Most researchers would agree that it is much easier to write a paper about an effect that is observed than it is to prove it doesn’t exist when no signal is observed. NIST JILA Fellow Ralph Jimenez found this to be the case, in contributing to a recent paper published in Physical Review Applied. The authors of this paper were originally hoping to observe the increased efficiency in two-photon absorption, a special type of process used in microscopy of living tissue, that had been reported by other research labs. This increased efficiency would be determined by an additional absorption signal than the one being produced by classical light. This additional signal came from using entangled photons. Instead, Jimenez and his team of collaborators from NIST found no additional signal in their measurements, indicating a lack of absorption entirely from the entangled photons. When speaking to other researchers around the world performing similar experiments, Jimenez found that the signal was either seen or it wasn’t. This randomness in signal appearance began a mystery for Jimenez to solve when it came to the process of using entangled photons in two-photon absorption.
A Brief History of two-Photon Absorption
According to NIST scientist Martin Stevens: "two-photon absorption is a very inefficient process. If two photons happen to be in the same place at the same time, and hit the molecule that you're trying to excite, there's a chance you'll get absorption." However, the chances of this absorption happening are very slim. That is why teams like Jimenez's and Stevens' use high-powered lasers with short light pulses. This increases the probability of two photons being next to each other and being absorbed. Previous studies suggested that the efficiency of two-photon absorption could be enhanced up to 10 orders of magnitude by using entangled photons. Quantum entanglement of light occurs when quantum states of two photons are not independent of each other. As Stevens explained: "The hope is that by generating entangled photon pairs, we can make photons that are highly correlated in energy and time, so they arrive at the same place at the same time with exactly the right energy, making the absorption process very efficient.” If an entangled photon pair would be absorbed, the instruments would be able to detect a signature of this absorption.
The Clues of Fluorescence
In order to test this theory, graduate students and postdocs from both the Jimenez and Stevens labs joined forces to build a state-of-the-art quantum optics laboratory from an empty room. This laboratory was specifically built by co-first authors Kristen Parzuchowski and Alexander Mikhaylov of the Jimenez laboratory. Stevens commented: "That's been a really rewarding part of this work with myself, Thomas Gerrits and Mike Mazurek with quantum optics backgrounds, interacting with Ralph his postdocs and students, who did not have this background and merging these different streams of talent together with these different areas of expertise." The team did several tests on this proposed efficiency by looking for the absorption of entangled photon pairs by molecules in liquids, without seeing any signals that corroborated previous studies.
After this initial testing with a transmission-based experiment didn’t turn up any clues, "we then realized that the more sensitive experiment would detect fluorescence." Jimenez stated. This would allow for more thorough testing. "In that case, if the molecules absorb a pair of photons, then a fluorescence photon will come out. If there's no two-photon absorption, you shouldn't see any signal, unless it's something else. So, we built this new experiment to detect two-photon absorption with fluorescence, and again, we didn't see a signal." Frustrated about their lack of signal, Jimenez and Stevens began informal discussions with other teams around the world to determine if other researchers were getting the same result that they were. These discussions, which includes a group in Geneva that has detected entangled two-photon absorption, evolved into a biweekly global seminar series, where different labs presented their findings. The results puzzled Jimenez and Stevens, as some labs have found a signal showing entangled photons being absorbed in two-photon absorption, while others haven't. Jimenez commented: "we thought there's something going on around the world as different people are seeing different things, such as a group in South America saw this process, and there's a group in Mexico that sees the same thing we do, with no signal. There's a group at the University of Oregon that also saw no signal. And we've been trying to understand what's going on here."
The Mystery Deepens:
In their fluorescence experiments, graduate student Kristen Parzuchowski and postdoc Alex Mikhaylov worked together with the NIST-campus team to show that even in the absence of a signal from entangled photon absorption, they could precisely calibrate their experiment and estimate upper bounds for the enhancement which are up to 10,000 times smaller than what was reported by others. The next steps are clear for both Jimenez and Stevens, who have a number of experiments lined up as they try to find the reason for why some labs are seeing a signal and others aren't. "I think what's come out of this is that we've been able to largely come to a consensus as a community about what measurements need to be done to verify this process." Stevens explained. With global established protocols, both Jimenez and Stevens are excited to try to find the answer to the case of the missing entangled photon absorption signal.
Written by Kenna Castleberry, JILA Science Communicator