A paper recently published in the journal Scientific Reports proposed a novel approach for quantum-secured image transmission.
(a) The images shown used the combined triggers from all four polarisation channels (HVDA). The projected image is obscured by increasing the background light level. (b) The signal measured in each of the receiver polarisation channels changed depending on the transmitted polarisation state. The signal shown was measured with the photon counting detectors while the images were recorded on the camera. Image Credit: https://www.nature.com/articles/s41598-024-62415-2
Background
The utilization of quantum-secure protocols is becoming crucial in encrypted communications, particularly with the increasing relevance of quantum key distribution (QKD)-based techniques in sectors such as finance and defense. These techniques are prized for their ability to detect eavesdroppers and securely exchange keys. However, one significant drawback is their low data transmission speed. Advances in quantum imaging, which exploit correlations between photons, are enabling image capture even in noisy environments and hold promise for enhancing data capacity.
Integrating these technologies to securely encode photons on a high-dimensional basis could significantly expand a communication system's data capacity and introduce innovative methods of image transmission. Using polarization states for encryption is a well-established approach that continues to propel the rapid development of quantum communications forward.
Research into higher-dimensional quantum secure communication has also been explored extensively. These advanced approaches focus on using mutually unbiased measurement bases, such as position versus transverse momentum, orbital angular momentum versus angle, and time versus frequency. Recently, secure image transmission has emerged as a critical area, laying the groundwork for future breakthroughs in high-dimensional communication technologies.
The Proposed Approach
In this research, a novel method for secure image transmission was explored, focusing on direct image transmission while ensuring the detection of any eavesdroppers, akin to Quantum Key Distribution (QKD) techniques. The researchers employed a photon-pair source enhanced by the deliberate addition of a thermal light source as background noise.
This setup allowed one photon of each pair to illuminate the object, effectively masking it from eavesdroppers by blending it with indistinguishable thermal photons. The companion photon served as a time reference, enabling the intended recipient to selectively filter out the image-carrying photons from the background.
To enhance security, the reference photons were encoded using conventional polarization-based QKD techniques. The security verification utilized a two-dimensional (2D) polarization basis, whereas the image information itself was encoded in a much higher-dimensional Hilbert space, specifically a pixel basis, which is information-rich.
Building on previous studies that concealed low-intensity images within background noise, this approach leveraged a photon-pair source to generate idler photons and a time-correlated signal. The signal beam was used to create an image through a programmable mask, while the idler beam activated a detector. This setup recorded the spatial position of the signal photons, reconstructing the image over multiple photon readings. Background light was intentionally added to obscure the image from casual observation.
However, in this system, only recipients possessing the idler photons to trigger the camera could successfully extract the actual image from the noise. The primary security concern was the potential interception of these trigger photons by an eavesdropper, who could extract and then re-transmit the image and the trigger photons to the intended recipient.
To address this vulnerability, researchers enhanced the system by employing polarization encoding on the trigger photons, inspired by the QKD-BB84 protocol. This encoding secured the idler photons by their polarization, providing crucial temporal information required to extract the spatial information of the time-correlated signal photons.
Additionally, the strategic use of background noise in the information transmission process made this approach particularly suitable for scenarios where such noise is an inherent part of operations, like optical transmission in free space. In a demonstration by the researchers, the system successfully managed an image comprising 152 independent pixels, illustrating the effectiveness of their security enhancements in a practical setting.
Significance of the Study
Researchers showed that images generated from a time-correlated photon-pair source could be hidden within an optical noise background. The polarization state encoding allowed a post-measurement comparison of states between the receiver and sender to ensure secure transmission of the heralding photons.
Additionally, the imaging system achieved covertness by concealing the image photons to ensure that these image photons could not be distinguished from the optical background through any polarization, wavelength, or temporal filtering. Thus, this image transmission could be used for covert information communication securely.
However, the timing and polarization were not inextricably linked, which could be exploited to attack the security of the proposed image transmission approach. Quantum teleportation is a potential way to exploit this vulnerability. In quantum teleportation, an input photon's quantum state, which is destroyed in the process, can be copied to the outgoing photon.
The Bell-state analyzer is the key to this copying process. However, all existing Bell-state analyzers depend on the photons' indistinguishability and the use of an entangled photon source synchronized to the incoming photon. Practically, either of these aspects can reduce the analyzer's probability of success and significantly decrease the photon rate or introduce errors in the polarization data's post-analysis, revealing the eavesdropper.
To summarize, the findings of this study demonstrated the feasibility of the proposed approach for the secure transmission of images. However, the specific implementation has yet to reach the data rate of any optimized and existing traditional QKD system.
Journal Reference
Johnson, S., Rarity, J., Padgett, M. (2024). Transmission of quantum-secured images. Scientific Reports, 14(1), 1-7. https://doi.org/10.1038/s41598-024-62415-2, https://www.nature.com/articles/s41598-024-62415-2
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Article Revisions
- May 29 2024 - Title changed from "Quantum-secured Image Transmission Boosts Data Security" to "Enhancing Data Security: A Novel Approach for Quantum-Secured Image Transmission"