BYLINE: Leah Hesla
Stanford collaborators at the Q-NEXT quantum center amp up the signal from tin atoms embedded in diamond, opening possibilities for quantum networking.
The future of tin-based qubits is brighter thanks to breakthrough work by Stanford University researchers supported through a quantum research center led by the U.S. Department of Energy’s (DOE) Argonne National Laboratory.
Qubits are the fundamental carriers of quantum information, and scientists worldwide are engineering atoms to create reliable, long-lived qubits for processing quantum data. Quantum engineering is expected to accelerate advances in areas as diverse as medicine and finance.
The Stanford team achieved their milestone on a type of qubit known as a tin vacancy center in diamond: replace two of diamond’s carbon atoms with one atom of tin and voila! A tin vacancy qubit is born.
But of course, it isn’t so simple.
A tin vacancy qubit can be in one of two states, called spin-up and spin-down. That spin signal tends to be fuzzy and hard to read, making tin vacancies less useful than other qubit types for transmitting information.
“Before working on this project, measuring this qubit’s spin was like trying to pick up a very, very weak light signal, like trying to squint at some dim light to determine whether the qubit was spin-up or spin-down,” said Eric Rosenthal, a Stanford University postdoctoral appointee.
But the Stanford team has now turned things around.
As detailed in Physical Review X, the group successfully boosted the tin vacancy qubit’s signal, reading its spin state with an impressive 87% accuracy. Typical tin vacancy measurements require averaging hundreds of readings. But the Stanford group was able to read the qubit’s spin state in a single shot. The high-accuracy, single-shot readout of the signal with reliable spin control is a first for tin-vacancy qubits.
The research was led by Jelena Vuckovic, the Jensen Huang professor of global leadership, professor of electrical engineering and by courtesy of applied physics at Stanford. It was supported by Q-NEXT, a DOE National Quantum Information Science Research Center led by Argonne.
The group’s advances will enable scientists to take full advantage of the tin vacancy qubit, which boasts long lifetimes and can operate at higher temperatures, helping cut down on cooling costs.
It’s also a promising qubit for a future quantum internet — a potential communication network, similar to the internet we’re familiar with, that transmits information through quantum principles. A quantum internet will require tight control of a qubit’s spin, high confidence in the spin state and compatibility with current communication infrastructure.
“In all of these aspects, the tin vacancy looks very promising as an emerging system for the quantum internet,” said Stanford postdoctoral scholar Souvik Biswas. “It checks most of the boxes.”
The team’s success followed in-depth studies of the way tin vacancies play with their electromagnetic surroundings. By tuning the relevant physics knobs, they solved tin vacancy’s metaphorical dim-light problem, cranking up the brightness of the qubit signal.
The most important knob was the tuner for the magnetic field around the qubit. The Vuckovic group showed that, by orienting the field in the right way, one can amp the tin-vacancy center’s quantum signal, like tilting a mirror at just the right angle to maximally reflect the light from a nearby bulb.
“You can have a magnetic field that is not oriented the right way, and then the qubit will not appear bright,” Biswas said. “We modified the physical environment using some knobs that people didn’t appreciate too much before this.”
The group also made major improvements to their experimental setup, especially to the arrangement of the optical instruments that help excite and read out the qubit’s spin states.
“It was like making a camera that can see really, really faint images,” Biswas said.
In making their measurements, the researchers overcame several engineering hurdles, including the difficulty of flipping a tin vacancy’s spin and the trickiness of working with diamond.
“This is generally a bit harder to do because you’re limited by the natural qubit itself,” Biswas said.
The Stanford researchers note that, by optimizing the diamond device further for light collection, the scientific community could improve on their qubit’s already impressive performance.
“The fact that it works that well with this unoptimized structure really speaks well of tin as a qubit,” Rosenthal said. “It means it’s an easy qubit to work with.”
In the same streamlined setup, researchers also carried out so-called weak measurements of the tin vacancy’s quantum states, laying out the ways that the very act of measuring spin can affect the system’s behavior.
Unlike a strong spin measurement, which forces the spin to fully “collapse” into an up or down state, a weak measurement doesn’t always collapse the spin. The extent to which this collapse happens – or doesn’t happen – ends up being a precise method to study how strongly the qubit interacts with light. The technique is an example of how quantum mechanics can be useful for sensing. And the method can be applied to other types of qubits.
The Stanford team worked with scientists at DOE’s Sandia National Laboratories to implant the tin atoms in diamond. The multiorganizational effort speaks to the goals of the broader quantum information science community to collaborate across institutions to create new materials for quantum devices.
“Not only can we measure if the tin vacancy qubit is up or down, but we can also control whether it’s up or down or somewhere in between very well,” Rosenthal said. “I hope our techniques will be helpful for the community at large.”
Added Biswas, “With the technical advancements made toward tin vacancy centers, the future for diamond-based quantum technology is exciting.”
This work was supported by the DOE Office of Science National Quantum Information Science Research Centers as part of the Q-NEXT center.
About Q-NEXT
Q-NEXT is a U.S. Department of Energy National Quantum Information Science Research Center led by Argonne National Laboratory. Q-NEXT brings together world-class researchers from national laboratories, universities and U.S. technology companies with the goal of developing the science and technology to control and distribute quantum information. Q-NEXT collaborators and institutions have established two national foundries for quantum materials and devices, develop networks of sensors and secure communications systems, establish simulation and network test beds, and train the next-generation quantum-ready workforce to ensure continued U.S. scientific and economic leadership in this rapidly advancing field. For more information, visit https://q-next.org/.
Argonne National Laboratory seeks solutions to pressing national problems in science and technology by conducting leading-edge basic and applied research in virtually every scientific discipline. Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.
The U.S. Department of Energy’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science.
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