The Southern University of Science and Technology team achieves non-diffractive

2024-06-06

Recently, Professor Chen Jiefei from the Southern University of Science and Technology and her collaborators have, for the first time, achieved a single photon wave packet that has no diffusion in both time and space.

The experimental results combine the most cutting-edge light quantum preparation and wave packet manipulation techniques, presenting an Airy wave packet in the truly single photon quantum realm.

Due to the important application prospects of the Airy wave packet in multiple basic research and application fields, such as quantum communication, microscope imaging, etc., this achievement will attract attention in the field of quantum optics, imaging, and other research directions.

It is reported that the achievement demonstrated this time is the high-precision manipulation of the spatial and temporal degrees of freedom of the quantum light source at the single photon level. This technology itself can be extended to arbitrary spatiotemporal reshaping of single photon quantum light sources, such as Bessel beams and other light fields with special structures.

Structured light in space and time can be used to increase information capacity through spatial and temporal wave division multiplexing, similar to the method of encoding quantum information in other degrees of freedom.At the same time, Airy light bullets can also be used for several specific applications related to quantum communication, such as quantum information transmission between the ground and the air, and underwater.

Advertisement

In addition, the optical Airy beams in the classical field have been used to achieve optical microscopy, and the related optical microscopes have been commercialized.

Chen Jiefei expects that Airy control can be applied to quantum imaging using entangled photon pairs, thereby reshaping the spatiotemporal patterns of photon pairs.

Within the quantum realm, Airy light bullets utilize photon correlations to improve imaging resolution and recoverability in noisy environments.The Ideal Light Form for Long-Distance Quantum State Transmission

Light is the most ideal carrier for the transmission of quantum information over long distances. It is widely believed that by utilizing the already mature optical fiber communication network, light can carry quantum information from one city to another, even across tens of thousands of kilometers to the other side of the Pacific.

Quantum information transmission in free space has developed even faster. As early as 2021, Professor Pan Jianwei's team at the University of Science and Technology of China demonstrated a space-ground integrated quantum communication network spanning 4600 kilometers.

However, in optical fiber communication, due to the existence of dispersion, light pulses will undergo pulse deformation and collapse after being transmitted for a certain distance, eventually resulting in the complete disappearance of the light signal.

Dispersion occurs because the light signal is actually composed of different frequencies, and these different frequency light waves propagate at different speeds in the medium, ultimately leading to the desynchronization and disintegration of different components of the light signal.In so-called "free spaces" such as air or the atmosphere, due to the absence of waveguide constraints on spatial modes, light signals even diverge and diffract perpendicular to the direction of propagation, leading to the inability of light energy to concentrate and reach distant places.

These problems are common in classical beams and originate from the spatiotemporal patterns of light. This is similar to how a laser can illuminate further than the light from a flashlight, because the spatiotemporal pattern of the laser has been shaped and limited from the time it is generated at the light source.

However, a collimated laser still has a certain coherence time and length, and after a certain distance, the laser beam inevitably begins to diverge.

Quantum states of light are loaded in these "boxes" with spatiotemporal patterns, and thus are not immune. The general idea to solve this problem is to reshape or restrict the spatiotemporal patterns of light.

Optical fibers are a typical solution to restrict the spatial mode of light transmission. But optical fibers are not suitable for all occasions, such as air-to-ground or underwater communication scenarios.In the broader field of optical applications, such as microscopic imaging and optical tweezers, where fiber optics are not applicable, addressing the issue of beam propagation and diffusion in free space is particularly important.

Since 2007, special light fields with "propagation invariance," represented by Airy beams, have begun to attract high attention in the field of optics.

These light fields, which have special intensity and phase distributions in space, exhibit a series of advantages conducive to transmission, such as shape invariance during propagation, no diffraction, and even the ability to self-repair when the spatial mode is disrupted during transmission.

It provides an ideal form of light for the long-distance transmission of quantum states. Based on this, Chen Jiefei and others have started this research work.The more persistent, the more unobstructed.

 

Chen Jiefei stated: "This research was initially determined after a discussion with another researcher in our group, Georgios Siviloglou."

 

Georgios has a rich experience in the research of Airy beams. In 2007, when he was still a Ph.D. student, he and his colleagues from other research groups first reported the preparation of Airy beams and demonstrated their diffraction-free characteristics.

 

After discussion, Georgios believed that Airy beams could be used to solve the problem of long-distance transmission of quantum light, but at that time, there were still very limited reports on the control of quantum light Airy beams.

 

Georgios is familiar with the generation of classical Airy beams, while Chen Jiefei specializes in the preparation of quantum light sources and the manipulation and detection of single photons.Thus, they hit it off immediately and quickly established a general approach and research objectives - that is, to demonstrate a single-photon quantum light source that can achieve Airy control in both time and space dimensions.

Subsequently, they combined experimental techniques of spatial light modulation and cold atom quantum optics, breaking down the spatiotemporal control of Airy single photons into independent spatial and temporal manipulations.

Chen Jiefei believes that the cold atom ensemble is very suitable for manipulating single-photon Airy bullets in the time dimension. The reason is that the waveform of the single photon in time can be controlled by the spatial mode of the pump light acting on the atomic ensemble.

Since they used a two-dimensional magneto-optical trap to prepare the atomic ensemble, the effective length of the final atomic ensemble can reach 2 centimeters. Therefore, on such a large spatial scale, the control of the spatial mode of the beam will be very convenient.

In addition, in the Airy control of the spatial dimension, Chen Jiefei and Georgios unanimously believe that the Airy control of the spatial mode can directly utilize the spatial light modulator to modulate the single-photon beam emitted by the atomic ensemble.Subsequently, they began conducting experiments. During this period, they first reduced the intensity of a laser beam to a weak light level where each pulse contained only a very small number of photons. Then, they placed a spatial light modulator in the path of this beam and used a high-sensitivity scientific-grade charge-coupled device (CCD) to detect the spatial distribution of photon signals.

However, at the beginning, the spatial light modulator they had purchased had not yet been shipped. Georgios proposed using a combination of three cylindrical lenses to perform Airy modulation.

This plan was quickly verified to be successful, and they obtained a photon distribution that met the two-dimensional Airy function in space.

The subsequent operations were more challenging, requiring a pump beam that had undergone one-dimensional Airy control to be accurately aligned with the center of the atomic ensemble and uniformly act on the atoms. At the same time, the atomic ensemble must be in a state of optimal atomic density.

This was the most difficult part of the entire experiment, requiring careful optimization of all experimental conditions and optical paths.Half a month later, the experimental results revealed the rudimentary form of the first two lobes of the Airy function. However, the experimental results were not stable; every day when the system was activated, the single-photon temporal waveform obtained was inconsistent.

After analysis, they believed that the combination of three cylindrical lenses required a high degree of stability for the optical path, coupled with the instability of the system, which made it difficult to effectively repeat the experimental data.

At that time, the spatial light modulator was about to arrive, so the research group first carried out system optimization while waiting for the spatial light modulator.

Whenever the experimental results obtained through the preset experimental plan did not meet expectations, it was necessary to use alternative schemes.

For example, when they were measuring the parabolic trajectory of the Airy single-photon bullet, they modified the detection plan.Initially, the plan was to use scientific-grade CCD to detect light signals, but in practice, due to the very low single-photon counting rate, the existing CCDs did not have sufficient sensitivity to measure the spatial trajectory of single photons.

Therefore, they utilized optical imaging techniques to modify the optical path and used optical fibers to collect and measure the photon trajectory with single-photon detectors.

During the process, the experimental system underwent slight changes due to variations in temperature and humidity within the laboratory, leading to the inability to reproduce experimental results.

At this point, it was necessary to re-optimize the atomic system and find the optimal interaction region again. In addition, single photons are very weak light signals, and the time required to accumulate a set of usable data is usually calculated in hours.

Therefore, in verifying the repeatability of the results, the first author of the paper, Wang Jianmin, endured a tedious and tormenting period of time.Later, Wang Jianmin summarized by saying: "Our attitude towards scientific research should be like an Airy beam, the longer we persist in the face of difficulties, the more naturally we will smooth out the impact of obstacles and achieve self-repair."

Ultimately, the related paper was published in Physical Review Letters[1] under the title "Spatiotemporal Single-Photon Airy Bullets," with Wang Jianmin as the first author and Chen Jiefei and Georgios Siviloglou as the co-corresponding authors.

However, this work is not the end, but the beginning of exploration. Subsequently, the research team wants to explore whether these types of propagation-invariant wave packets can be directly stored in atomic ensembles and applied to quantum optical storage.

One advantage of atomic ensembles is their narrow spectral linewidth, which allows for the realization of long coherence time photons, which may be used to explore quantum repeaters based on structured light.Additionally, since the team's spatiotemporal manipulation technology for single photons has already matured, they are currently exploring the use of machine learning and spatial light modulator technology to obtain the most suitable spatiotemporal patterns of light quantum for storage in atomic media.

Another exciting possibility is to utilize media with a broader spectrum, such as nonlinear crystals, and to achieve time-only single photons within the femtosecond range, where temporal dynamic effects are more pronounced. It is expected that this will enable them to observe the direct acceleration form of single-photon wave packets.

Furthermore, they also plan to use AI to optimize the experimental system, hoping to achieve more complex quantum systems that can provide real-time feedback and self-optimization, thereby reducing human intervention.

"Being one's own super postdoctoral fellow"It is also reported that there are not many female scholars studying physics. So, how did Chen Jiefei embark on this path?

It is said that Chen Jiefei's doctoral advisor is Professor Du Shengwang. At that time, when Chen Jiefei was in her second year of graduate studies at the Hong Kong University of Science and Technology, Professor Du Shengwang had just joined the faculty and was in need of students.

As a result, Chen Jiefei's master's advisor recommended her to Professor Du Shengwang's research group, where she became the latter's first graduate student.

Professor Du Shengwang's research direction is atomic and molecular optics and quantum physics, so it was natural for Chen Jiefei to take this path.

Chen Jiefei said: "The first year after I joined was a key period for the construction of Professor Du's laboratory. At that time, there were only me, doctoral student Hou Dong (now an associate professor at Xi'an Jiaotong University), and Professor Du himself working in the research group. We spent almost all our time in the laboratory setting up a cold atom system."That year was the year when Chen Jiefei absorbed knowledge and skills in her studies and research at the fastest pace, feeling that she learned more in that year than in the previous two decades.

When she saw the glowing cold atom clusters on the monitoring screen, Chen Jiefei felt a great sense of achievement, which was the "miracle" they built up one optical component at a time.

With the beginning of Chen Jiefei's independent academic career, like most "young teachers," difficulties also came one after another. "At that time, I was my own super postdoctoral, and there was no mentor to rely on," she said.

In fact, after establishing an independent group, Chen Jiefei was often confused when setting topics. At first, she didn't know what to do. When she finally knew what to do, she began to worry about how to do it.

Fortunately, during this period, Chen Jiefei also gradually grew up, and colleagues and family members have given a lot of help.She said: "In the past few years since joining the Southern University of Science and Technology's Institute for Quantum Science, with the help of the dean, Academician Yu Dapeng, I have gradually built my own team. The country also strongly supports the development of quantum science and technology, and I look forward to contributing more."

Leave a comment

Submit