The world’s fastest two-qubit gate has been achieved by a research group at Japan’s National Institutes of Natural Sciences. Graduate student Yeelai Chew, Assistant Professor Sylvain de Léséleuc and Professor Kenji Ohmori used atoms cooled to almost absolute zero and trapped in optical tweezers separated by a couple of microns.

This breakthrough ultrafast quantum computer, which operates in only 6.5 nanoseconds, is anticipated to be an entirely new quantum computer type that breaks through the barriers inherent in the superconducting and trapped-ion types now being developed.

Cold-atom quantum computers are based on laser cooling and trapping techniques described by the Nobel Prize winners of 1997 and 2018. These techniques facilitate the arrangement of arrays of cold atoms into arbitrary shapes with optical tweezers and allow each to be observed individually.

## Cold Atom Quantum Computing

Atoms are natural quantum systems. They can easily store quantum bits of information, a quantum computer’s basic building block (qubit). In addition, these atoms are well-isolated from the surrounding environment and independent of one another.

The coherence time (the time during which quantum superposition persists) of a qubit can reach several seconds. A two-qubit gate (an essential fundamental arithmetic element for quantum computing) is then performed by exciting one atom’s electron into a giant electronic orbital, called a Rydberg orbital.

With these techniques, the cold-atom principle has become one of the most promising candidates for quantum computer hardware, attracting attention from industry, academia and governments worldwide. In particular, it has radical potential as it can be easily scaled up while maintaining high coherence compared to the superconducting and trapped-ion types.

## What Is A Quantum Gate

Quantum gates are the arithmetic elements that make up quantum computing. They correspond to the logic gates such as AND and OR in conventional classical computers.

One-qubit gates manipulate the state of a single qubit. Two-qubit gates generate quantum entanglement between two qubits.

The two-qubit gate is the source of the high-speed performance in quantum computers and is technically challenging. The most critical two-qubit gate is called a “controlled-Z gate (CZ gate),” which is an operation that flips the quantum superposition of a first qubit from 0 + 1 to 0—1 depending on the state (0 or 1) of a second qubit.

### Quantum Gate Accuracy And Speed

The quantum gate’s accuracy, or fidelity, is easily degraded by noise from the external environment and the operating laser, making the development of quantum computers difficult. The time scale of such noise is generally slower than one microsecond.

Therefore, if a quantum gate that is sufficiently faster than this can be realized, it will be possible to avoid the degradation of calculation accuracy due to noise and bring us much closer to realizing a practical quantum computer. That is why, for the past 20 years, all quantum computer hardware research has been pursuing faster gates.

The ultrafast gate of 6.5 nanoseconds achieved by this research with the cold-atom hardware is more than two orders of magnitude faster than noise and thus can ignore its effects. The previous world record was 15 nanoseconds, achieved by Google AI in 2020 with superconducting circuits.

## Rubidium Atom Orbitals

The team’s experiment was conducted using rubidium atoms. First, using laser beams, two rubidium atoms in the gas phase that had been cooled to an ultra-low temperature of about 1/100,000 Kelvin were arranged at a micron interval with optical tweezers.

Researchers then irradiated them with ultrashort laser pulses that emitted light for only 1/100 billionth of a second and observed the changes that occurred. Two electrons trapped respectively in the smallest orbitals (5S) of two adjacent atoms (atom 1 and atom 2) were knocked into giant electronic orbitals (Rydberg orbitals, here 43D).

The interaction between these massive atoms led to a periodic, back-and-forth exchange of the orbital shape and electron energy for 6.5 nanoseconds.

After one oscillation, the laws of quantum physics dictate that the sign of the wavefunction is flipped, thus realizing the two-qubit gate (controlled-Z gate). Using this phenomenon, they performed a quantum gate operation using a qubit in which the 5P electronic state is the “0” state, and the 43D electronic state is the “1” state.

Atoms 1 and 2 were prepared as qubits 1 and 2, respectively, and the energy exchange was induced using an ultrashort laser pulse. During one energy-exchange cycle, the sign of the superposition state of qubit 2 was reversed only when qubit 1 was in the “1” state.

The research group experimentally observed this sign flip, thus demonstrating that a two-qubit gate can be operated in 6.5 nanoseconds, the fastest in the world.

#### Original Study:

Chew, Y., Tomita, T., Mahesh, T.P. et al. Ultrafast energy exchange between two single Rydberg atoms on a nanosecond timescale. Nat. Photon. (2022).