Entangled Photons Boost Radar Signal Detection Beyond Single-Photon Limits

Sunghwa Kang and colleagues at Naval Information Warfare Centre Pacific demonstrate that two-photon entangled states improve quantum radar cross section (QRCS) measurements. Signal-signal entanglement offers enhancement over both single-photon and separable two-photon QRCS approaches. The evaluation of performance across different target geometries derives a QRCS formula applicable to biphoton states with varying degrees of entanglement and calculates the resultant scattering patterns, representing a key step towards advanced radar technology.

Entangled photons sharply improve radar detection of low-reflectivity targets

A 30% enhancement in quantum radar cross-section (QRCS) is achieved using maximally entangled two-photon states, surpassing the limitations of single-photon and separable two-photon methods. This gain allows for the detection of targets with lower reflectivity, a capability beyond the reach of conventional radar systems. A new formula, derived from two-photon scattering theory and a double-Gaussian approximation, enables precise control over the quantum signal by calculating QRCS for biphoton states with varying entanglement levels. The significance of this improvement lies in the potential to detect targets that would otherwise be invisible to traditional radar due to their small size or radar-absorbent materials, offering a crucial advantage in surveillance and defence applications.

The team modelled photon scattering using the complex mathematical technique known as the ‘T-matrix’, a standard approach in scattering theory used to describe how particles scatter from a potential. The T-matrix formalism allows for a rigorous calculation of the scattering amplitude, crucial for determining the QRCS. Prior to work, notably Brandsema’s PhD Thesis (2017), had shown signal-idler entanglement offered no benefit to QRCS enhancement. This is because the correlation between signal and idler photons does not contribute to a stronger reflected signal. Consequently, scientists focused on this more effective approach of signal-signal entanglement to improving detection sensitivity. Simulations across square, circular, and triangular targets in both monostatic and bistatic radar configurations reveal a distinct scattering pattern and enhanced side-lobe sensitivity, particularly for smaller objects. Analysis of a 31×31 atom square target, for example, showed enhancement around ±0.5 radians, differing from the main lobe observed in single-photon returns. Circular and triangular targets, comprising 709 and 462 atoms respectively, exhibited even more pronounced side-lobe enhancement. These improvements remained consistent across varying target sizes and at a wavelength of 4λ, and also held true in spatially separated bistatic configurations. Pump divergence and phase-matching bandwidth parametrised entanglement control, allowing for precise manipulation of the quantum signal. These parameters directly influence the degree of entanglement, and therefore the resulting QRCS. However, these calculations assume ideal conditions and do not yet account for atmospheric interference, such as turbulence and absorption, or detector limitations, such as noise and efficiency, which would sharply impact real-world performance. Atmospheric effects introduce decoherence, reducing the degree of entanglement and diminishing the QRCS enhancement. Detector limitations, particularly low detection efficiency, can obscure the weak signals from low-reflectivity targets.

Entangled photons enhance radar detection despite current modelling limitations

Quantum entanglement, a phenomenon linking two particles regardless of distance, is being explored to refine radar technology. This non-classical correlation between photons offers the potential to overcome limitations inherent in classical radar systems. Conventional radar relies on bouncing electromagnetic signals off objects and measuring the reflected power, but this work investigates how manipulating entangled photons could sharpen detection, especially for smaller or stealthier targets. The underlying principle is that entangled photons exhibit correlations that can be exploited to improve signal-to-noise ratio and enhance the detection of weak signals. Translating these theoretical gains into practical systems remains a vital hurdle, as current models depend on a double-Gaussian approximation, simplifying photon behaviour and limiting the scope to two-dimensional scenarios. The double-Gaussian approximation models the spatial distribution of the electromagnetic field, but it may not accurately represent the complex field distributions created by realistic targets.

Despite reliance on a simplified, two-dimensional model and the double-Gaussian approximation, this work establishes an important theoretical foundation. Utilising entangled photons can indeed enhance radar cross-section measurements, offering improved detection compared to conventional or separable photon approaches. This enhancement validates the core principle and justifies further investigation into more complex, realistic scenarios. The observed enhancement is not merely a theoretical curiosity; it demonstrates the potential for quantum radar to outperform classical radar in specific scenarios. Quantum radar’s potential to identify subtle targets remains a compelling area of research, with implications for applications ranging from remote sensing and environmental monitoring to security and defence. The ability to detect low-reflectivity targets could revolutionise these fields.

Over the coming decade, further development will explore more complex, realistic scenarios, building on the established formula for calculating the quantum radar cross section, a measure of radar detectability. This formula provides a quantitative link between the degree of entanglement and the resulting QRCS, enabling researchers to optimise the quantum radar system for specific applications. The resulting calculations, validated across various target shapes and radar configurations, reveal a distinct scattering pattern compared to conventional radar, suggesting improved ability to discriminate between objects. This difference in scattering patterns could be exploited to develop advanced signal processing algorithms for target identification and classification. This work specifically focuses on ‘signal-signal’ entanglement, where linked photons enhance the signal, and provides a key step towards realising the potential of quantum radar technology. Future research will need to address the challenges of maintaining entanglement in noisy environments and developing efficient detectors capable of capturing the weak signals from entangled photons. Furthermore, extending the model to three-dimensional scenarios and incorporating more realistic target geometries will be crucial for translating these theoretical gains into practical applications. The development of quantum radar represents a significant advancement in radar technology, with the potential to enhance our ability to detect and track objects in a wide range of environments.

The research demonstrated that utilising two-photon entangled states can enhance the quantum radar cross section when compared to single-photon or separable two-photon approaches. This improvement suggests quantum radar has the potential to outperform classical radar in certain detection scenarios. Calculations were performed for two-dimensional target geometries, revealing a unique scattering pattern from entangled photons. The authors intend to expand this model to three-dimensional scenarios and address the challenges of maintaining entanglement in real-world conditions.