A method developed by scientists at the University of Rochester overcomes the limitations of electron spin resonance.

in a published article Natural PhysicsA team of researchers at the University of Rochester, including associate professor of physics John Nicol, describes a new method for controlling the spin of electrons in silicon quantum dots—tiny nanoscale semiconductors with remarkable properties—as a means of manipulating information in a quantum system. .

“The results of the study provide a promising new mechanism for electron spin-based coherent qubit control in semiconductor quantum dots, which may pave the way for the development of a practical silicon-based quantum computer,” said Nicol.

Using quantum dots as qubits

A typical computer consists of billions of transistors called bits. Quantum computers are based on quantum bits called qubits. Unlike ordinary transistors, which can be either “0” (off) or “1” (on), qubits are governed by the laws of quantum mechanics and can be both “0” and “1” at the same time.

Scientists have long considered using silicon quantum dots as qubits; controlling the spin of electrons in quantum dots would offer a way to manipulate the transmission of quantum information. Each electron in a quantum dot has an intrinsic magnetism, like a tiny bar magnet. Scientists call this “electron spin”—the magnetic moment associated with each electron—because each electron is a negatively charged particle that behaves like it’s spinning rapidly, and it’s this actual motion that gives rise to magnetization.

Electron spin is a promising candidate for information transfer, storage, and processing in quantum computing because it offers long sequence times and high fidelity, and is compatible with advanced semiconductor fabrication techniques. The coherence time of a qubit is the time before the quantum information is lost due to interaction with a noisy environment; a long sequence means a longer time to perform calculations. High gate fidelity means that the quantum operation the researchers are trying to perform is performed exactly as they want it to be.

However, one of the main challenges in using silicon quantum dots as qubits is controlling the electron spin.

Control the spin of electrons

A standard method for controlling electron spin is electron spin resonance (ESR), which involves applying radio-frequency magnetic fields to qubits. However, this method has several limitations, including the need to precisely generate and control oscillating magnetic fields in cryogenic environments where most electron spin qubits are mined. Typically, to create oscillating magnetic fields, researchers send a current through a wire that generates heat that can disrupt cryogenic environments.

Nicol and his colleagues describe a new method for controlling the spin of electrons in silicon quantum dots that does not rely on oscillating electromagnetic fields. The method is based on the phenomenon called “spin-valley coupling”, which occurs during the transition of electrons from one spin state to another in silicon quantum dots. While the spin state of an electron refers to its magnetic properties, the valley state refers to a different property related to the electron’s spatial profile.

Researchers apply a voltage pulse to exploit the spin-valley coupling effect and manipulate spin and valley states by controlling the spin of electrons.

“This method of coherent control via spin-valley coupling allows for universal control over qubits and can be implemented without the need for oscillating magnetic fields, which is a limitation of ESR,” says Nicol. “This opens up a new way for us to use silicon quantum dots to manipulate information in quantum computers. »