Another major breakthrough in creating ultra-fast quantum computers

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A 2-qubit gate is the simplest component of a quantum computer: it makes it possible to create quantum entanglement between two qubits. Superpositions of qubits are unfortunately very sensitive to external perturbations. To circumvent the problem, scientists are trying to create faster and faster quantum gates. A team from the Institute of Molecular Sciences in Okazaki, Japan, has broken a new record in the field thanks to a new technology.

A quantum computer uses the quantum properties of matter to perform operations on data, including the concept of superposition of states: the basic units of information, qubits, can be in several states at once, called quantum superposition—which distinguishes them from traditional qubits used in computing, which can hold the value 0 or 1 precisely. This layer is particularly sensitive to the environment; It degrades quickly and the slightest interaction will result in a change of state and thus errors in the calculation.

To limit these effects, one solution might be quantum gate acceleration. The disturbances observed during the experiments are on the order of microseconds; A quantum gate faster than that would theoretically be able to “bypass” the spurious noise for calculations. To do this, physicist Yeelai Chew and his collaborators built a quantum gate out of large atoms, known as Rydberg atoms.

The world’s fastest binary qubit gate

Rydberg atoms, with their massive electron orbitals, exhibit dipole interactions up to the gigahertz range at a distance of one micrometer, making them prime candidates for performing ultrafast quantum processes. ” These strong interactions between two single atoms have not been exploited so far due to the stringent requirements regarding the variability of atom positions and the necessary excitation strength. The researchers note.

The qubits used in this experiment are gaseous rubidium atoms. The researchers cooled them to a temperature close to absolute zero (to immobilize them), then placed them at a very precise distance from each other (on the order of micrometres) using 3D optical tweezers. They then applied ultrashort (10 picoseconds) laser pulses to simultaneously excite a pair of these nearby atoms into a Rydberg-like state.

A schematic diagram of a qubit using rubidium atoms. © Dr. Takafumi Tomita (IMS)

When the atoms were irradiated by laser pulses, two trapped electrons were successively thrown into the smallest orbitals (denoted by 5p, closest to the atomic nucleus) from two neighboring atoms (denoted as atom 1 and atom 2) in the giants of electron orbitals (Rydberg orbitals, indicated here 43 Dr). The 5p electronic state constitutes state “0” and the 43d electronic state is state “1”; The atoms 1 and 2 were prepared as qubits 1 and 2, respectively. The interaction between these giant atoms then led to a bidirectional exchange of orbital shape and electron energy, occurring in the 6.5 nanosecond period.

advanced quantum computing devices

Thus the researchers succeeded in creating a two-qubit gate called a “controlled Z gate” (or CZ gate), a process that reverses the quantum superposition of the first qubit from 0 +1 to 0 -1 depending on the state (0 or 1) of the second qubit. Usually this type of gate is easily degraded by external noise – especially that inherent in laser operation. But this time, the researchers broke speed records: their quantum gate works in just 6.5 nanoseconds! The team notes that this is more than two times faster than ambient noise by volume, so its effects can be ignored.

Turn on the quantum gate. When atom 1 is in state “0”, nothing happens. When atom 1 is in state 1, the superposition sign of atom 2 changes from positive to negative. This process is at the heart of the algorithm running on quantum computers. © Dr. Takafumi Tomita (IMS)

The previous world record was 15 nanoseconds, the team said in a statement, which was set by Google for artificial intelligence in 2020 with superconducting circuits.

Atoms are natural quantum systems, so they can easily store qubits. In lattices of cold atoms, each well isolated from the surrounding environment and independent of the other, the coherence time of a qubit—that is, the time that quantum superposition persists—can be up to several seconds (which is much higher than quantum systems based on superconductors or trapped ions).

As a result, cold atom rigs are among the most promising candidates for quantum computing devices and are attracting the attention of industry, academia, and governments around the world today. In fact, it goes beyond certain limits for superconducting and trapped quantum computers, which are currently the most advanced types of quantum computers. To further improve the quantum architecture, Chew and colleagues now plan to replace the used laser with a more precise one.

Source: Y. Chew et al., Nature Photonics

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