A research group led by Assistant Professor Yuhei Sekiguchi and Professor Hideo Kosaka of the Yokohama National University's (YNU) Faculty of Engineering /Institute of Advanced Studies has successfully controlled spin qubits consisting of nitrogen-vacancy centers (NV centers) in diamond with high spatial resolution and high fidelity using an optical address quantum gating method that combines microwaves and lasers. As a core technology for large-scale integrated quantum memory and quantum processors using diamond, this accomplishment will contribute to dramatic improvements in the performance of quantum computers and quantum communications and pave the way for the construction of the quantum Internet. Professor Kosaka said, 'So far, we have been conducting research from the material to the individual device stage, but in the future, we would like to proceed with system construction simultaneously.' The findings were published in the journal Nature Photonics.
An NV center has one carbon replaced by nitrogen (N) and the other by a vacancy (V) in two adjacent carbons in a diamond. NV centers have electron spin and nuclear spin. These characteristics are used as spin qubits. Their memory time is much longer than other materials, such as superconducting qubits and semiconductor quantum dots, reaching a maximum of more than one second. There are particularly high expectations for this material as a material for quantum repeaters, which require long-life quantum memory.
Typically, microwaves are used to control NV center qubits. However, since microwaves affect a fixed area, they can impact multiple NVs, making it challenging to manipulate just one qubit. On the other hand, light-based manipulation, while capable of manipulating a single qubit, had low fidelity. This characteristic made it difficult to use for high-precision manipulation.
Focusing on the three-level structure of the NV center's electron and nuclear spins, the group devised a new principle (optical addressable quantum gate) that activates or deactivates the quantum gate of a spin qubit by tuning the excess energy levels through optical Stark-shifts. Specifically, by applying 3 gigahertz microwaves, which do not change spin, and then applying a 637 nanometer laser to the NV center to be manipulated, only the spin of that NV center is changed.
The group's experiment demonstrated the operation of electron spin qubits with all of the fundamental quantum controls of initialization, gate manipulation, readout, and retention. It also demonstrated that spatial resolution of approximately 300 nanometers and fidelity of up to 95% could be achieved. This result means that correct control can be achieved 95% of the time without affecting NV centers that are more than approximately 300 nanometers away from the NV center to be controlled. Furthermore, the group showed that nuclear spin qubits could also be activated by light using electron spin as a mediator and succeeded in generating quantum entanglement of electron-nuclear spin qubits.
Professor Sekiguchi said, 'We are currently operating one NV center at a time, but in the future, we will aim for control in conjunction with neighboring NV centers.' Professor Kosaka said, 'For actual use as memory, the nuclear spin of carbon-13 in diamond will be manipulated through quantum teleportation by light striking electrons. We are already on track to realize this technique. During this experiment, we operated at 5 Kelvin, but we can raise the temperature to about 20 Kelvin.'
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