A revolutionary method that reveals images hidden in noise

This innovative quantum-inspired imaging technology excels in low-light conditions, offering new frontiers in medical imaging and art conservation.

Researchers at the University of Warsaw School of Physics with colleagues from Stanford University and Oklahoma State University present a quantum-inspired phase imaging method based on measurements of the correlation of strong light intensity to phase noise. The new imaging method can work even with very low light, and could be useful in emerging applications, such as infrared and X-ray interferometry, quantum and matter-wave interferometry.

A revolution in photography techniques

Regardless of whether you're taking pictures of cats with your smartphone or taking pictures of cell cultures with an advanced microscope, you're doing so by measuring the intensity (brightness) of light in pixels. Light is characterized not only by its intensity, but also by its phase. Interestingly, transparent objects can become visible if you are able to measure the phase lag of light they introduce.

Phase contrast microscopy, for which Fritz Zernecke won the Nobel Prize in 1953, revolutionized biomedical imaging because of the possibility of obtaining high-resolution images of various transparent and optically thin specimens. The field of research that emerged from Zernike's discovery includes modern imaging techniques such as digital holography and quantitative phase imaging.

“It enables quantitative, label-free characterization of living samples, such as cell cultures, and could find applications in neurobiology or cancer research,” explains Dr. Radek Lapkiewicz, Head of the Quantitative Imaging Laboratory at the Faculty of Physics at the University of Warsaw.

Noise-resistive phase imaging with intensity correlation, credit: Faculty of Physics, University of Warsaw

Challenges and innovations in the photography stage

However, there is still room for improvement. “For example, interferometry, which is a standard measurement method for precise thickness measurements at any point of the investigated object, only works when the system is stable, not exposed to any shocks or disturbances,” explains Jerzy Szoniewicz, a doctoral student at the Faculty of Physics at the University of Warsaw. It is very difficult to perform such a test, for example, in a moving car or on a vibrating table.

Researchers from the University of Warsaw's School of Physics together with colleagues from Stanford University and Oklahoma State University decided to address this problem and develop a new method for phase imaging that is immune to phase instabilities. The results of their research have been published in the prestigious journal Advancement of science.

Back to old school

How did researchers come up with the idea of ​​the new technology? Leonard Mandel and his group demonstrated in the 1960s that even when interference is not detectable in intensity, correlations can reveal its presence.

“Inspired by the classic Mandel experiments, we wanted to study how intensity correlation measurements can be used in phase imaging,” explains Dr. Lapkiewicz. In correlation measurement, we look at pairs of pixels and observe whether they become brighter or darker at the same time.

“We have shown that such measurements contain additional information that cannot be obtained using a single image, i.e., densitometry. Using this fact, we have demonstrated that in interference-based phase microscopy, observations are possible even when standard interferometry patterns lose all phase information and do not There is a registered margin of severity.

“With the standard approach, one might assume that there is no useful information in such an image. However, it turns out that the information is hidden in the correlations and can be recovered by analyzing multiple independent images of an object, allowing us to obtain ideal interferograms, on Although normal interference is undetectable due to noise,” adds Labkiewicz.

“In our experiment, light passing through a phase object (our target, which we want to examine) is fitted with a reference light. A random phase delay is introduced between the rays of the object and the reference light – this phase delay mimics a disturbance that hampers standard phase imaging methods.

“Therefore, no interference is observed when measuring intensity, i.e., information about the phase object cannot be obtained from intensity measurements. However, the spatially dependent intensity-density correlation displays a marginal pattern that contains the full information about the phase object.

“This intensity-to-intensity correlation is not affected by any time-phase noise that varies slower than the detector speed (about 10 ns in the experiment) and can be measured by accumulating data over an arbitrarily long time period – which is a game-changer – the longer measurement It means more photons, which translates to higher Accuracy“, explains Jerzy Ssoniewicz, first author of the work.

Simply put, if we were to record a single frame of film, that single frame would not give us any useful information about the shape of the object under study. “So, we first recorded a complete series of these frames with the camera and then multiplied the measurement values ​​at each pair of points from each frame. We averaged these correlations, and recorded a complete image of our body,” explains Jerzy Szuniewicz.

“There are many possible ways to recover the phase profile of an observed object from a series of images. “However, we have demonstrated that our method based on intensity-intensity correlation and the so-called off-axis holographic technique provides optimal reconstruction accuracy,” says Stanislaw Kurdzialek. , the second author of this paper.

A bright idea for dark environments

The intensity correlation-based phase imaging approach can be widely used in very noisy environments. The new method works with both classical (laser and thermal) and quantum light. It can also be implemented in Photon Counting system, for example using single photon avalanche diodes. “We can use it in cases where there is little light available or when we cannot use high light intensity so as not to damage the object, for example, a delicate biological specimen or a work of art,” explains Jerzy Zuniewicz.

“Our technology will expand the horizons in phase measurements, including emerging applications such as infrared and X-ray imaging, quantum and matter wave interferometry,” concludes Dr. Lapkiewicz.

Reference: “Noise-resistant phase imaging with intensity correlation” by Jerzy Szoniewicz, Stanisław Kurdzialek, Sanjukta Kondo, Wojciech Šoliński, Radosław Čapkiewicz, Majukh Lahiri, and Radek Lapkiewicz, 22 September 2023, Advancement of science.
doi: 10.1126/sciadv.adh5396

This work was supported by the Polish Science Foundation within the framework of the I-Team project “Spatiotemporal photon correlation measurements for quantization and super-resolution microscopy” co-financed by the European Union under the European Regional Development Fund (POIR.04.04.00-00)-3004/17 -00). Jerzy Szuniewicz also acknowledges support from the National Science Centre, Poland, Grant No. 2022/45/N/ST2/04249. S. Kurdzialek acknowledges support from National Science Center Grant (Poland) No. 2020/37/B/ST2/02134. M. Mahiri. Acknowledges support from the United States Office of Naval Research under Award Number N00014-23-1-2778.