Record-breaking experiment could solve huge challenge in quantum computing

Two atoms blown up to almost comical size and cooled to a fraction above absolute zero have been used to generate a robust and incredibly fast two-qubit quantum gate that could help overcome some of the lingering challenges in quantum computing.

Given that a two-qubit gate is the cornerstone of efficient quantum computers, this breakthrough has huge implications. This could lead to a new type of quantum computer architecture that would overcome the current limits of noiseless quantum operations.

Qubit is a contraction, abbreviation of the term “quantum bit”. It is the quantum computing equivalent of a conventional bit – the basic unit of information on which computing technology is based.

To solve an old-fashioned problem, the information (and the logic used to calculate it) is represented by a binary system. Like a light switch, the units that make up this system are all in an exclusive on or off state. Or, as they are often described, as one or zero.

What makes quantum computing so much more powerful is that qubits can be both simultaneously, in a state known as quantum superposition. By itself, a qubit is not really a computer. Combined (or entangled) with the superpositions of other qubits, however, they can represent very powerful algorithms.

The two-qubit gate is a logic operation based on the quantum state of two entangled qubits. It is the simplest component of a quantum computer, allowing qubits to be both entangled and read.

Scientists have been experimenting with quantum gates based on different materials for some time and have achieved some extraordinary breakthroughs. However, one problem remained prominent: superpositions of qubits can degrade quickly and easily through external sources that also become entangled.

Accelerating the gate is the best way to solve this problem: since this intrusion is usually slower than a millionth of a second (a microsecond), a quantum gate faster than this will be able to “overrun” the noise to produce calculations.

To achieve this goal using a slightly different approach than usual, a team of researchers led by physicist Yeelai Chew of Japan’s National Institutes of Natural Sciences turned to a complicated setup.

The qubits themselves are atoms of the metal rubidium in a gaseous state. Using lasers, these atoms were cooled to near absolute zero and positioned a precise micron-scale distance from each other using optical tweezers – laser beams that can be used to manipulate atomic scale objects.

Then the physicists pulsed the atoms with lasers. This threw electrons from the closest orbital distance to each of the atomic nuclei into a very wide orbital split, swelling the atoms into objects known as Rydberg atoms. This produced a periodic 6.5 nanosecond exchange of orbital shape and electronic energy between the now huge atoms.

By using more laser pulses, the research team was able to perform a quantum gate operation between the two atoms. The speed of this operation was 6.5 billionths of a second (nanoseconds) – more than 100 times faster than any previous experiments with Rydberg atoms, the researchers said, setting a new record for quantum gates based on on this particular type of technology.

That doesn’t quite beat the global record for the fastest two-qubit quantum gate operations to date. This was achieved in 2019, using phosphorus atoms in silicon, achieving a mind-blowing time of 0.8 nanoseconds; but the new work involves a different approach that could work around some of the limitations of other types currently in development.

Additionally, exploring different architectures could lead to clues that help minimize the shortcomings of other types of hardware.

The next steps, according to the team, are pretty clear. They should replace the commercial laser with a specially designed laser, to improve accuracy, as the laser can contribute noise; and implement better control techniques.

The research has been published in Nature Photonics.

Sherry J. Basler