December 7, 2024 by Ingrid Fadelli , Phys.org
Collected at: https://phys.org/news/2024-12-gain-group-delay-multiphoton-pulses.html
Spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing are powerful nonlinear optical processes that can produce multi-photon beams of light with unique quantum properties. These processes could be leveraged to create various quantum technologies, including computer processors and sensors that leverage quantum mechanical effects.
Researchers at the National Research Council of Canada and École Polytechnique de Montréal recently carried out a study observing the effects emerging in the SPDC process. Their paper, published in Physical Review Letters, reports the observation of a gain-induced group delay in multi-photon pulses generated in SPDC.
“The inspiration for this paper came from studying a process called SPDC,” Nicolás Quesada, senior author of the paper, told Phys.org. “This is a mouthful to say that certain materials are able to take a violet photon (the particle light is made of) and transform it into two red photons.
“This is a super versatile phenomenon that allows physicists to generate light with interesting correlations, as the two ‘daughter’ red photons are born at the same time and need to have exactly the energy and momentum of their ‘mother’ violet photon.”
Over the past decades, SPDC has been the focus of numerous physics studies. So far, this process has been primarily studied in a particular regime, where researchers converted one violet photon into two red photons approximately once out of 100 times during each experimental run.
“During my PhD, I studied what happens when the probability of making two daughter photons starts to approach unity and then, beyond this point, when you make more than a pair of photons for each run of the experiment,” said Quesada.
“We found that the color in which the daughter photons are born starts to slightly change and, moreover, that the efficiency of the process (how many ‘red’ photons are born per ‘violet’ photon) also changes.”
When Quesada first started exploring the possibility of creating two or more daughter photons for each experimental run, he had not yet identified ways to measure this experimentally. This year, however, his colleague Guillaume Thekkadath noticed that slight changes in color could also be reflected in different arrival times for the daughter photons, as one goes from barely making a pair to making many pairs.
“We observed that increasing the number of photons generated by the SPDC process caused a shift in the arrival time of the two daughter photons,” explained Thekkadath.
“To investigate this effect, we made two key modifications to the conventional SPDC experimental setup. First, we used a high-power laser capable of delivering ultrashort (femtosecond) pulses, compressing its energy into extremely short bursts. These pulses were further amplified to achieve the high intensities necessary for generating multiple pairs of daughter photons in the SPDC crystal. Second, we implemented a technique called ‘spectral interferometry’ to measure the photons’ arrival times with high precision.”
Thekkadath, Quesada and their colleagues passed photons generated by their high-power laser through an optical fiber that was several km long, which temporally stretched the photon pulse. Subsequently, they used superconducting nanowire detectors, highly sensitive devices that can detect single photons with exceptional timing resolutions, to record the photons’ arrival times.
The results they collected confirmed the presence of a gain-induced group delay between the multiphoton pulses generated in their SPDC source. This observation could have important implications for the future development of devices that leverage quantum interference.
“Our results imply that one has to be extra careful when interfering with the light coming from SPDC sources that produce photon pairs at different brightnesses (that produce different numbers of pairs, on average),” said Quesada.
“If one is not careful and has photons from two different sources arriving at an interferometer at different times, they are unable to perform a quantum mechanical feat known as Hong-Ou-Mandel interference. This interference is the one that allows quantum computers made of light to surpass the capabilities of classical computers.”
One of the co-authors of this recent paper, namely Martin Houde, has recently been trying to design better SPDC sources in which photons come out simultaneously, irrespective of the brightness of emitted laser pulses. Quesada and his colleagues at École Polytechnique de Montréal have also been trying to determine how the different arrival times of photons, which could be a source of error, affects the functioning of photonic quantum computers.
“Our SPDC source was relatively ‘bright,’ generating hundreds of daughter photon pairs compared to most sources, which typically produce only a single pair,” added Thekkadath.
“However, many of these photons were lost before reaching the optical detector. These losses occur for various reasons, such as reflections from optical elements like lenses or incomplete capture by the optical fibers. Losing one photon from a pair is problematic as it disrupts the quantum correlations essential for technologies like quantum-enhanced sensors.”
As part of their next studies, Thekkadath and his colleagues at the National Research Council of Canada are trying to devise strategies to minimize optical losses in quantum devices. In addition, they are trying to determine how the photon-pair sources they have been examining could be leveraged for quantum sensing and computing, irrespective of any associated photon losses.
More information: Gain-induced group delay in spontaneous parametric down-conversion. Physical Review Letters(2024). DOI: 10.1103/PhysRevLett.133.203601. On arXiv: DOI: 10.48550/arxiv.2405.07909
Journal information: Physical Review Letters , arXiv
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