A new approach to laser spectroscopy based on quantum physics makes it cheaper and easier to observe ultrafast processes inside materials, say A*STAR researchers.
In general, spectroscopy involves shining a laser on to a sample and observing the effect on the laser light to work out what is going on inside the sample. However, some processes in materials happen so quickly that very expensive lasers and sophisticated detector systems are needed to observe them. Powerful lasers can also damage samples.
Now, Dmitry Kalashnikov and colleagues at the A*STAR Data Storage Institute, Moscow State University, and the Russian Academy of Sciences, have demonstrated a method that exploits the strange phenomenon of quantum entanglement to measure extremely short-lived processes inside materials without expensive equipment.
“By measuring the time over which a sample responds to electromagnetic fields, we can learn about the connections between components in the substance,” says Kalashnikov. “In particular, the so-called dephasing time describes how long different atoms or molecules of the substance respond as a coherent ensemble. For many substances this time lies within the femtosecond time scale (10-15 seconds) and is not easy to measure.”
Instead of investing in costly femtosecond lasers, Kalashnikov’s team used a continuous-wave laser configured for Hong-Ou-Mandel (HOM) interference. This interference effect describes what happens when quantum-entangled photons, which are effectively indistinguishable and dependent on one another, hit a semi-transparent mirror called a beamsplitter.
“When HOM interference was discovered in the 1980s it became the true manifestation of quantum mechanics,” says Kalashnikov. “When two entangled photons hit a beamsplitter from different sides they always exit together from one or another beamsplitter outputs; they never exit separately.”
After very carefully assembling and aligning their HOM setup, the team could measure the level of entanglement in a beam by placing two photodetectors at the beamsplitter outputs. If both detectors clicked at the same time, then there was no entanglement, but if only one detector clicked, the photons were entangled.
“When a sample is placed into the path of one of the photons, it causes the photon to become less identical to its twin,” explains Kalashnikov. “Therefore, the distribution of clicks changes, and we can estimate the dephasing time.” Through this method, the researchers measured dephasing times as short as 100 femtoseconds, in neodymium-doped crystals and silicon nanodisks.
“In the future we will use this technology to study chemical and biological samples, with even shorter dephasing times,” says Kalashnikov. “Our work could also be important for developing quantum computer memory.”