Scientists have demonstrated a time-resolved spectroscopy technique that enables the study of very fast processes in samples on the femtosecond (fs) time scale without the need for an fs laser or a complex detection system. The method works by analyzing quantized light transmitted through a sample. It relies on single photons to study the interactions and processes occurring in the samples and ordinary lasers to produce the photons. Interferometer developed at Moscow State University. Courtesy of Elizaveta Melik-Gaikazyan. Researchers from Lomonosov Moscow State University (MSU) demonstrated that by exploiting quantum two-photon interference of entangled photons, it is possible to measure the dephasing time of a sample on the fs time scale, down to 100 fs, using a continuous wave laser and single-photon counting. Researchers built an experimental setup to measure the two-photon interference pattern with and without a test sample. A basic interferometer was used to measure the interference. A nonlinear crystal was situated on the cw-laser path in the assembled system, and pairs of entangled photons were created. A test sample was set inside one arm of the interferometer. One photon of the entangled pair passed through the interferometer and, hitting the beam splitter, met its counterpart, which meanwhile had passed through the interferometer’s second arm. The photons then fell onto one of two detectors. The detectors reacted to single photons only, allowing the construction of a coincidence circuit. If both photons went to the same detector, it was zero coincidence. If the photons went to different detectors, it was coincidence of one. At the point where the delay between the two arms became absolutely identical, the effect of quantum interference occurred, and coincidence disappeared, since the photons could never fall on both detectors simultaneously. When the sample was set onto the photons’ path, the pattern of the quantum interference started to change, i.e., the pairs of entangled photons that came to the beam splitter became less ‘identical’ than in a situation without a sample. The photon reception statistics on the two detectors changed. By observing the change in these statistics, scientists were able determine the nature of the interactions in the sample. For example, they could estimate the transition time from the excited to the unexcited state. The researchers tested and verified the method on two samples: an Nd:YAG crystal (an aluminum-yttrium garnet with neodymium) and a matrix of dielectric nanoparticles. In contrast to high-powered fs lasers, the MSU researchers’ design is not expensive and can be used without risking photo-damage to the sample under investigation. It is easy to use. The MSU design allows the measurement of dephasing times in optically thick samples, for which application of transmission spectroscopy is limited. The technique operates at a single photon level and could be useful for measurements of fragile biological, chemical and nanostructured samples. The researchers believe that their technique will contribute to further development of ultrafast time-resolved spectroscopy. “The new method of analyzing unknown substances can be used in chemistry, biology and materials science,” researcher Elizaveta Melik-Gaykazyan said. “In addition, it can be useful when creating a quantum computer, and when trying to understand how to use quantum light in information technology.” The research was published in Scientific Reports (doi: 10.1038/s41598-017-11694-z).