom in laser light. Others have experimented with embedding qubits into diamonds and other crystals. Companies like IBM and Google have begun designing quantum computer chips using qubits made from super­conductors. Under the right circum­stances, superconductors will emit quantum signals in the form of photons, that oscillate at microwave frequencies. And that’s where the problem starts, Delaney said. 

To send those kinds of quantum signals over long distances, researchers would first need to convert microwave photons into optical photons. But when it comes to quantum computers, achieving that transformation is tricky, said Konrad Lehnert. In part, that’s because one of the main tools you need to turn microwave photons into optical photons is laser light, and lasers are the nemesis of super­conducting qubits. If even one stray photon from a laser beam hits your qubit, it will erase completely. “The fragility of qubits and the essential incom­patibility between super­conductors and laser light makes usually prevents this kind of readout,” said Lehnert.

To get around that obstacle, the team turned to a go-between: a thin piece of an electro-optic transducer. Delaney explained that the team begins by zapping that wafer, which is too small to see without a microscope, with laser light. When microwave photons from a qubit bump into the device, it wobbles and spits out more photons – but these ones now oscillate at a completely different frequency. Microwave light goes in, and visible light comes out 

In the latest study, the researchers tested their transducer using a real super­conducting qubit. They discovered that the thin material could achieve this switcheroo while also effectively keeping those mortal enemies, qubits and lasers, isolated from each other. In other words, none of the photons from the laser light leaked back to disrupt the super­conductor. “Our electro-optic transducer does not have much effect on the qubit,” Delaney said. 

The team hasn’t gotten to the point where it can transmit actual quantum information through its tranducer. Among other issues, the device isn’t parti­cularly efficient yet. It takes about 500 microwave photons, on average, to produce a single visible light photon. The researchers are currently working to improve that rate. Once they do, new possi­bilities may emerge in the quantum realm. Scientists could, theoretically, use a similar set of tools to send quantum signals over cables that would auto­matically erase their information when someone tries to listen in. (Source: UC Boulder)

Reference: R. D. Delaney et al.: Superconducting-qubit readout via low-backaction electro-optic transduction, Nature 606, 489 (2022); DOI: 10.1038/s41586-022-04720-2

Link: JILA (K. Lehnert), National Institute of Standards and Technology and the University of Colorado, Boulder, USA