Quantum research provides promise for a variety of technological applications, including the development of hacker-proof communication networks and quantum computers that could speed up the discovery of novel drugs. These applications necessitate the use of a quantum version of a computer bit called a qubit, which holds quantum data.
However, researchers are currently working out how to read the information stored in these qubits and dealing with qubits’ short memory period, or coherence, which is typically measured in microseconds or milliseconds.
A team of researchers from the Department of Energy’s (DOE) Argonne National Laboratory and the University of Chicago have made two key breakthroughs to overcome these frequent obstacles for quantum systems. They were able to read out their qubit on demand and then maintain the quantum state for more than five seconds, which is a new record for this class of gadget.
Furthermore, the qubits developed by the researchers are constructed of silicon carbide, a common material used in lightbulbs, electric vehicles, and high-voltage electronics.
“It’s uncommon to have quantum information preserved on these human timescales,” said David Awschalom, senior scientist at Argonne National Laboratory, director of the Q-NEXT quantum research center, Liew Family Professor in Molecular Engineering and Physics at the University of Chicago, and principal investigator of the project.
“Five seconds is long enough to send a light speed signal to the moon and back. That’s powerful if you’re thinking about transmitting information from a qubit to someone via light. That light will still correctly reflect the qubit state even after it has circled the Earth almost 40 times paving the way to make a distributed quantum internet.”
It’s uncommon to have quantum information preserved on these human timescales. Five seconds is long enough to send a light speed signal to the moon and back. That’s powerful if you’re thinking about transmitting information from a qubit to someone via light. That light will still correctly reflect the qubit state even after it has circled the Earth almost 40 times paving the way to make a distributed quantum internet.
David Awschalom
The researchers seek to offer a new path for quantum innovation by developing a scalable and cost-effective qubit system that can be built with conventional electronics.
“This essentially brings silicon carbide to the forefront as a quantum communication platform,” said University of Chicago graduate student Elena Glen, co-first author on the paper. “This is exciting because it’s easy to scale up, since we already know how to make useful devices with this material.”
The findings were published on Feb. 2 in the journal Science Advances.
‘10,000 times more signal’
The researchers’ initial achievement was making silicon carbide qubits easier to read. Every computer must be able to read data stored in its bits.
The conventional readout approach for semiconductor qubits, such as the ones observed by the team, is to address the qubits with lasers and measure the light emitted back. However, this process is difficult since it necessitates a high level of efficiency in detecting single photons of light.
Instead, the researchers employ precisely tailored laser pulses to add a single electron to their qubit, which can be either zero or one, depending on its starting quantum state. The qubit is then read out using a laser, same like before.
“Only now, the emitted light reflects the absence or presence of the electron, and with almost 10,000 times more signal,” Glen said.
“By converting our fragile quantum state into stable electronic charges, we can measure our state much, much more easily. With this signal boost, we can get a reliable answer every time we check what state the qubit is in. This type of measurement is called ‘single shot readout,’ and with it, we can unlock a lot of useful quantum technologies.”
With the single shot readout method, the researchers could concentrate on making their quantum states persist as long as possible, which is a known issue for quantum technology because qubits often lose their information owing to noise in their surroundings.
The researchers developed highly pure silicon carbide samples to decrease background noise that can interfere with qubit performance. They then extended the period of time that their qubits kept their quantum information by applying a sequence of microwave pulses to them, a concept known as “coherence.”
“These pulses decouple the qubit from noise sources and errors by rapidly flipping the quantum state,” said Chris Anderson of the University of Chicago, co-first author on the paper. “Each pulse is like hitting the undo button on our qubit, erasing any error that may have happened between pulses.”
Even longer coherences, according to the researchers, should be achievable. Extending coherence time has major implications, such as the complexity of operations that a future quantum computer can execute or the sensitivity of a quantum sensor to detect minuscule signals.
“For example, this new record time means we can perform over 100 million quantum operations before our state gets scrambled,” Anderson said.
The scientists see multiple potential applications for the techniques they developed.
“The ability to perform single-shot readout unlocks a new opportunity: using the light emitted from silicon carbide qubits to help develop a future quantum internet,” Glen said. “Essential operations such as quantum entanglement, where the quantum state of one qubit can be known by reading out the state of another, are now in the cards for silicon carbide-based systems.”
“We’ve essentially made a translator to convert from quantum states to the realm of electrons, which are the language of classical electronics, like what’s in your smartphone,” Anderson said.
“We want to create a new generation of devices that are sensitive to single electrons, but that also host quantum states. Silicon carbide can do both, and that’s why we think it really shines.”
The research used resources of the UChicago Materials Research Science and Engineering Center, the Pritzker Nanofabrication Facility, and the Research Computing Center.
This work was supported by the DOE Office of Basic Energy Sciences, Materials Science and Engineering division, DOE National Quantum Information Science Research Center, the National Science Foundation, Boeing, Swedish Research Council, Japan Society for the Promotion of Science, European Commission, Air Force Office of Scientific Research and Knut and Alice Wallenberg Foundation.
