Record-breaking experiment could solve quantum computing’s grand challenge

A fraction of two atoms expanded to an almost ridiculous size and cooled to above absolute zero has been used to generate a powerful, fast two-qubit quantum gate that could help overcome some of the ongoing challenges of quantum computing.

Since two-qubit gates are fundamental building blocks of efficient quantum computers, the breakthrough has enormous implications. It could lead to a new type of quantum computer architecture that pushes the limits of current noiseless quantum operations.

Qubit is an acronym for the word “qubit”. It is the quantum computing equivalent of the traditional bit – the fundamental unit of information on which computing technology is based.

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

What makes quantum computing so powerful is that qubits can exist simultaneously, known as quantum superpositions. By itself, a qubit is not a computer. However, combined with superposition (or entanglement) of other qubits, they can represent some very powerful algorithms.

Two-qubit gates are logical operations based on the quantum states of two entangled qubits. It is the simplest component in a quantum computer, allowing qubits to be entangled and read at the same time.

Scientists have been experimenting with quantum gates based on different materials for some time and have achieved some remarkable breakthroughs. However, one problem remains serious: The superposition of qubits can degenerate quickly and easily because the external sources also become entangled.

Acceleration gates are the best solution to this problem: since this intrusion is typically slower than a millionth of a second (a microsecond), a quantum gate faster than this will be able to “override” the noise to produce accurate calculations.

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

The qubits themselves are gaseous atoms of the metal rubidium. Using a laser, these atoms are cooled to almost absolute zero and positioned with each other at precise micrometer-scale distances using optical tweezers – a laser beam that can be used to manipulate objects on the atomic scale.

Then, physicists pulse the atoms with a laser. This bumps the electrons from the orbital distance closest to each nucleus into a very wide orbital separation, expanding the atom into objects known as Rydberg atoms. This creates a 6.5-nanosecond periodic exchange of orbital shapes and electron energies between the now-giant atoms.

Using more laser pulses, the team was able to perform quantum gate operations between the two atoms. The researchers say the operation took place at a speed of 6.5 billionths of a second (nanosecond) — more than 100 times faster than any previous Rydberg atomic experiment — and set a new record for a quantum gate based on this particular technique.

That doesn’t quite break the overall record for the fastest two-qubit quantum gate operation. This was achieved in 2019, using phosphorus atoms in silicon, in an astonishing 0.8 nanoseconds; but the new work involves a different approach that circumvents some of the limitations of other types currently under development.

Additionally, exploring different architectures may uncover clues that help reduce other types of hardware flaws.

The team says the next steps are fairly clear. They need to replace the commercial laser with a specialized laser to improve accuracy, since the laser produces noise; and implement better control techniques.

The research has been published in nature photonics.

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