From 2010 to 2015, I worked with colleagues at the University of Vienna/Austrian Academy of Sciences and collaborators from other institutions to build an experimental test of Bell’s inequality that was free of significant loopholes. The plan was to generate polarization-entangled optical photons at a central source station and distribute them through optical fibers to two distant measurement stations known as Alice and Bob. (See Physics Today, January 2016, page 14 and December 2022, page 14.)
To balance the constraints imposed by the various loopholes we wanted to close, we concluded that we’d need three small labs to house the stations, with about 60 meters between Alice and Bob and a line of sight between all three stations. The space also needed to support power and water adequate for operating the cryostats to cool the detectors.
Aiming to keep the experimental setup close to our home base at the university, we pored over a Google Maps aerial view of Vienna and looked for suitably dimensioned spaces. We found a few options, but each had fundamental shortcomings: For example, one was an attic space without climate control and with questionable elevator access.
After discussions with the Austrian Ministry of Castles, we found a home for our experiment inside a tunnel in the subbasement of the Hofburg, a former imperial palace in the center of Vienna.
We moved our work into the Neue Burg wing of the castle. The area directly above our experimental setup location houses Vienna’s Weltmuseum (Museum of Ethnography). The tunnels underneath the museum were originally built as an air-conditioning system: They open to the nearby Burggarten (castle gardens) to let in fresh air, and due to their underground positioning, they remain fairly cool even in the summer. Open windows in the upper level allow hot air to escape, while cool air is pulled up from the basement and subbasement.
We installed electrical power, plumbing, and internet, plus three lab spaces made of drywall and temporary walls. Roughly 50 meters of yellow and black plastic and rubber cable protectors on the floor shielded the various cables and fibers that ran the length of our experiment.
With its proximity to the university and relatively undisturbed location, the Hofburg subbasement was in some ways an ideal location. It was not, however, without some challenges. For one thing, one must navigate a subterranean labyrinth to reach the lab space, and once inside the building there is no mobile phone service. We were equipped with one cabled phone at the Alice end of the lab, so any phone communication in or out of the lab happened there. A 50-meter dash to answer the phone was a common experience, and many calls were missed.
At first we had only one key shared among three scientists, building access was restricted to weekdays between 8:00am and 5:00pm, and there was no accessible sink or toilet. Those issues were mitigated within a few weeks: We received a couple more keys, managed to get off-hours access after establishing relationships with the security guards (who were friendly if perplexed by our activities and working schedule), and got permission to enter a floor of the museum that was equipped with a sink and toilet. On one late-night occasion, we startled a substitute security guard who wasn’t aware of our scientific presence and assumed we were there to commit an armed robbery.
The roughly 2-centimeter-thick layer of dust that had accumulated over the preceding century was the most persistent menace to our efficiency-critical optics experiments. Countermeasures included extensive wet and dry vacuuming (including the short-lived service of an overworked robot vacuum cleaner), plastic foils on the walls, air filtration units in the labs, and a strict shoe policy: slip-on shoes to get between labs, rugs and sticky mats at the entrance to each lab, and no shoes allowed in the labs themselves. Given our distance from the museum’s sink, we eventually installed our own sink in the subbasement to increase the frequency of handwashing.
The Hofburg’s central location meant that an electric tram line ran essentially right over our experiment, and multiple subways came close. Despite shielding, this added significantly to the noise seen on the SQUID amplifiers we used for the readout from our single-photon detectors. There was a period of a few hours each night (about 1:30–4:30am) when the detectors became much quieter, but we ultimately learned to operate with the standard ambient noise—it would have been impractical to restrict our work to a few hours per day.
The tunnels under the castle remain open to the Burggarten during the summer, and occasionally birds would wander in and get lost. (The chirp of those birds sounded very similar to that of a broken rotary valve!) Directing anxious birds toward the exit of the underground maze was surprisingly challenging. During the entire run of the experiment, I saw only one mouse: It was attracted, of course, to our precious optical fibers and showed its tiny self during a lab tour. Also, we were not the only humans interested in the subbasement: One morning we encountered a pool of fake blood along our path to the lab space and then discovered that several pieces of lab equipment had been moved “out of the way” so a group of students could film a dramatic scene in an adjacent tunnel.
One winter morning, we entered the lab to find our underground space at a balmy 28 °C. We discovered that the city’s Fernwärme (district heating) system ran directly through our lab, providing an unexpected source of extra heat. At least this heating effect was fairly stable. During the summer, our lab was subject to the temperature fluctuations of the outdoors, which translated directly to instability in the polarization transformations imposed by the optical fibers.
Despite the obstacles, it was on a summer evening after weeks of warm weather that we finally reached the performance and stability to collect our culminating data set, one that would ultimately provide strong evidence for the violation of Bell’s inequality.
Marissa Giustina is a quantum electronics engineer and research scientist at Google’s Quantum AI laboratory. Giustina has a PhD in experimental quantum information from the University of Vienna and is the first author of “Significant-loophole-free test of Bell’s theorem with entangled photons,” the 2015 paper that resulted from the experiment described in this article.
