Diamond quantum computing breakthrough first complete universal quantum gate
The ability to individually control many spins in solid-state crystals (e.g., diamond) is a promising technology for the development of large-scale quantum processors and memories. Local laser fields provide spatial selectivity for electron spin manipulation via spin-orbit coupling, but it is difficult to achieve both precise and universal manipulation.
01Optical methods and microwave control combined
"Microwaves are usually used for individual quantum control, but separate wiring of microwave lines is required." says Hideo Kosaka, corresponding author of the paper and director of the Quantum Information Research Center and professor of physics at the Institute of Advanced Science, Yokohama National University [2], "On the other hand, it is possible to manipulate quantum bits locally, but imprecisely, with light."
Kosaka and other researchers were able to demonstrate that by combining microwave manipulation and local optical shifts in atomic-molecular leap frequencies (Stark shift, Stark shift), using nitrogen vacancy centers in diamond, to manipulate electron spin and thus control quantum bits. In other words, they were able to combine the optical approach, which relies on laser light emission, with microwaves to overcome previous limitations.
The researchers were also able to show that this control of electron spin in turn can control the nuclear spin of the nitrogen atom in the nitrogen vacancy center, as well as the interaction between the electron and the nuclear spin. This makes sense because it enables precise control of quantum bits without wiring problems.
(a)Principle of optically addressable universal integral gate and geometry of the device; (b) Structure of the electronic level with NV-centered spin sublevels; (c) Optically detected magnetic resonance (ODMR) spectra showing frequency shifts; (d) Optical Stark shift versus laser power; (e) Spin dynamics of lasers without addressing; (f) Spin dynamics of lasers with addressing.
Experimental demonstration of optically addressable universal single quantum bit operation. (a,b,c) χ matrix reconstructed by quantum process tomography showing Pauli-X,Y and Z gate operations; (d,e) schematic and experimental results of optically addressable quantum state preparation are shown; (f) experimental results showing optically addressable quantum state initialization; (g) experimental results showing spin with optically addressable Pauli-Y gate.
"Simultaneous irradiation of light and microwaves enables individual and precise control of quantum bits without the need for wiring." Kosaka said, "This paves the way for large-scale quantum processors and quantum memories, which are crucial for the development of large-scale quantum computers."
02Making connections between photons and quantum bits
In addition, the researchers were able to create "quantum entanglement" between electrons and nuclear spins to transfer a photon state to a nuclear spin state. This allows quantum bit-to-bit connections with photons, which will eventually require less computing power and enable the transfer of information to quantum processors and quantum memory through the principle of quantum invisible transfer of states.
Generation of optically addressable entanglement. (a,d) pulse sequence for optically addressable entanglement generation; (b,e) energy level structure and state transition of the electron-nitrogen nucleus spin at the center of NV; (c,f) absolute value of the density matrix obtained by quantum tomography of states.
The new method satisfies all the "DiVincenzo criteria", which are required for the operation of quantum computers, including scalability, initialization, measurement, universal gates, and long coherence. It can also be applied to magnetic field scenarios other than Stark shift to manipulate quantum bits individually in these scenarios, and can prevent common types of computational errors such as gate errors or environmental noise.
03Towards large-scale quantum computing
"The reason for the improved fidelity of our scheme over the all-optical scheme is the use of redundant degrees of freedom that are easier to control," Kosaka said, with degrees of freedom referring to the number of variables that can be controlled using this method.
This advancement is a step toward larger-scale quantum computing.
"By further increasing the resolution of individual quantum operations and entanglement operations, it will enable large-scale integration of diamond quantum computers, quantum storage and quantum sensors," Kosaka said, "and it will also improve quantum relay networks for long-range quantum communication, distributed quantum computer networks or quantum Internet for data transfer capabilities."
Reference links：
https://www.nature.com/articles/s41566-022-01038-3
https://sciencesources.eurekalert.org/news-releases/959951
