Electrically controlling quantum bits in silicon may lead to large quantum computers

April 15, 2015

Electron wavefunction of a donor under an electrostatic gate. A positive voltage applied to the gate attracts the electron towards the Si-SiO2 interface. This modifies the hyperfine coupling, shifts the resonance frequencies of electron and nucleus, and allows addressing of individual donor qubits. (credit: A. Laucht, UNSW Australia)

A UNSW-led research team has encoded quantum information in silicon using simple electrical pulses for the first time, bringing the construction of affordable large-scale quantum computers one step closer to reality.

The idea is to exploit the advanced fabrication methods developed in semiconductor nanoelectronics and create quantum bits (qubits) that are both highly coherent and easy to control and couple to each other — a challenging task.

The findings were published in the open-access journal Science Advances.

The UNSW team, which is affiliated with the ARC Centre of Excellence for Quantum Computation & Communication Technology, was first to demonstrate single-atom spin qubits in silicon, reported in Nature in 2012 and 2013. The team later improved the control of these qubits to an accuracy of above 99% and established the world record for how long quantum information can be stored in the solid state, as published in Nature Nanotechnology in 2014.

Controlling individual qubits with electric fields

The researchers have now demonstrated a key step that had remained elusive since 1998: using electric fields instead of pulses of oscillating magnetic fields.

Lead researcher Andrea Morello, a UNSW Associate Professor from the School of Electrical Engineering and Telecommunications, said the method works by distorting the shape of the electron cloud attached to the atom, using a very localized electric field. “This distortion at the atomic level has the effect of modifying the frequency at which the electron responds. Therefore, we can selectively choose which qubit to operate.”

The findings suggest that it would be possible to locally control individual qubits with electric fields in a large-scale quantum computer using only inexpensive voltage generators, rather than requiring expensive high-frequency microwave sources.

Moreover, this specific type of quantum bit can be manufactured by placing qubits inside a thin layer of specially purified silicon, containing only the silicon-28 isotope. “This isotope is perfectly non-magnetic and, unlike those in naturally occurring silicon, does not disturb the quantum bit,” Morello said.

The purified silicon was provided through collaboration with Keio University in Japan.


Abstract of Electrically controlling single-spin qubits in a continuous microwave field

Large-scale quantum computers must be built upon quantum bits that are both highly coherent and locally controllable. We demonstrate the quantum control of the electron and the nuclear spin of a single 31P atom in silicon, using a continuous microwave magnetic field together with nanoscale electrostatic gates. The qubits are tuned into resonance with the microwave field by a local change in electric field, which induces a Stark shift of the qubit energies. This method, known as A-gate control, preserves the excellent coherence times and gate fidelities of isolated spins, and can be extended to arbitrarily many qubits without requiring multiple microwave sources.