Hanbury Brown and Twiss effect

Ghost imaging is a technique that allows an object to be imaged using light that never actually touched the object. Traditionally, this required highly controlled laboratory settings and entangled photon pairs produced by spontaneous parametric down-conversion. The recent experiment shifts this paradigm by replacing expensive, fragile quantum sources with the sun, proving that the chaotic nature of sunlight contains enough correlation to perform high-resolution imaging.

The Mechanics of Solar Correlation

The experiment relies on the second-order correlation of light, a phenomenon rooted in the Hanbury Brown and Twiss effect. While sunlight appears as a constant stream of illumination, it is actually composed of countless independent emitters. This creates a complex, fluctuating speckle pattern across space and time. These fluctuations are not random; they are correlated across different points in the light field.

In the setup, the solar beam is split into two paths: a signal arm and a reference arm. The signal arm contains an object and a bucket detector. This detector is a single-pixel sensor that collects all light passing through the object but cannot resolve any spatial detail. It simply records the total intensity of light hitting it at a specific moment.

Simultaneously, the reference arm sends an identical portion of the sunlight to a high-resolution camera. This camera never sees the object. By correlating the intensity fluctuations recorded by the bucket detector with the spatial patterns captured by the camera, the researchers can mathematically reconstruct the image of the object. The image emerges not from a direct projection, but from the statistical coincidence of photons.

Experimental Architecture and Resolution

To make the experiment viable, the team had to overcome the massive noise inherent in ambient light. Sunlight is broadband, meaning it contains a wide spectrum of wavelengths that would normally wash out the necessary correlations. The researchers employed a narrow-band interference filter centered at 532 nanometers to isolate a specific frequency of light, ensuring the coherence length was sufficient for the correlation to be detectable.

The target object was placed 10 meters from the splitting optics. The bucket detector recorded the integrated intensity, while the reference arm utilized a CMOS sensor with a pixel pitch of 3.45 microns. The resulting image showed a clear alphanumeric character, confirming that the spatial resolution was limited by the aperture of the system rather than the nature of the light source.

The ability to use a natural, non-coherent source like the sun removes the need for complex cryogenic cooling or high-power pump lasers that typically restrict quantum imaging to the laboratory.

Dr. Li Wei, Lead Researcher at the University of Science and Technology of China

The team noted that the signal-to-noise ratio improved as the integration time increased. Because solar fluctuations occur on a femtosecond scale, the detectors must be capable of capturing these correlations over millions of frames to produce a crisp image, a process that requires significant computational power for the cross-correlation calculations.

Distinguishing Thermal from Entangled Imaging

A central point of analysis in the study is the distinction between quantum ghost imaging and thermal ghost imaging. Original ghost imaging used entangled photons, where the position and momentum of two photons are strictly linked. In that scenario, the correlation is nearly perfect, allowing for high-contrast images with very few photons.

Sunlight is a thermal source, meaning its photons are not entangled. Instead, they exhibit photon bunching, a quantum statistical property where photons tend to arrive in groups. The correlations in sunlight are weaker than those found in entangled pairs, resulting in a lower inherent contrast. However, the researchers demonstrated that the mathematical framework for ghost imaging remains identical regardless of whether the correlation is based on entanglement or thermal statistics.

This finding suggests that the ghost effect is a general property of light correlation rather than a unique feature of entanglement. By utilizing the Bose-Einstein statistics of thermal light, the researchers achieved an imaging result that is functionally equivalent to quantum-source imaging for most practical applications.

Applications in Remote Sensing

The shift to sunlight as a source has immediate implications for remote sensing and covert surveillance. Because the light source is external and ambient, the imaging system does not emit its own radiation. This makes the sensor passive and virtually undetectable to the target being imaged.

First, the system can image through turbid media, such as fog or smoke. Because the bucket detector only needs to collect some of the light that passed through the object, the spatial distortion caused by scattering in the signal arm does not degrade the final image. The spatial information is preserved in the reference arm, which travels through clear air.

Second, the technology could be applied to long-range atmospheric monitoring. By using the sun as a source, sensors could potentially image distant objects or atmospheric anomalies without the power requirements of a high-energy laser system.

The current limitation remains the requirement for a synchronized reference beam. To image a distant object in a real-world setting, the system must be able to split the incoming sunlight perfectly or find a way to correlate the solar pattern at the target with the pattern at the sensor. The January 2026 study used a beam splitter for precision, but the team is now exploring the use of computational ghost imaging, where the reference pattern is simulated or predicted based on the known properties of solar radiation.

Future iterations of the experiment will aim to increase the distance between the source and the object. The researchers intend to test the system in outdoor environments to determine how atmospheric turbulence affects the correlation of solar speckle patterns over kilometer-scale distances.