Researchers at Cornell University (USA) have developed a laser cooling method that could remove one of the main obstacles to larger and faster quantum computers. The work was published on November 9 in the journal PRX Quantum and was led by Karan Mehta, an assistant professor in the Department of Electrical and Computer Engineering at the Duffield College of Engineering.
In trapped-ion quantum computers, information is processed by individual charged atoms (ions) held by electromagnetic fields. Such systems provide very precise control of quantum states, but even the smallest unwanted vibrations interfere with calculations. Therefore the ions must be constantly cooled by lasers almost to the lowest energy state - and this, as the authors note, takes up a surprisingly large share of the computer's operating time.
"One might think that most of the time goes to logic operations," Mehta says. "But cooling, especially to the quantum ground state, turns out to be the main bottleneck in advanced systems."
The team used integrated photonic technology: light is routed along microscopic pathways on the same chip where the atoms are located. The researchers created a standing wave - a pattern of alternating regions of high and low light intensity - by directing two laser beams toward each other. By placing a calcium ion at the point of minimum intensity, they were able to remove motional energy from it and suppress unwanted interactions that usually cause heating.
The method, called electromagnetically induced transparency cooling, brought the target motional mode of the ion close to the quantum ground state in about 150 microseconds. For comparison: a conventional laser scheme requires about 500 microseconds. In addition, the new technique simultaneously cooled all three measured motional modes of the ion in a frequency range of about 5 megahertz. This is important because an ion can oscillate in three dimensions, and each direction can have its own frequency; traditionally, separate cooling operations are required for different types of motion.
"It is usually difficult to cool all modes with a single mechanism," Xing explains. "But we showed that if the frequencies are not too far apart, they can be effectively cooled to the ground state." The researchers also demonstrated cooling below the conventional Doppler limit, experimentally confirming a long-standing theoretical prediction.
A key feature of the work is that the optical components are built into the chip rather than assembled from bulky external lasers. The chip delivers light at several wavelengths needed to control the ion; many groups are developing this approach to scale systems to a larger number of qubits. The chips were fabricated commercially in the Netherlands in collaboration with ETH Zurich, while packaging and preparation of the devices were carried out at Cornell facilities, including the Cornell NanoScale Science and Technology Facility.
Although the experiments were conducted with a single ion, the approach is also applicable to larger trapped-ion quantum computers. According to Mehta, the main significance of the work is that it shows technologies being developed to scale up quantum computers can simultaneously improve the fundamental operations on which those computers depend.