The pitfalls of UV lasers

Dimitris never goes diving without sunscreen. “I have my mother’s German skin,” he jokes. But it’s not just summers by the sea that pose a risk of sunburn; working in an optical lab without natural light does too. To excite atoms into Rydberg states, high-energy photons from the ultraviolet spectrum are needed, explains the physicist. “The wavelength of my UV laser lies within the solar spectrum.” As a very small amount of the UV light is scattered uncontrollably in various directions by dust particles, imperfections on mirrors, and so on, face protection must be worn while working on the ongoing experiment. Perhaps sunscreen would also be a solution for his lab, Dimitris muses with amusement.

However, working with UV lasers presents far greater challenges than just protecting oneself from sunburn. For example, when using optical fibers. In optical experiments, these offer the advantage that light can be guided very flexibly from one point to another. This means that the laser and the target the laser beam is intended to hit – in Dimitris’s case, the atoms in the vacuum cell – can be positioned at a distance from one another. A glass fiber also allows the laser beam to be aligned more stably with the target. “Actually, for us, optical fibers are just a tool. We buy them, we use them,” the physicist explains. However, fibers for UV light, he adds, are more like research objects than tools and are not commercially available. The reason for this is an effect known as solarization. The high-energy UV radiation triggers chemical reactions in the glass that cause its light transmittance to decrease significantly. “If I take a normal optical fiber and pass UV light through it, the transmittance drops to zero within hours. Then nothing gets through anymore.”

The solution to the problem lies in a kind of “passivation process,” the doctoral student continues, in which the optical fiber is first enriched with hydrogen. Due to the presence of additional hydrogen molecules in the glass, the UV light, which passes through the fiber in a second step, triggers other reactions whose outcome does not affect UV transmission. “The glass fiber then remains permanently transparent to UV light.” Dimitris explains that they had the hydrogen enrichment carried out by a company that has the necessary equipment. “This involves storing the fibers for two weeks at over 40 degrees in a pressure chamber filled with hydrogen gas. It requires 100 to 200 bar!” In water, that would correspond to the pressure at a depth of 1,000 to 2,000 meters. But handling the fibers after their “hydrogen bath” – especially coupling the UV light into the fiber – is also challenging. Together with a colleague, Dimitris invested a great deal of time and patience in producing UV fibers. However, when he talks about the difficulties they faced, the problem itself never seems to be the main focus. Rather, his enthusiasm prevails as he gradually gets to the bottom of the issue, and the solution gradually takes shape. By now, they’ve successfully used UV fibers in experiments for several weeks, says the doctoral student. However, outsourcing hydrogen enrichment to a company makes the whole process tedious and inflexible. “Switching to new UV wavelengths or errors in the passivation process are thus associated with months-long wait times,” Dimitris emphasizes, and the look in his eyes behind his round glasses reveals that he finds such a process far too inefficient. “When I asked for permission to build a hydrogen pressure chamber in the MPQ’s garden during a meeting, my postdoc just pulled out his phone and showed us all a photo of the Hindenburg explosion,” he says with a laugh. Of course, the doctoral student adds, he realizes that one cannot simply build such a facility at the MPQ. For the time being, their findings have led them to a pragmatic solution: forgo the advantages of UV fibers and place the UV laser close to the vacuum chamber containing the atoms. For their experiment, he notes, that’s sufficient for now.

A major breakthrough in quantum computing with trapped atoms

It is all the more gratifying that they achieved a major breakthrough in another area: “I’m especially proud that we, as a team, finally solved the problem with the electric fields.” The physicist begins his explanation by noting that electric charges could be present anywhere in the experiment where non-conductive materials are used, particularly on the glass walls of the vacuum chamber. This happens automatically because the laser light repeatedly knocks electrons out of the material, he continues. Since these charges cannot escape non-conductive materials, such as glass, an electric field forms inside the glass cell. Atoms in the Rydberg state, acting as electric dipoles, react very sensitively to these unwanted electric fields, Dimitris explains further. “The energy level of the Rydberg state – and thus the required laser frequency with which I excite the atom into this state – is constantly shifting.”  Several times a day, they have to remeasure what the correct transition frequency is at that moment and then adjust the laser accordingly. “Otherwise, nothing works with the two-qubit gates anymore.” Labs around the world were struggling with this issue. Eventually, a Chinese lab came up with a simple yet clever idea: Some of the many atoms, which initially form a disordered “cloud” in the vacuum cell, are ionized using a laser. The resulting free charges are drawn along the field lines toward the walls of the glass cell until a counterfield builds up that cancels out the unwanted field. Only then are atoms from the “cloud” captured using additional lasers and arranged at regular intervals to form the quantum processor. Building on this idea from China, the team from Dimitris’s lab developed a significantly more efficient, more versatile, and less expensive solution for ionizing the atoms and eliminating the interference field. The lab in China used a UV laser for ionization, Dimitris explains. His lab, on the other hand, was able to demonstrate that ionization actually works even faster when using a simple blue laser diode, which is not only significantly cheaper than a UV laser but also much easier to handle. “You take the diode, plug it in, and aim it at the cloud of atoms. That’s it.” They were also able to demonstrate that their method works for all relevant elements, regardless of whether a lab is working with strontium, ytterbium, rubidium, or cesium atoms, for example. “We worked on this for over a year. It was a big deal.” Dimitris is currently working on the paper about it.