In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields as well as properties such as rotation, acceleration, and distance, with far more precision than classical sensors. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with pinpoint positional accuracy.
As many real-world quantum sensing devices are emerging, one promising direction is to use microscopic defects inside diamond to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.
Researchers at MIT and elsewhere have developed a technique that enables them to identify and control large numbers of these microscopic defects. This could help them build a larger system of qubits that could perform quantum sensing with greater sensitivity.
Their method creates a central defect inside the diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then with microwave pulses. Can control. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that cannot be seen with the laser, called dark spins.
The researchers want to detect and control large numbers of dark spins through a network of connected spins. Starting from this central NV spin, researchers build this chain by combining the NV spin with a nearby dark spin, and then use this dark spin as a probe to find and control more distant spins. which cannot be directly sensed by the NV. , The process can be repeated on these more distant spins to control longer chains.
“One lesson I learned from this work is that searching in the dark can be quite discouraging when you can’t see the results, but we were able to take this risk. It is possible, with some courage, to search in places where people have not looked before and potentially find more advantageous qubits,” says member Alex Unger, PhD student in electrical engineering and computer science. Quantum Engineering Group at MIT, who is the lead author of a paper On this technology, which is published today prx quantum,
His co-authors include his advisor and corresponding author, Paola Cappellaro, Ford Professor of Engineering and professor of physics in the Department of Nuclear Science and Engineering; as well as Alexander Cooper, a senior research scientist at the Institute for Quantum Computing at the University of Waterloo; and Won Q. Kelvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.
Scientists implant nitrogen into a diamond sample to create NV centers.
But adding nitrogen to a diamond creates other types of nuclear defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which arise from valence electrons around the defect site. Valence electrons are those that are in the outermost shell of an atom. The interaction of a defect with an external magnetic field can be used to create a qubit.
Researchers can use these electronic spins from neighboring defects to create more qubits around the same NV center. This large collection of qubits is known as a quantum register. Having a larger quantum register increases the performance of a quantum sensor.
Some of these electronic spin defects are linked to the NV center through magnetic interactions. In previous work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is stable for only a short period of time, a principle called coherence. It can be used to control only those few spins that can be accessed within this coherence range.
In this new paper, the researchers use an electronic spin defect that occurs near the NV center as a probe to find and control an extra spin, creating a chain of three qubits.
They use a technique called spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all the electronic spins interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with a nearby spin.
Unlike NVs, these neighboring dark spins cannot be excited or polarized by laser light. This polarization is an essential step in controlling them with microwaves.
Once researchers find and characterize the first-layer spin, they can transfer the polarization of the NV to this first-layer spin through magnetic interaction by applying microwaves to both spins simultaneously. Then once the first layer spin is polarized, they repeat the SEDOR process on the first layer spin, using it as a probe to identify the second layer spin that is interacting with it.
Controlling the Dark Spin Chain
This repeated SEDOR process allows researchers to detect and characterize a new, specific defect located outside the coherence range of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center to this second layer spin along the series.
“This is setting the stage for building larger quantum registers for higher-layer spins or longer spin chains,” says Unger, “and it’s also showing that we can find these new defects that haven’t been discovered before.” “
To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Small changes in the experimental setup, due to temperature or vibration, can distort the microwave pulse.
The researchers were able to optimize their protocol to send precise microwave pulses, says Unger, enabling them to effectively detect and control the spin of the second layer.
“We are searching for something in the unknown, but at the same time, the environment may not be stable, so you don’t know if what you are searching is just noise. Once you start seeing promising things, you can try your best in that one direction. But before you get there, it’s a leap of faith,” Cappellaro says.
While they were able to effectively demonstrate the three-spin chain, the researchers estimate that they could extend their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale to more than 10 layers.
In the future, they plan to enhance their technique to efficiently characterize and probe other electronic spins in the environment and detect different types of defects that can be used to create qubits.
This research is partially supported by the US National Science Foundation and the Canada FIRST Research Excellence Fund.
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