Australian team achieves new silicon quantum bits for electrical signal control

Recently, the University of New South Wales Sydney (UNSW) team demonstrated a new type of silicon quantum bit [1], the flip-flop quantum bit, which could facilitate the construction of large-scale quantum computers.

Dr. Tim Botzem, Prof. Andrea Morello and Dr. Rostyslav Savytskyy in the Quantum Computing Lab at UNSW.

Electrical signal control of single-atom quantum bits

A team led by Professor Andrea Morello has just demonstrated the operation of a new type of quantum bit: the trigger quantum bit, which combines the quantum properties of single atoms and can be easily controlled using electrical signals.

"Sometimes, new quantum bits, or new modes of operation, are discovered by lucky 'accident'. But this time the new quantum bits were completely designed." Professor Morello said [2], "Our group had excellent quantum bits ten years ago, but we wanted something that could be controlled electrically for maximum ease of operation. Therefore, we had to invent something completely new."

Professor Morello's group is the first in the world to demonstrate the use of electron spin as well as the nuclear spin of a single phosphorus atom in silicon as a unit of quantum bits. He explained that while both quantum bits are very good on their own, their operation requires the support of oscillating magnetic fields (oscillating magnetic fields). "Magnetic fields are difficult to locate on the nanoscale, which is the typical size of a single quantum computer component. That is why the first proposal for silicon quantum bits envisioned placing all the quantum bits in a uniform oscillating magnetic field; applying it to the entire chip and then using the local electric field to select which quantum bits are operated."

A few years ago, Prof. Morello's team realized that by defining quantum bits as a combined up-down/down-up orientation of electrons and nuclei, it would allow to control such quantum bits using electric fields only. Today's results confirm this idea, says Dr. Rostyslav Savytskyy, one of the main experimental authors of the paper: "Such new quantum bits are called 'trigger' quantum bits, because they consist of two spins belonging to the same atom: the electron and the nuclear spin , and that they always point in opposite directions."

"For example, if the |0⟩ state is 'electron-down/nucleus-up' and the |1⟩ state is 'electron-up/nucleus-down', from the |0 ⟩ to |1⟩ means that the electron 'flips' up and the nucleus 'flips' down. Hence the name!" The theory predicts that arbitrary quantum states of the trigger quantum bits can be programmed by displacing the electron relative to the nucleus.

Trigger quantum bits

"Our experiments confirm this prediction with perfect accuracy," said Dr. Tim Botzem, the other lead experimental author. "Most importantly, such an electron displacement is obtained simply by applying a voltage to a small metal electrode, rather than irradiating the chip with an oscillating magnetic field . This is a method that more closely resembles the electrical signals within conventional silicon computer chips, as we use every day in computers and smartphones."

Promising applications for large quantum processors

The electrical control of trigger quantum bits by moving electrons away from the nucleus is accompanied by a very important side effect: when a negative charge (electron) is moved away from a positive charge (nucleus), an electric dipole is formed; placing two (or more) electric dipoles near each other creates a strong electrical coupling between them, which can mediate quantum logic operations with multiple quantum bits -- an operation that is necessary to perform useful quantum computations.

Coherent electrical drive. The data clearly show the dynamics of the trigger quantum bits: that is, the directions of the two spins are swapped several times as the electric drive signal is gradually applied.

"The standard approach to coupling spin qubits in silicon is to place the electrons very close to each other so that they effectively 'touch'," says Prof. Morello.

"This requires placing quantum bits on a grid with a spacing of tens of nanometers; the engineering challenges of doing so are quite severe. In contrast, electric dipoles do not need to 'touch': they interact from a distance. Our theory suggests that 200 nm is the optimal distance for fast, high-fidelity quantum manipulation."

In conclusion, Professor Morello said, "This will probably be a game-changing development. Because 200 nm is far enough away to allow the insertion of various control and readout devices between quantum bits, making the processor easier to wire and operate."

Reference links：

[1]https://www.science.org/doi/10.1126/sciadv.add9408

[2]https://newsroom.unsw.edu.au/news/science-tech/flip-flop-qubit-realisation-new-quantum-bit-silicon-controlled-electric-signals