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A hot 85Rb vapor cell is used as the necessary non-linear medium for the FWM process which generates related quantum twin beams, which we call probe and conjugate. The FWM is based on a dual configuration in the D1 line of 85Rb as shown in the upper inset. The beam from the pump is reflected by an SLM which impresses on it a phase pattern to obtain the necessary momentum distribution (angular spectrum) for the pump. The phase structure pump is then imaged in the center of the cell by means of an optical system 4f. Finally, the momentum distribution of the probe and conjugate beams is mapped to a position distribution on a far-field EMCCD camera using an f-to-f imaging system. The images acquired with the EMCCD are then used to measure the distribution of spatial correlations and extract the information encoded in the twin bundles. To generate bright twin beams, we seed the FWM with an input probe beam to obtain a photon flux of 1014 photons/s per output beam, which is limited by the saturation of the EMCCD. SLM – spatial light modulator; EMCCD – Electron Multiplier Charge Coupled Device. Credit: The progress of science (2023). DOI: 10.1126/sciadv.adf9161

Researchers at the University of Oklahoma conducted a recently published study on The progress of science this demonstrates the principle of using spatial correlations in quantum entangled light beams to encode information and enable its secure transmission.

Light can be used to encode information for high-speed data transmission, long-distance communication, and more. But for secure communication, encoding large amounts of information in the light presents additional challenges to ensure the privacy and integrity of the transferred data.

Alberto Marino, Ted S. Webb Presidential Professor at the Homer L. Dodge College of Arts, led the research with OU graduate student and first author Gaurav Nirala and co-authors Siva T. Pradyumna and Ashok Kumar. Marino also holds positions at the Center for Quantum Research and Technology at OU and at the Quantum Science Center, Oak Ridge National Laboratory.

‘The idea behind the project is to be able to use the spatial properties of light to encode large amounts of information, just like an image contains information. However, being able to do so in a way that is compatible with quantum networks for the secure information transfer. When you look at an image, it can be constructed by combining basic spatial patterns known as modalities, and depending on how these modalities are combined, the image or the encoded information can be modified,” Marino said.

“What we’re doing here that’s new and different is we’re not just using those modalities to encode information; we’re using the correlations between them,” he added. “We’re using the additional information about how these modalities are related to encode the information.”

Frames (A) and (E) show the target information to be encoded in the spatial cross-correlation of the twin beams. The target is used to calculate the corresponding CGH (B) and (F) with a MRAF algorithm. The SLM pixel size (12.5m12.5m) and its 8-bit resolution along with the f-to-f imaging system are taken into account to calculate the simulated cross-correlations in frames (C) and (G) . Spatial cross-correlations measured between the probe and conjugate intensity fluctuations reveal the encoded information, as shown in frames (D) and (H). Except for frames (B) and (F), each pixel value is normalized to the sum of the squared amplitude of all pixels in the image to provide a better comparison between the simulation and the experiment. The maximum values ​​for the cross-correlations of the experimental and simulated data are larger than the target due to non-uniform distributions that result from a non-ideal setup and CGH. A small rotation ( 5 ) can be seen in the measured spatial cross-correlations, which is due to experimental alignment imperfections. All digits, except the CGH, are in EMCCD pixel basis, with a pixel size of 16m16m. The color bar for CGH frames (B) and (F) corresponds to the 8-bit phase coding in the range 0 to 2. For a detailed explanation of the measurement procedure and calculation of spatial cross-correlations. Credit: The progress of science (2023). DOI: 10.1126/sciadv.adf9161

The researchers used two beams of light that are intertwined, meaning that the light waves are interconnected with stronger correlations than can be achieved with classical light and remain interconnected despite their distance.

“The advantage of the approach we introduce is that it is not possible to recover the encoded information unless we make joint measurements of the two entangled beams,” Marino said. “This has applications like secure communication, since if you were to measure each beam by itself, you wouldn’t be able to extract any information. You have to get the information shared between both beams and combine it in the right way to extract the encoded information.”

Through a series of images and correlation measurements, the researchers demonstrated the results of successfully encoding information in these quantum light beams. Only when the two beams were combined using the intended methods did the information resolve into recognizable information encoded in the form of images.

“The experimental result describes how one can transfer spatial patterns from one optical field to two newly generated optical fields using a quantum mechanical process called four-wave mixing,” said Nirala. ‘The encoded spatial pattern can only be retrieved by joint measurements of the generated fields. An interesting aspect of this experiment is that it offers a new method of coding information in light by changing the correlation between the various spatial modes without affecting the temporal correlations.’

“What this could in principle enable is the ability to securely encode and transmit a lot of information using the spatial properties of light, just as an image contains much more information than simply turning the light on and off,” Marino said. “Using spatial correlations is a new approach to coding information.”

“Encoding of information in the spatial correlations of entangled twin beams” was published in The progress of science.

More information:
Gaurav Nirala et al, Encoding of information in spatial correlations of entangled twin beams, The progress of science (2023). DOI: 10.1126/sciadv.adf9161

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