Observation of gain-induced group delay between multiphoton pulses generated in spontaneous downconversion sources

Spontaneous parametric downconversion (SPDC) and spontaneous four-wave mixing are powerful nonlinear optical processes that can generate multiphoton light beams with unique quantum properties. These processes can be exploited to create a variety of quantum technologies, such as computer processors and sensors that exploit quantum mechanical effects.

Researchers from the National Research Council of Canada and Polytechnique de Montréal recently conducted a study looking at the effects of the SPDC process. Their paper, published in Physical Review Letters, reports the observation of gain-induced group delay in multiphoton pulses generated in SPDCs.

“The inspiration for this paper came from the study of a process called SPDC,” Nicolás Quesada, senior author of the paper, told Phys.org. “To say that a certain material can take a violet photon (particle light material) and convert it into two red photons is just a mouthful.

“This is a very versatile phenomenon that allows physicists to generate light with interesting correlations. Two ‘daughter’ red photons are born at the same time and have exactly the same energy and momentum as the ‘mother’ purple photon. Because you need to have it.”

Over the past few decades, SPDCs have been the focus of numerous physics studies. So far, this process has mainly been studied in a specific regime, where the researchers converted one violet photon into two red photons about once in 100 during each experimental run.

“During my PhD, I studied what happens when the probability of producing two daughter photons starts to approach 1, and then beyond this point you produce more than one photon per experimental run. ” said Quesada.

“We found that the color in which the daughter photons are born begins to change slightly, and the efficiency of the process (how many ‘red’ photons are born per ‘purple’ photon) also changes. ”

When Quesada first started considering the possibility of producing more than one daughter photon per experiment, he had not yet identified a way to measure this experimentally. But this year, his colleague Guillaume Teckadas suggested that small changes in color, from making few pairs to making many pairs, could also be reflected in differences in the arrival times of the daughter photons. I noticed that there is.

“We observed that as the number of photons produced by the SPDC process increases, the arrival times of the two daughter photons become staggered,” Thekkadath explained.

“To investigate this effect, we made two important changes to the traditional SPDC experimental setup. First, we used a high-power laser that can send ultrashort (femtosecond) pulses, and we They compressed them into bursts. These pulses were further amplified, and second, they implemented a technique called “spectral interferometry” to measure the photon arrival times with high precision. ”

Thekkadath, Quesada, and their colleagues passed photons produced by a high-power laser through an optical fiber several kilometers long, stretching the photon pulse in time. They then recorded the photon’s arrival time using a superconducting nanowire detector, a highly sensitive device that can detect single photons with extraordinary timing resolution.

The results they collected confirmed the existence of a gain-induced group delay between the multiphoton pulses generated in the SPDC source. This observation could have important implications for the future development of devices that exploit quantum interference.

“Our results suggest that particular care should be taken when interfering with light from SPDC sources that produce photon pairs at different brightnesses (and on average produce different numbers of pairs). ” said Quesada.

“If we’re not careful, if photons from two different sources arrive at the interferometer at different times, we won’t be able to perform a quantum mechanical feat known as Hon-Ow-Mandel interference. , enabling quantum computers to exceed the capabilities of classical computers.”

Martin Houde, one of the co-authors of this recent paper, has recently been trying to design better SPDC light sources in which photons are emitted simultaneously, regardless of the brightness of the emitted laser pulse. Quesada and his colleagues at École Polytechnique de Montréal are trying to understand how differences in the arrival times of photons, which can cause errors, affect the functionality of optical quantum computers.

“Our SPDC light source was relatively ‘bright’ and produced hundreds of daughter photon pairs compared to most light sources, which typically produce only a single pair,” Tekkadas added.

“However, many of these photons are lost before reaching the photodetector. These losses can occur for a variety of reasons, including reflection from optical elements such as lenses or incomplete capture by optical fibers. Quantum correlation is essential for technologies such as quantum-enhanced sensors. Losing one photon from a pair disrupts photon detection.

As part of their next study, Thekkadass and colleagues at the National Research Council of Canada are trying to devise strategies to minimize optical loss in quantum devices. Additionally, they seek to determine how the photon pair sources they are studying can be exploited for quantum sensing and computing, regardless of the associated photon losses.

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