Researchers at Cornell University find a faster way to cool the atoms powering quantum computers

Researchers at Cornell University have developed a laser-cooling technique that could help address a major operational bottleneck in the development of larger quantum computers.

The method uses specially designed laser beams delivered through a microfabricated chip to cool electrically charged atoms, known as ions, while controlling multiple types of atomic motion simultaneously.

The findings, published in the journal PRX Quantum on Nov 9, demonstrate how integrated optical components could improve the efficiency of trapped-ion quantum computing systems.

The research was led by Karan Mehta, assistant professor of electrical and computer engineering at Cornell's Duffield College of Engineering.

Why cooling matters in quantum computing

Trapped-ion quantum computers use electrically charged atoms held in electromagnetic fields to process quantum information. These ions act as qubits, the basic units of information in quantum computing.

Unlike conventional computer bits, which represent information as either zero or one, qubits can exist in combinations of states. This property allows quantum computers to approach certain computational problems differently from conventional machines.

However, maintaining precise control over qubits is challenging. Even small amounts of unwanted atomic motion can interfere with quantum operations, making it necessary to repeatedly cool ions close to their lowest possible energy state, known as the quantum ground state.

These cooling operations can consume a significant portion of the time required to operate trapped-ion quantum computers.

Mehta said cooling, particularly to near the quantum ground state, has become a major bottleneck in leading systems.

A chip-based approach to faster cooling

To address this challenge, the Cornell team developed an integrated photonic approach that guides laser light through microscopic pathways on the same chip that hosts the trapped ions.

The researchers used two laser beams travelling in opposite directions to create a standing wave, a pattern of alternating regions of high and low light intensity.

By positioning a trapped calcium ion at a point of minimum light intensity, the researchers could remove energy from its motion while reducing unwanted interactions that could otherwise cause heating.

The technique combined this arrangement with a method called electromagnetically induced transparency (EIT) cooling.

Using the approach, the team brought the ion's targeted motion close to its quantum ground state in approximately 150 microseconds. A conventional laser arrangement required around 500 microseconds to achieve a comparable result.

The technique also cooled all three measured modes of the ion's motion simultaneously, across a frequency range of approximately 5 megahertz. This is significant because ions can oscillate in three dimensions, with each direction potentially having a different frequency. Conventional techniques often require separate cooling operations to address these different modes.

Doctoral students Zhenzhong Xing and Hamim Mahmud Rivy were the co-first authors of the study. The work translated theoretical predictions developed by Xing into an experimental demonstration.

The researchers also demonstrated cooling below the conventional Doppler limit, experimentally confirming a long-standing theoretical prediction.

What this could mean for larger quantum computers

An important feature of the research is the integration of optical components into the ion-trap chip, reducing reliance on complex external laser arrangements.

The chip can deliver multiple wavelengths of light, ranging from ultraviolet to near-infrared, which are required to control the trapped ion. The researchers also developed packaging that allowed the devices to operate at cryogenic temperatures.

The chips were commercially fabricated in the Netherlands through a collaboration with ETH Zurich, while Cornell facilities were used to prepare the devices for the experiments.

The demonstration involved a single calcium ion, rather than a complete multi-ion quantum processor. However, the researchers said the approach could be applied to larger trapped-ion quantum computers.

As researchers work towards increasing the number of qubits in quantum processors, improving the efficiency of basic operations such as cooling will be important. Simultaneously addressing multiple types of atomic motion could reduce the time spent preparing ions for quantum operations.

The findings point to how integrated photonics, which uses light-based components built into chips, could contribute not only to scaling quantum computing systems but also to improving the operations on which they depend.

The study was supported by the US National Science Foundation, the Army Research Office and the Alfred P. Sloan Foundation.