Nature Laser-cooled atoms for hybrid quantum systems

Neutral atoms, sometimes called "cold atoms", are composed of a series of individual atoms that are trapped in a room-temperature vacuum and are cooled by using a laser as an optical "tweezer" to restrict the motion of the individual atoms (hence the name "cold atom" reference). These neutral atoms can be brought into highly excited states by firing certain laser pulses at them, which will expand the radius of the outer electrons (the Rydberg state), which can be used to entangle them with each other, among other features.

While there are some significant differences in the methods used by the neutral atom players, there are also many similarities. The figure below highlights an atomic calculation setup representing the generalized cold atom approach. It includes two sets of lasers and associated controllers as well as an AOD (acousto-optical deflector), a vacuum chamber, and a photon-sensitive camera for reading the results.

In the neutral-atom quantum processor, atoms are first heated into a gaseous cloud and then suspended in ultra-high vacuum by a tightly focused laser array of specific wavelengths (often called "optical tweezers").

Each element responds to very specific wavelengths of light and can therefore be manipulated by tuning the laser to those specific wavelengths. These optical tweezers can also be used to configure atoms into specific geometric arrays. For gate-based digital computation, single and multi-gate implementations can be programmed with different light pulses.

Although the use of neutral atoms in quantum computing is relatively new, neutral atom technology has been successfully deployed in other physics research and has been powering the world's most accurate atomic clocks for many years. The laser cooling technique is based on research that won the 1997 Nobel Prize, while optical tweezers are based on research that won the 2018 Nobel Prize.

On March 22, Kumar, A. et al. published a paper in natureQuantum-enabled millimetre wave to optical transduction using neutral atoms, which means that laser-cooled atoms bring quantum computer networks a step closer.

Specifically, photons can be used for local quantum operations, but they cannot transmit information over long distances. This is because, at room temperature, low-energy photons are abundant in the thermal radiation of the environment, making photons carrying quantum information indistinguishable from the thermal background. Photons with wavelengths in the visible and near-infrared frequency ranges do not have such problems; they have higher energies and can carry information over long distances through fibers with minimal information loss. Converting quantum information between low-energy and high-energy photons will be the key to building quantum computer networks.

To address this problem, the Kumar, A. team reports a system that couples a cold 85Rb atomic aggregate to both a currently unprecedented optically accessible 3D superconducting resonator and a vibrationally suppressed optical cavity in a low temperature (5K) environment. The atom's valence (outer) electrons can compensate for the energy difference by absorbing or emitting photons, thus switching between energy levels.

"Coupling neutral atoms to superconducting resonators, while maintaining deterministic control, has been an active goal of the quantum optics community for decades. Not only for photonic conversion, but also for the variety of quantum experiments it can enable. This research is the result of a long-term effort in our lab and required seminal contributions from members of the research group."

"We had to overcome several technical challenges: designing a superconducting resonator with optical channels; stabilizing the optical cavity in a high-vibration cryogenic vacuum over a length range of less than 100 picometers; finely controlling the superconducting cavity; tuning the magnetic field within the superconducting structure; and so on. The first really striking discovery was when we observed strong coupling between the superconducting millimeter-wave cavity and rubidium atoms." Team members L.T. and A.K. spoke of.

The huge range of possible energy conversions makes atoms an ideal medium for interconversion between photons with vastly different energies. Rubidium atoms convert on the energy scale of both photons at optical wavelengths and photons in the millimeter-wave to microwave range (Figure 1a). In this platform, laser-cooled rubidium atoms are placed inside a resonator that acts as a trap for the photons (Figure 1b). The trap allows more interaction between the photons and the rubidium atoms and, therefore, is more efficient than the interconversion that occurs in free space.

Conversion between optical photons and millimeter-wave photons. When the outer electrons of a rubidium atom change energy levels, it absorbs or emits photons of different energies (wavelengths). This property is useful when converting photons at optical wavelengths to millimeter-wave photons.a, Left, Energy levels (nS and nP subshells) of electrons in a rubidium atom (85Rb), and the associated wavelengths of light (left). On the right, a schematic of the interchanger, with the Rb atom (purple) and the millimeter-wave photon (green-yellow) in the center. The atom interacts with a resonator device that captures both optical frequency photons (red) and millimeter-wave photons (green). Blue (481 nm) and ultraviolet (UV; 297 nm) lasers are docked between the low and high energy level electrons of the Rb atom. b, Cross section of the superconducting and optical resonator showing the access of millimeter-wave and optical photons (780 nm) and laser beams (481 nm and 297 nm). The mirror holder assembly and the millimeter wave waveguide in the device are also shown.

The oxygen ion generator captures photons by repeatedly reflecting them between two mirrors facing each other. It also has a hollow cavity within a superconducting niobium structure that captures millimeter-wave photons. To evaluate the performance of the interchanger, the team fed millimeter-wave photons into the superconducting cavity and counted the number of photons converted in the optical resonator.

The interchanger converted an average of 58% of the input photons, consistent with the team's previous theoretical predictions. The reverse conversion of optical photons to millimeter-wave photons was also consistent with predictions. The average thermal background is expected to be about 0.6 millimeter-wave photons at 5 Kelvin, the required operating temperature of the superconducting cavity, and the team confirmed this low level of noise by monitoring the converted photons without millimeter-wave photon input.

The interconverter has a good impact outside the laboratory. When integrated with existing superconducting quantum bit platforms, it can be used to form a quantum computer network. The interposer can also be used as a sensor for individual millimeter-wave photons in place of a device known as a bolometer. Its ability to efficiently convert millimeter-wave signals into optical signals could be valuable in certain fields: in cosmology, it could help in the search for dark matter, and in astrophysics, it could detect millimeter-level photons from celestial sources.

In contrast to microwaves, millimeter-wave photons remain largely unexplored in the context of quantum science. While most superconducting platforms operate at microwave frequencies around 5 GHz, this experiment is operating at millimeter-wave photons close to 100 GHz, and this platform has the potential to be combined with emerging quantum circuit technologies operating at millimeter-wave frequencies. In addition, the interposer can also operate with microwaves, either using a microwave-to-millimeter-wave link4 or directly with microwaves by using atoms with electrons showing different energy transitions and larger superconducting resonators cooled to below 0.5 Kelvin to suppress the background thermal noise at microwave frequencies.

"The authors demonstrate coherent conversion between millimeter-wave photons and optical photons. This conversion has been a hot topic of recent research, motivated by the need to connect superconducting circuits to optical photons, the fastest and most reliable means of quantum communication. More broadly, this work demonstrates a significant advance in hybrid quantum systems composed of different components with complementary capabilities for quantum information processing, storage and communication."

Federico Levi, senior editor and team manager at Nature, commented on the article, "This work makes a compelling case for the potential of hybrid approaches. In this approach, different quantum systems are coupled to take advantage of their complementary strengths for quantum technologies."

Reference links:

[1]https://www.nature.com/articles/d41586-023-00323-7
[2]https://www.nature.com/articles/s41586-023-05740-2

[3]https://thequantuminsider.com/2023/03/22/pasqal-launches-first-neutral-atoms-quantum-computing-exploration-platform/