Qubits are a basic building block for quantum computers, but they鈥檙e also notoriously fragile鈥攖ricky to observe without erasing their information in the process. Now, new research from 抖阴旅行射听Boulder and the (NIST) may be a leap forward for handling qubits with a light touch. 听
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鈥攁nd without destroying the qubit at the same time.
Artist's depiction of an electro-optic transducer, an ultra-thin device听that can capture and transform the signals coming from a superconducting qubit. (Credit: Steven Burrows/JILA)
The group鈥檚 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.听
The study, , was led by between 抖阴旅行射 Boulder and NIST.
鈥淐urrently, there鈥檚 no way to send quantum signals between distant superconducting processors like we send signals between two classical computers,鈥� said Robert Delaney, lead author of the study and a former graduate student at JILA.
Quantum computers, which run on qubits,听get their power by tapping into the properties of quantum physics, or the physics governing very small things. Delaney explained 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 a property called 鈥渟uperposition,鈥� exist as zeros and ones at the same time.听
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 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鈥攖he same sort of light that already carries digital signals from city to city through fiberoptic cables.
鈥淩esearchers have done experiments to extract optical light from a qubit, but not disrupting the qubit in the process is a challenge,鈥� said study co-author Cindy Regal, JILA fellow and associate professor of physics at 抖阴旅行射 Boulder.
Fragile qubits
There are a lot of different ways to make a qubit, she added.听
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.
A quantum computer chip designed by IBM that includes four superconducting qubits. (Credit: npj Quantum Information,听2017)
Superconductors are materials that electrons can speed around without resistance. Under the right circumstances, superconductors will emit quantum signals in the form of tiny particles of light, or 鈥減hotons,鈥� that oscillate at microwave frequencies.
And that鈥檚 where the problem starts, Delaney said.听
To send those kinds of quantum signals over long distances, researchers would first need to convert microwave photons into visible light, or optical, photons鈥攚hich can whiz in relative safety through networks fiberoptic cables across town or even between cities. But when it comes to quantum computers, achieving that transformation is tricky, said study co-author Konrad Lehnert.
In part, that鈥檚 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.听
鈥淭he fragility of qubits and the essential incompatibility between superconductors and laser light makes usually prevents this kind of readout,鈥� said Lehnert, a NIST and JILA fellow.
Secret codes
To get around that obstacle, the team turned to a go-between: a thin piece of material called an electro-optic transducer.
Delaney explained 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鈥攂ut these ones now oscillate at a completely different frequency. Microwave light goes in, and visible light comes out听
In the latest study, the researchers tested their transducer using a real superconducting qubit. They discovered 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.听
鈥淥ur electro-optic transducer does not have much effect on the qubit,鈥� Delaney said.听
The team hasn鈥檛 gotten to the point where it can transmit actual quantum information through its microscopic telephone booth. Among other issues, the device isn鈥檛 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 was trying to listen in. Mission Impossible made real, in other words, and all thanks to the sensitive qubit.
