Researchers are developing a protocol to extend the lifetime of quantum coherence

For years, researchers have tried different ways to trick quantum bits—or qubits, the basic building blocks of quantum computers—to stay in their quantum state for long periods of time, a key step in creating devices like quantum sensors, gyroscopes, and memories. .

A group of physicists from MIT has taken an important step forward in this need, and to do so, they borrowed an idea from an unlikely source – noise canceling headphones.

Led by Ju Li, Battelle Energy Alliance Professor of Nuclear Engineering and professor of materials science and engineering, and Paola Cappellaro, Ford Professor of Engineering in the Department of Nuclear Science and Engineering and Electronics Research Lab, and professor of physics, the team described how to achieve a two-fold increase in 20 times the compatibility of nuclear weapons in orbit.

The work is described in a paper published in the Physical review letters. The first author of the study is Guoqing Wang Ph.D. ’23, a postdoctoral fellow in Cappellaro’s lab who is now a postdoc at MIT.

“This is one of the main problems in quantum information,” Li said. “Nuclear spin (ensembles) are very attractive platforms for quantum sensors, gyroscopes, and quantum memory, (but) they have correlation times on the order of 150 microseconds in the presence of electronic spins… and information It just disappears. What we have. It’s shown that, if we can understand the interaction, or the noise, in these processes, we can do a lot better.”

Increasing correlation and ‘unbalanced echo’

In the same way that noise-cancelling headsets use certain sounds to filter out the surrounding noise, the team has developed a method they call “unbalanced echo” to increase the compatibility time of the system.

By showing how a certain source of noise—in this case, heat—affects the nuclear quadrupole interaction in the system, the team was able to use that same noise source to eliminate the nuclear electron interaction, increasing the coherence times from 150 microseconds to microseconds. 3 milliseconds long.

That improvement, however, may be only the beginning. More progress is possible, said Wang, the first author of the study that came up with the safety protocol, as they explore other potential sources of noise.

“In theory, we could even improve it to hundreds or thousands of times longer. But in practice there may be other sources of noise in the system, and what we have shown is that if we can define it, we can cancel it. .”

The paper will have a “significant impact” on future work on quantum devices, says Dmitry Budker, leader of the Matter-Antimatter Division of the Helmholtz Institute Mainz, professor at the Johannes Gutenberg University and the University of California at Berkeley, who was not present. participate in research.

“[This group is] world leaders in the field of quantitative understanding.” They are constantly inventing new ways to promote progress in this successful field. In this work, they demonstrate an efficient way to extend the nuclear coherence time by an order of magnitude for the spin-echo technique that should be easy to implement in applications.”

Cornell University professor of applied physics and engineering Gregory Fuchs calls the work “innovative and impactful.”

“This thing [work] “It is important because although the nuclear spin can have a long-lasting coherence principle than the electron spins born in NV centers, it has been a challenge for anyone to see long-lived nuclear spin ensembles in the experiments of the diamond NV center,” he says. Professor Cappellaro and his students have shown that it is an unexpected strategy to do so. This approach could have major implications for applications of nuclear spin ensembles, such as spin detection. [a gyroscope].”

Building a sensor using ’10 billion clocks’

The experiments and calculations described in the paper deal with a large assembly—about 10 billion—of atomic-scale impurities in diamond known as nitrogen centers, or NV centers, each of which exists in a specific quantum spin position of nitrogen-14. the nucleus, and the electron of the nearest place.

While they have already been identified as a suitable candidate for quantum sensors, gyroscopes, memories and more, the challenge, Wang explains, lies in working out how to get large ensembles of NV centers to work together.

“If you think that each spin is like a clock, these 10 billion clocks are slightly different… and you can’t measure them all individually,” Wang said. “What we see when you set up all these clocks, they start synchronizing with each other at first, but after a while, they completely lose their phase.

“The goal is to use billions of clocks but achieve the same stopping time as one clock,” he continues. “That allows you to get improvements in measuring multiple clocks, but at the same time you maintain phase coherence, so you don’t lose your quantum information too quickly.”

The basic theory of the heterogeneity of de-phasing, which is related to the structures of the devices, was first described in March by a group of researchers including Li, Cappellaro, Wang, and other MIT graduate students. That paper, published in Journal of Physical Chemistry Lettersdescribed a theoretical method to calculate how temperature and pressure affect different types of interactions that can lead to incompatibility.

The first, known as the nuclear quadrupole interaction, occurs because the nitrogen nucleus acts as an imperfect nuclear dipole – essentially a subatomic magnet. Because the nucleus is not perfectly spherical, Wang explains, it deforms, disrupting the dipole, which interacts effectively with itself.

Similarly, the hyperfine interaction is the result of the nucleus magnetic dipole interacting with the nearby electron magnetic dipole. Both of these two types of interactions can vary from place to place, and when considering an assembly of nuclear spin qubits, de-phasing can occur because “clocks in different places can get different phases.”

Based on their previous paper, the team said that if they can show how those interactions are affected by temperature, they will be able to eliminate the effect and increase the system’s compatibility times.

“Temperature or gravity affects both of these interactions,” Wang said. “The theory we described predicted how heat or gravity could affect the quadrupole and the hyperfine, and so the unbalanced echo that we developed in this work essentially cancels the spectral drift due to the interaction of one body using another different body interaction, using their alignment caused by the movement visible. the same noise.”

The main novelty of this work, compared to existing spin echo methods commonly used in the quantum community, is to use different interaction noises to cancel each other out so that the noise to be canceled can be very selective. “The most exciting thing is that we can use this system in other ways,” he continues. “Therefore, we can use this to understand the temperature or mass of the spatiotemporal heterogeneity. This can be good for something like biological processes, where minute temperature changes can have important consequences.”

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Those applications, Wang said, barely scratch the surface of the system’s possible uses.

“This system can be used to monitor electric currents in electric vehicles, and because it can measure pressure points, it can be used for non-destructive health testing,” Li said. “You can imagine a bridge, if it had these sensors on it, we can understand what kind of pressure it receives. In fact, diamond sensors are already used to measure the heat transfer on the surface of the material, because it can be very difficult. , a sensor with a high vision of the area.”

Another application, Li says, could be in biology. Researchers have previously shown that the use of quantum sensors to map neuronal activity from electromagnetic fields can provide potential improvements, enabling a better understanding of other biological processes.

The process described in the paper could also represent a significant leap forward in quantum memory.

While there are ways to increase the coherence time of qubits for use in quantum memory, those processes are complex, and usually involve “reversing”—or reversing the spin—of the NV centers. Although that process works to reverse the spectral drift that causes inaccuracy, it also results in the loss of any encoded information.

By eliminating the need to change the spin, the new system not only increases the time of coherence of the qubits, but also prevents data loss, a key step forward in quantum computing.

Going forward, the team plans to investigate additional sources of noise – such as electric field disturbances – in the system with the aim of counteracting them to further increase the synchronization time.

“Now that we have achieved a 20-fold improvement, we are looking at how we can improve it further, because internally, this uneven echo can achieve infinite improvement,” Li said.

“We are also looking at how we can use this system in the creation of a quantum gyroscope, because the coherence time is another key parameter for building a gyroscope, and there are other parameters that we are trying to increase (to understand) the sensitivity. we can reach compared to previous methods.”