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    Critical thoughts on quantum technologies

    An Innovative Approach: Enhancing Quantum Devices by Suppressing Noise

    ByThemba Hadebe

    Nov 16, 2023
    An Innovative Approach: Enhancing Quantum Devices by Suppressing Noise

    Noise cancellation technology has recently made an unexpected leap forward, this time not in the world of music, but in the realm of quantum computing. A group of physicists at MIT, led by Ju Li and Paola Cappellaro, has successfully utilized noise-canceling principles to improve the coherence times of quantum devices.

    Quantum computers rely on qubits, or quantum bits, to process and store information. However, qubits are notoriously fragile, quickly losing their quantum state due to interactions with their environment. Prolonging the coherence time of qubits is a crucial step in developing practical quantum technologies such as sensors and memories.

    The MIT team discovered a method, inspired by noise-canceling headphones, to achieve a remarkable 20-fold increase in coherence times for nuclear-spin qubits. The researchers developed an “unbalanced echo” approach that mitigates the detrimental effects of noise on the qubits.

    By understanding how heat-induced noise affects nuclear quadrupole interactions in the system, the team harnessed this very noise to counteract the nuclear-electron interactions. This ingenious technique extended coherence times from 150 microseconds to an impressive 3 milliseconds.

    Leading author Guoqing Wang, who devised the protection protocol, believes further advancements are possible. By exploring other sources of noise, the improvements could potentially be extended by orders of magnitude. The team hopes that their findings will open up new avenues of research and improve the efficiency of quantum devices.

    Some experts in the field are highly optimistic about the impact of this research. Dmitry Budker, a professor at Johannes Gutenberg University and the University of California at Berkeley, praises the MIT group as world leaders in quantum sensing. He believes their practical approach to extending nuclear coherence time will have a significant influence on future quantum device development.

    Gregory Fuchs, a professor at Cornell University, also recognizes the importance of this work. Fuchs applauds the team’s unexpected strategy for achieving long-lived nuclear-spin ensembles, which has been challenging in previous experiments. He highlights the potential applications of this technique in rotation sensing, or gyroscopes.

    To conduct their experiments, the researchers focused on atomic-scale impurities in diamond called nitrogen vacancy centers (NV centers). These NV centers consist of a nitrogen-14 nucleus and a nearby electron, each in a specific quantum spin state. Working with large ensembles of approximately 10 billion NV centers, the team aimed to synchronize their quantum states.

    The analogy of clocks is used to describe the challenge they faced. Each NV center’s quantum state behaves like an individual clock, but they are not perfectly in sync with each other. Over time, these clocks lose their phase coherence, resulting in the loss of quantum information. The researchers aimed to achieve the same de-phasing time as a single clock while utilizing the enhanced measurement capabilities of multiple synchronized clocks.

    The team’s earlier theoretical work on temperature heterogeneity-induced de-phasing set the foundation for this experimental breakthrough. They investigated how temperature and strain affect different types of interactions, leading to decoherence. Two key interactions, the nuclear quadrupole interaction and the hyperfine interaction, were found to vary across space and time, causing de-phasing in the nuclear spin qubits.

    The MIT researchers now look forward to further exploring and characterizing different sources of noise to enhance the coherence times even more. Their work is a testament to the creative application of noise-canceling concepts to tackle one of the most critical challenges in quantum information.

    Frequently Asked Questions (FAQ)

    What are qubits?

    Qubits, short for quantum bits, are the fundamental building blocks of quantum computers. Unlike classical bits, which can represent either a 0 or a 1, qubits can exist in both states simultaneously due to the principles of quantum mechanics. This property, called superposition, allows qubits to perform complex calculations much faster than classical bits.

    What is coherence time?

    Coherence time refers to the duration in which a qubit can maintain its quantum state before decoherence occurs. Decoherence happens when a qubit interacts with its environment, causing it to lose its superposition and become entangled with surrounding particles. Increasing coherence time is crucial for the stability and accuracy of quantum devices.

    What is noise cancellation?

    Noise cancellation is a technique used to reduce unwanted background noise by generating an opposing sound wave that cancels out the original noise. This technology is commonly employed in devices such as headphones to enhance the listening experience by minimizing external disturbances.

    What are nitrogen vacancy centers (NV centers)?

    Nitrogen vacancy centers are atomic-scale defects found in diamond crystals. They consist of a nitrogen atom substituting a carbon atom in the diamond lattice, resulting in a missing neighboring carbon atom. The nitrogen atom and the adjacent vacancy, along with an electron, form a specific quantum system that can be manipulated and utilized for various applications in quantum information processing.

    – [MIT: Canceling noise to improve quantum devices](https://news.mit.edu/2021/canceling-noise-improve-quantum-devices-1203)