FEATURE QUANTUM COMPUTING
Speeding up the quantum game
Researchers have developed quantum computing hardware that dramatically improves on the quantum gate speeds of prior technologies
Martin Weides, Professor of Quantum Technologies at the University of Glasgow, remarked on the development
E
arlier this year, researchers from the National Institutes of
Natural Sciences’ Institute for Molecular Science in Okazaki, Japan announced in Nature Photonics their execution of what they say is the world’s fastest two- qubit gate, which operates in just 6.5 nanoseconds. This was achieved using a new method in which an ultrafast picosecond laser was used to manipulate rubidium atoms cooled to almost absolute zero. The previous record, 15 nanoseconds, was achieved by Google AI in 2020 using superconducting circuits. According to the
researchers, the work represents a breakthrough in the form of new cold- atom quantum computer hardware that breaks through the limitations of other superconducting and trapped- ion formats currently under development.
Quantum gates explained For the past two decades, quantum computer hardware innovation has been chasing faster gates: the basic arithmetic elements that make up quantum computing. These gates correspond to the logic gates such as AND and OR used in conventional classical computers. There are one-qubit gates
that manipulate the state of a single qubit, and two-qubit gates that generate quantum entanglement between two qubits. The latter is the source
30 Electro Optics November 2022
Conceptual diagram of the world’s fastest two-qubit gate. Two atoms captured in optical tweezers (red light) with a separation of a micrometre are manipulated by an ultrafast laser pulse (blue light) shone for only 10 picoseconds
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of the high-speed performance of quantum computers. The accuracy (fidelity) of a quantum gate can easily be degraded by noise from the external environment and the operating laser, which is one of the many factors making the development of quantum computers so challenging. This is why researchers around the world are striving for faster gates – to escape the effects of external noise.
Outpacing noise In the researchers’ new method, two rubidium atoms in the gas phase are cooled to an ultra-low temperature of about 1/100,000 of a Kelvin using laser beams, and then arranged at a micron interval via optical tweezers – a laser beam tightly focused to a size of less than a micron, towards which atoms are attracted and trapped. The
atoms are then irradiated with ultrashort laser pulses of only 10 picoseconds in duration. This knocks two electrons
trapped respectively in the smallest orbitals of the two adjacent atoms, into giant electronic orbitals – turning them into what’s known as Rydberg atoms. The interaction between these giant atoms then leads to a periodic back and forth exchange of the orbital shape and electron energy, occurring with a period of 6.5 nanoseconds. After one oscillation, the laws of quantum physics dictate that the sign of the wavefunction is flipped, thus realising the two-qubit gate. According to the researchers,
since the time-scale of external noise is generally slower than one microsecond, the new, ultrafast quantum gate is able to avoid the degradation of calculation accuracy due to noise, which they say brings
the world much closer to realising a practical cold-atom quantum computer. Cold-atom based quantum computers have been rapidly attracting attention from industry, academia, and governments around the world in recent years, as revolutionary hardware that breaks through some of the limitations of superconducting and trapped- ion quantum computers – currently the most advanced types of quantum computers.
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