Solid-state spin defects, especially nuclear spins with potentially achievable long coherence times, are compelling candidates for quantum memories and sensors. However, their current performances are still limited by dephasing due to variations in their intrinsic quadrupole and hyperfine interactions.
MIT scientists proposed an unbalanced echo to overcome this challenge. The team developed a protocol to extend the life of quantum coherence. Their method achieves a 20-fold increase in the coherence times for nuclear-spin qubits.
The team created a strategy they called an “unbalanced echo” to increase the system’s coherence time, much as how noise-cancelling headphones use particular sound frequencies to filter out background noise.
The team was able to extend coherence times from 150 microseconds to as long as 3 milliseconds by understanding how one type of noise—in this case, heat—affected nuclear quadrupole interactions in the system.
Guoqing Wang, Ph.D. ’23, a recent doctoral student in Cappellaro’s lab who is now a postdoc at MIT, said, “In theory, we could even improve it to hundreds or even thousands of times longer. But in practice, there may be other noise sources in the system, and we’ve shown that if we can describe them, we can cancel them.”
Dmitry Budker, leader of the Matter-Antimatter Section of the Helmholtz Institute Mainz, professor at the Johannes Gutenberg University and the University of California at Berkeley, said, “The paper will have a “significant impact” on future work on quantum devices.”
“In this work, they demonstrate a practical way to stretch nuclear coherence time by an order of magnitude with an ingenious spin-echo technique that should be relatively straightforward to implement in applications.”
The experiments and calculations discussed in the paper deal with a sizable ensemble of nitrogen-vacancy centers, or NV centers, which are atomic-scale impurities in diamond. Each center has a nearby localized electron and a unique quantum spin state for the nitrogen-14 nucleus.
The difficulty was figuring out how to get big ensembles of NV centers to cooperate. However, they have long been recognized as an ideal choice for quantum sensors, gyroscopes, memory, and more.
Wang says. “What we see is when you prepare all these clocks, they are initially in sync with each other at the beginning, but after some time, they completely lose their phase. We call this their de-phasing time.”
“We want to use a billion clocks but achieve the same de-phasing time as a single clock. That allows you to get enhancements from measuring multiple clocks, but at the same time, you preserve the phase coherence, so you don’t lose your quantum information as fast.”
The underlying theory of temperature heterogeneity-induced de-phasing, which relates to the material’s properties, described a theoretical approach for calculating how temperature and strain affect different types of interactions, which can lead to decoherence.
The nitrogen nucleus behaves as an incomplete nuclear dipole, or in other words, as a subatomic magnet, which causes the initial interaction, known as nuclear quadrupole interaction. Wang argues that the dipole essentially interacts with itself, is disrupted because the nucleus is not exactly spherical.
The interaction between the magnetic dipoles of the adjacent electron and nucleus causes hyperfine interaction. These interactions are spatiotemporally variable, and dephasing can occur when a group of nuclear spin qubits is considered because “clocks at different locations can get different phases.”
Scientists theorized that characterizing how those interactions were affected by heat would allow them to offset the effect and extend coherence times for the system.
Wang said, “Temperature or strain affects both of those interactions. The theory we described predicted how temperature or strain would affect the quadrupole and hyperfine. Then the unbalanced echo we developed in this work essentially cancels out the spectral drift due to one physical interaction using another different physical interaction, utilizing their correlation induced by the same noise.”
“The key novelty of this work, compared to existing spin echo techniques commonly used in the quantum community, is to use different interaction noises to cancel each other such that the noises to be canceled can be highly selective. What’s exciting, though, is that we can use this system in other ways.”
“So, we could use this to sense temperature or strain field spatiotemporal heterogeneity. This could be good for biological systems, where even a very minute temperature shift could have significant effects.”
This technology can monitor strain fields, which makes it useful for non-destructively assessing the structural health of buildings. It can also be used to check electrical currents in electric vehicles. If a bridge had these sensors, for example, we could determine its stress level. In reality, diamond sensors are already employed to assess temperature distribution on the surface of materials due to their potential as a susceptible, high spatial resolution sensor.
Li says, “Another application may be in biology. Researchers have previously demonstrated that using quantum sensors to map neuronal activity from electromagnetic fields could offer potential improvements, enabling a better understanding of some biological processes.”
“The system described in the paper could also represent a significant leap forward for quantum memory.”
Even though there are several current methods for increasing the coherence time of qubits for use in quantum memory, those procedures are difficult and frequently require “flipping” the NV centers or reversing their spin. While reversing the spectral drift that causes decoherence, that process also results in the loss of any information encoded in the system.
The new system advances quantum computing by preventing data loss and extending the coherence period of the qubits by doing away with the requirement to reverse the spin.
The scientists will continue to look into new noise sources in the system, such as fluctuating electrical field interference, to find ways to mitigate them and extend coherence time.
Li said, “Now that we’ve achieved a 20-fold improvement, we’re looking at how we can improve it even more because intrinsically, this unbalanced echo can achieve an almost infinite improvement.”
“We are also looking at how we can apply this system to the creation of a quantum gyroscope because coherence time is just one key parameter to building a gyroscope, and there are other parameters we’re trying to optimize to (understand) the sensitivity we can achieve compared to previous techniques.”