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Technique Could Improve Sensitivity of Quantum Detection Devices

February 9, 2024

(Nanowerk News) In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties such as rotation, acceleration and distance, with much more precision than classical sensors. The technology could enable devices that image the brain in unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is using microscopic defects within diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.Researchers use microscopic defects within a diamond to build a chain of three qubits (represented as small circles with arrows) that they can use for quantum sensing. They start from a central defect, combine it with a nearby defect, and then use this second defect to find and control a third defect. (Image: Courtesy of the researchers)

Researchers at MIT and elsewhere have developed a technique that allows them to identify and monitor a larger number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensors with greater sensitivity.

Their method is based on a central defect within a diamond, known as the nitrogen vacancy (NV) center, which scientists can detect and excite using laser light and then monitor with microwave pulses. This new approach uses a specific microwave pulse protocol to identify and extend that monitoring to additional defects that cannot be seen with a laser, which are called dark spins.

Researchers seek to control a greater number of dark spins by locating them through a network of connected spins. Starting from this central NV gyrus, the researchers build this chain by coupling the NV gyrus to a nearby dark gyrus, and then use this dark gyrus as a probe to find and monitor a more distant gyrus that the NV cannot detect directly. . 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 don’t see results, but we were able to take this risk. “It is possible, with some bravery, to look in places where people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a doctoral student in electrical and computer engineering and member of the Quantum Engineering Group at MIT. , lead author of an article on this technique, published in quantum PRX (“Control of an environmental spin defect beyond the coherence limit of a central spin”).

His co-authors include his advisor and corresponding author, Paola Cappellaro, Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, senior research scientist at the Institute for Quantum Computing at the University of Waterloo; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a diamond sample.

But the introduction of nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, may harbor what are known as electron spins, which originate from valence electrons around the defect site. Valence electrons are those found in the outermost shell of an atom. The interaction of a defect with an external magnetic field can be used to form a qubit.

Researchers can take advantage of these electron spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum record. Having a larger quantum register increases the performance of a quantum sensor.

Some of these electron spin defects are connected to the NV center through magnetic interaction. In previous work, researchers used this interaction to identify and monitor nearby spins. However, this approach is limited because the NV center is only stable for a short period of time, a principle called coherence. It can only be used to control the few turns that can be achieved within this consistency limit.

In this new paper, the researchers use an electron spin defect found near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin double echo resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all the electron spins that interact with it. Then, they selectively apply another microwave pulse to match the NV center with a nearby spin.

Unlike the NV, these neighboring dark spins cannot be excited or polarized with laser light. This polarization is a necessary step to control them with microwaves.

Once researchers find and characterize a first-shell spin, they can transfer the NV polarization to this first-shell spin through magnetic interaction by applying microwaves to both spins simultaneously. Then, once the first-shell spin is polarized, they repeat the SEDOR process on the first-shell spin, using it as a probe to identify a second-shell spin that interacts with it.

Controlling a chain of dark turns

This repeated SEDOR process allows researchers to detect and characterize a new and distinct defect located outside the coherence boundary of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that allow them to transfer polarization from the NV center along the chain to this second-shell spin.

“This is laying the groundwork for building larger quantum records for higher-shell spins or longer spin chains, and it also shows that we can find these new defects that haven’t been discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Small variations in the experimental setup, due to temperature or vibrations, can alter the microwave pulses.

The researchers were able to optimize their protocol to send precise microwave pulses, allowing them to effectively identify and control the second layer spins, Ungar says.

“We’re looking for something in the unknown, but at the same time, the environment may not be stable, so you don’t know if what you’re finding is just noise. Once you start seeing promising things, you can put all your best efforts in that direction. But before you get there, it’s a leap of faith,” says Cappellaro.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate that they could scale 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 up to 10+ layers.

In the future, they plan to continue improving their technique to efficiently characterize and test other electronic spins in the environment and explore different types of defects that could be used to form qubits.

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