Japanese researchers conduct experiment to enable next generation of quantum computers

Yokohama National University demonstrate technology that could lead to a fault-tolerant universal quantum computer

Researchers at the Yokohama National University in Japan have conducted an experiment that, they say, could help enable the next generation of fast and reliable quantum computers.

The experiment, which was part of a study published in the journal Nature Communications, demonstrated non-adiabatic and non-abelian holonomic quantum gates under zero-magnetic field at room temperature.

Holominic, which refers to the ability to move in all directions and rotate independently, means the gates were able to move over a geometric spin qubit on an electron or nitrogen nucleus.

This, the researchers said, could give way to building a universal quantum computer - machines that have the potential to solve complex problems much faster than today's conventional computers.

"The geometric phase is currently a key issue in quantum physics," the report states. "A holonomic quantum gate manipulating purely the geometric phase in the degenerate ground state system is believed to be an ideal way to build a fault-tolerant universal quantum computer."

The geometric phase gate or holonomic quantum gate has been experimentally demonstrated in several quantum systems, including nitrogen-vacancy centres in diamond.

However, previous experiments required microwaves or light waves to manipulate the non-degenerate subspace, leading to the degradation of gate fidelity due to unwanted interference of the dynamic phase.

"To avoid unwanted interference, we used a degenerate sub-space of the triplet spin qutrit [quantum trit] to form an ideal logical qubit, which we call a geometric spin qubit, in an NV [nitrogen vacancy] centre," explained the study's author Professor Hideo Kosaka.

"This method facilitated fast and precise geometric gates at a temperature below 10 Kelvin [-263.15 degrees celsius], and the gate fidelity was limited by radiative relaxation.

"Based on this method, in combination with polarised microwaves, we succeeded in manipulation of the geometric phase in an NV centre in diamond under a zero-magnetic field at room temperature."

The group also demonstrated a two-qubit holonomic gate to show universality by manipulating the electron-nucleus entanglement. This rendered a purely holonomic gate without requiring an energy gap, which would have induced dynamic phase interference to degrade the gate fidelity, and thus enabled a more precise and fast control over long-lived quantum memories.

This is something the researchers say is ideal for "realising quantum repeaters interfacing between universal quantum computers and secure communication networks".