UC Riverside Researchers Advance Quantum Computing with Spin Centers

Physicists at the University of California, Riverside, working with colleagues at the University of Iowa and Chongqing University, have set out a design for a quantum simulator made from spin centers, tiny defects in a crystal that act like individual quantum magnets. The work is a theoretical proposal backed by large-scale computer calculations, not a laboratory demonstration. It appeared in Physical Review B on July 8, 2024, and UC Riverside described it in a press release on July 11.

What the researchers proposed

A quantum simulator is a purpose-built quantum device. Unlike a general-purpose quantum computer, which runs arbitrary programs on qubits, a simulator is designed to reproduce one family of physical models. Researchers then change its settings and measure how it responds, learning about systems that are too complex for ordinary computers to calculate. Tsai explained in the university's release that giving up full programmability can make simulators easier to build and useful sooner.

The paper, titled "Quantum simulation of the spin-1/2 XYZ model using solid-state spin centers," has six authors: Troy Losey and Shan-Wen Tsai of UC Riverside, Denis R. Candido, Yannick Meurice and Michael E. Flatté of the University of Iowa (Flatté is also affiliated with Eindhoven University of Technology), and Jin Zhang of Chongqing University. According to the paper's abstract, the team considered an evenly spaced line of spin-1 defects that interact through magnetic dipole-dipole forces, the same basic attraction and repulsion that acts between bar magnets. With an external magnetic field applied, the authors show that this chain behaves like a simpler, well-studied model from condensed-matter physics called the XYZ spin chain.

Two settings act as control knobs: the strength of the magnetic field and the angle between the chain and the defects' symmetry axis. Adjusting them moves the system between different textbook regimes, including the isotropic Heisenberg model and the class of behavior described by the transverse-field Ising model, a standard reference point for quantum phase transitions.

The floating phase and why it matters

Using a numerical method called the density-matrix renormalization group, the team mapped the system's phase diagram. Between the familiar regimes they found a line of "critical floating phases," where the spins form a density wave whose repeat length does not match the spacing of the crystal. These phases are bounded by two kinds of exotic transitions, known as Berezinskii-Kosterlitz-Thouless and Pokrovsky-Talapov transitions. Floating phases have been studied with cold atoms and proposed for Rydberg-atom arrays. The authors say theirs is the first proposal to realize them with spin centers in a solid.

Losey said in the release that phase transitions are interesting because very different systems can behave identically at those points, which hints at shared underlying physics. He also said the fabrication techniques involved overlap with those for spin-center quantum computers, which are seen as a leading route toward quantum hardware that could run at room temperature. That is a long-term motivation for the field, not something this paper demonstrates.

Caveats

The paper is candid about the obstacles. The authors identify nitrogen-vacancy centers in diamond as the best-understood candidate, but note that the nuclear spin of nearby nitrogen atoms would swamp the weak dipole coupling. For this reason they favor silicon-vacancy centers in diamond or divacancies in silicon carbide. The chain would need defects spaced about 10 nanometers apart, nearly in a perfect line. The best reported precision for implanted nitrogen-vacancy centers is about 16 nanometers, plus or minus 5.

The biggest hurdle is energy scale. For spin centers 10 nanometers apart, the coupling is equivalent to about 2 microkelvin, a temperature the authors call "currently unachievable" in experimental setups. They suggest getting around this by boosting the interactions through photons, phonons or magnons, the quantized waves in magnetic materials. Those hybrid schemes are themselves still in development. Losey acknowledged in the release that researchers are only now getting close to the level of control over spin centers, crystal purity and low temperatures that would be needed to build the device.

The result is a useful blueprint that adds spin centers to the list of platforms that could, in principle, explore exotic quantum magnetism. Whether it works in practice will depend on experiments that have not yet been done.