Stability in asymmetry: Scientists extend qubit lifetimes

What happened

Scientists have demonstrated that they can extend the lifetime of a molecular qubit by altering the surrounding crystal's structure to be less symmetrical.

The asymmetry protects the qubit from noise, enabling it to maintain information for five times longer than if it were housed in a symmetrical structure. The research team achieved a coherence time -- the time the qubit maintains information -- of 10 microseconds, or 10 millionths of a second, compared to the 2 microsecond coherence time of a molecular qubit in a symmetrical crystal host.

"This newfound ability to chemically control the host environment opens up new space for targeted applications of molecular qubits." -- Danna Freedman, MIT

The result, published in Physical Review X, comes from a team of researchers at the U.S. Department of Energy's (DOE) Argonne National Laboratory, MIT, Northwestern University, The University of Chicago and the University of Glasgow. The result is supported in part by Q-NEXT, a DOE National Quantum Information Science Research Center led by Argonne.

A bit of background

• A qubit is the fundamental unit of quantum information, the quantum analog of a traditional computing bit.
• Qubits can hold on to information for only so long before noise, or interfering signals, destroys the information. Extending the length of time the information remains stable, known as the coherence time, is one of the biggest challenges in quantum information science.
• Qubits come in different types, one of which is a lab-engineered molecule. Molecular qubits are modular, meaning they can be moved from one environment and placed in another with ease. By contrast, other types of qubits, such as those made of semiconductors, are highly wedded to their environment.

Why it matters

• Longer coherence time: Longer coherence times make for more useful qubits in applications such as computing, long-distance communication and sensing in areas such as medicine, navigation and astronomy.
• Modularity: Because the coherence time can be stretched by altering the qubit's housing or by placing it in a more asymmetrical position relative to its housing, there's no need to change the qubit itself to achieve longer lifetimes. Just change its situation.

"Molecular chemistry enables us to swap out the crystalline material that hosts the qubit as well as modify the qubit itself," said Danna Freedman, F.G. Keyes Professor of Chemistry at MIT and paper co-author. "Adding in this new level of control is very powerful."

"The change was realized just by interchanging single atoms on the host molecules, one of the smallest changes you could get, and it gave rise to the five-fold enhancement in coherence time," said University of Glasgow's Sam Bayliss, who co-authored the study. "It's a nice demonstration of this atomic-level tunability that you get with molecules. Chemical techniques intrinsically provide control on the level of single atoms, which is a dream in a lot of modern technologies."

• Variability: The effectiveness of this symmetry-breaking technique means that molecular qubits can operate in a wide variety of environments, even those in which noise can't be reduced.

"We've created a new handle for modifying coherence properties in molecular systems," Freedman said. "This newfound ability to chemically control the host environment opens up new space for targeted applications of molecular qubits."

"While 10 microseconds may not sound extremely long compared to some systems, keep in mind that we didn't do anything to reduce the noise sources. In the environments we measured, the noise is very significant. So even though there's noise there, the qubits basically don't see it," Bayliss said. "And why don't we just remove the noise source? In practical cases, it's not always possible to operate in an environment that is ultrapure. So having a qubit that can operate intrinsically in a noisy environment can be advantageous."

The details

• The team's qubit is made of a chromium-based ion attached to carbon-based molecules.
• For a molecular qubit, the main source of noise is the magnetic fields in its surroundings. The magnetic fields tend to jostle the qubit's energy levels, which encode the information. The crystal's asymmetry shields the qubit from the potentially disruptive magnetic fields, and the information is preserved for longer.
• In addition to improving the qubit's properties, the team developed a mathematical tool that accurately predicts any molecular qubit's coherence time based on the structure of the host crystal.

"This is incredibly exciting for us," Bayliss said. "One of the very exciting things was just how much of an advancement could be made with these systems over a short space of time, and how small some of the modifications to the host matrix can be to get quite a significant improvement."

"I'm delighted to observe a new, exciting feature of molecular chemistry," Freedman said.

"This is an important development. Being able to precisely tune a qubit's environment is a unique advantage of molecular qubits. This can't be easily done within other material systems," said Q-NEXT Director and paper co-author David Awschalom, who is also an Argonne senior scientist, vice dean of Research and Infrastructure and the Liew Family Professor of Molecular Engineering and physics at the University of Chicago's Pritzker School of Molecular Engineering, and the director of the Chicago Quantum Exchange. "Knowing we can extend a qubit's lifetime by engineering its environment opens new possibilities for applications across quantum computing, sensing, and communication."

This work was supported by the U.S. Department of Energy Office of Science National Quantum Information Science Research Centers and the Office of Basic Energy Sciences.