Seeing the Light

Last month, an outstanding paper, describing an experiment that combines temporal and quantum optics, was published in Physical Review Letters, one of the world’s top scientific journals. This paper is the product of two forces joining together here, at the Faculty of Engineering: Prof. Moti Fridman, a world-leading temporal optics scholar, and Dr. Eliahu Cohen, who specializes, among other things, in quantum optics, a field that explores optical phenomena that escape classical explanation, i.e. in terms of waves alone, and ventures into the single photon level. “Even prior to the publication, the topic had been presented in several international conferences, across Europe and the U.S., to raving reception,” the two scholars recount. “The journal’s reviewers themselves indicated that this was a truly innovative, fascinating concept. It is not often that a paper submitted for review in prestigious magazine earns such positive feedbacks.”

The idea for the joint project first came up at a Bar Ilan’s Institute of Nanotechnology and Advanced Materials (BINA) conference, held in 2019 at Herzliya, while the collaboration between the pair was taking shape over time. “The project was inspired by the work of Prof. Avi Pe’er of Bar Ilan’s Physics Department, which shows that the optical bandwidth of entangled photons can be significantly extended and utilized,” they tell us.

Interferometer is a common, efficient optical device, which allows light to expose interference, while utilizing it for a range of sensing purposes. The quantum interferometer offers highly sensitive measurements of crucial properties associated with light and the system it goes through, with emphasis on phase. “We integrated the discipline of sensitive, broadband quantum interferometers - which has to do with quantum optics - with the years’ long research performed by my team, which explores optical components known as time lenses,” explains Prof. Fridman. “The common lenses in optics, as well as in everyday life, are spatial lenses: they can take a beam of light, and either focus or diverge it in space. The component we have been working on and developing for years is called a temporal lens, which does the same thing, only in time – it can take a pulse of light and either contract or stretch it in time, i.e., increase or decrease its duration. Over the years, we have developed different devices that examine pulses through temporal lenses, using nonlinear classical optics, to study some very fleeting events in time. The person who helped us understand that this tool could work great with quantum optics’ instruments and devices was Eli. On top of that, he’s a theoretician, while I’m an experimentalist, so it was only natural that we should work together.”

And indeed, together the two managed to develop the theoretical model of the idea, as well as the actual experiment. The then-sophomore undergraduate Sara Meir - now about to complete her master’s degree and embark on her doctoral studies, supervised by Prof. Fridman - was entrusted with building the components. “In the process of working on the project, we received a Ministry of Science and Technology grant, while Sara earned a women-in-hi-tech scholarship, which allowed this study to go ahead,” the two note, while also citing the support received from the Faculty of Engineering and the Institute of Nanotechnology and Advanced Materials.

As part of the joint project, a new, first of its kind type of quantum interferometer was built in Prof. Fridman’s lab. Instead of the traditional use of two non-linear crystals, this interferometer employed two time lenses. “The experiment in fact incorporates fiber optics and non-linear optics, while the application of the temporal lens employed some highly non-linear fibers,” the scholars elaborate. “Examining a ray of light, you’d normally look into its intensity, high or low, but a ray of light is in fact a wave, it has a phase as well, and it is this phase that the interferometer measures. However, this tool is not without its limits, while a quantum interferometer allows to handle these limits and measure with higher sensitivity. We took it a step further, by inventing a device that allows to measure a given phase, even when it undergoes a super-rapid change.”

The tool devised by the two scholars allows to perform, for the first time, a quantum-enhanced measurement of physical scales and effects that have up until now been hard to sense. ”With this interferometer, you can, for instance, look into the phase generated by laser pulse when it just emerges from its source, or examine what happens when two strong light pulses ‘clash’ within an optical fiber. We’re discussing two ultra-fast events that are very hard to measure in any other way, not least when it comes to their phase,” explains Prof. Fridman. “With this tool, we can now talk not only about what happens to the phase when two pulses clash together, but also get down to the actual photon level and see what happens to the photon’s phase when it’s generated, or what happens to the phase when two photons interact.”

“While developing the tool, we had a rapid chemical or biological process in mind, where the light’s phase changes rapidly, but can nevertheless inform us about the process,” says Dr. Cohen. “The system built by Sara in Prof. Fridman’s lab combines the quantum interferometer’s sensitivity, which allows it to trace tiny phases, with the time lenses, which allow to extend the duration of the process, even if it’s really fast to begin with. As a result, the tool can sense incredibly rapid phase changes, which provides us in turn with the power to learn about unique light modes, i.e. to perform state tomography or process tomography of external quantum processes that have an impact on light. In the future, this may have implications for various light-based applications, including detection, sensing, imaging, and communication.”