The technique could improve the sensitivity of quantum sensing devices MIT News

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

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects within diamonds 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 helps detect and control large numbers of these microscopic defects. This will enable them to build a large qubit system that can perform quantum sensing with high sensitivity.

Their method creates a central defect within the diamond, called a nitrogen-vacancy (NV) center, which scientists can detect and excite with laser light and control with microwave pulses. This new approach uses a specific protocol of microwave pulses to detect and extend additional defects that cannot be seen with the laser, called dark spins.

Researchers seek to control large numbers of dark vortices by tracing them through a network of connected vortices. Starting from this central NV vortex, researchers build this chain by connecting the NV vortex with a nearby dark vortex, and then use this dark vortex to find and control a distant vortex that cannot be directly sensed by the NV. . The process can be repeated at these distant loops to control longer chains.

„One lesson I learned from this work is that searching in the dark can be very discouraging when you can't see the results, but we were able to take this risk. It's possible with some bravery to look where people haven't seen before and find highly beneficial qubits,” says Alex, a PhD student in electrical engineering and computer science and member. Unger. Quantum Engineering Group At MIT, he is a principal lecturer Paper In this technique, published today PRX Quantum.

His co-authors include his advisor and corresponding author, Poola Cappellaro, Ford Professor of Engineering and Professor of Physics in the Department of Nuclear Science and Engineering; Alexandre Cooper, Senior Research Scientist at the University of Waterloo's Quantum Computing Institute; and Von Q. Calvin Son, a former researcher in Cappellaro's group who is now a postdoc at the University of Illinois at Urbana-Champaign.

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Diamond defects

To create NV centers, scientists attach nitrogen to a sample of diamond.

But the introduction of nitrogen into diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host so-called electronic spins that originate from the valence electrons surrounding the defect site. Valence electrons are in the outermost shell of an atom. The interaction of a defect with an external magnetic field is used to create a qubit.

Researchers can use these electronic spins from neighboring defects to create more qubits around an NV core. This large collection of qubits is called a quantum record. Having a large quantum register increases the efficiency of the quantum sensor.

Some of these electronic spin defects are attached to the NV core by magnetic interaction. In past work, researchers have used these interactions to identify and control nearby vortices. However, this approach is limited because the NV center can only be stable for a short period of time, which is called asynchrony. It can only be used to control certain rotations that can be achieved within this synchronization range.

In this new paper, the researchers use an electronic spin defect near the NV center to detect and control an additional spin, which creates 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 core from all the electronic spins interacting with it. Then, they select another microwave pulse to connect the NV core to a nearby vortex.

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Unlike 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 the researchers discovered and characterized the first-order spin, they could switch the polarity of the NV to this first-order spin through magnetic interaction by applying microwaves to both spins simultaneously. 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 interacting second-layer spin.

Controlling the chain of dark cycles

This SEDOR process allows researchers to identify and characterize a new, unique defect located outside the coherence range of the NV core. To control this distant spin, they carefully use a specific series of microwave pulses that help transfer the polarity along the chain from the NV center to this second-layer spin.

„This sets the stage for building large quantum registers for high-order spins or long spin chains, and shows that we can find these new defects that were previously undiscovered by scaling this technique,” says Unger.

To control a vortex, the microwave pulses must be very close to the resonant frequency of that vortex. Small drifts in the test setup due to temperature or vibration can throw off microwave pulses.

The researchers were able to optimize their protocol to send precise microwave pulses, which enabled them to effectively identify and control second-layer vortices, Ungar says.

„We're looking for something unknown, but at the same time, the environment isn't static, so you don't know if what you're finding is just noise. Once you start seeing promising things, you can make your best effort in that one direction. But before you get there, it's a leap of faith,” Cappellaro said. says

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Although they were able to effectively demonstrate a three-loop chain, the researchers estimate that they can scale their method to the fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further upgrades, they can scale over 10 layers.

In the future, they plan to continue to develop their technique to efficiently characterize and probe other electronic spins in the environment and explore the different types of defects that can be used to create qubits.

This research was supported in part by the US National Science Foundation and the Canada First Research Special Fund.

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