Towards quantum-enhanced phase imaging
Quantum imaging and sensing provide important advantages in many measurement applications in fields ranging from fundamental physics to biology and microscopy to optical sensors. Since optical phase measurement is so valuable in a wide range of scientific fields, scientists have expended considerable effort to exploit quantum entanglement or squeezing for this purpose. This has resulted in major improvements in phase estimation and imaging in interferometric schemes – settings involving measurement using the phenomenon of wave interference – beyond the usual limits. However, interferometric schemes are not suitable for multi-parameter wide-field imaging, since they require raster scanning for extended samples. Researchers supported by the EU-funded Qu-Test project have now developed a technology that uses quantum correlations to enhance imaging of phase profiles in a non-interferometric way. As reported in a news item posted on ‘Newswise’, the proposed scheme “can be directly applied to wide-field transmission microscopy settings, to obtain a full-field phase retrieval in real time, and it is intrinsically more stable than an interferometric setup.” Furthermore, as the news item goes on to explain, the enhanced sensitivity could enable more information to be retrieved from samples “than classically allowed at a fixed photon exposure or, equivalently, at a fixed measurement time.” The method of exploiting entanglement to enhance imaging of a pure phase object in a non-interferometric setting is based on the so-called transport of intensity equation (TIE). TIE is a computational approach to reconstruct the phase of a complex wave in optical and electron microscopy, and it describes the internal relationship between a wave’s intensity and phase distribution. The TIE-based method is quantitative, providing the phase’s absolute value without any previous knowledge of the object and operating in wide-field mode. It therefore does not require time-consuming raster scanning.
Improved image quality and phase estimation
The method also involves a pair of entangled light beams. The quantum correlations between the two beams are so strong that they are identical at the single-photon level. The scientists used these correlations to reduce the intrinsic noise fluctuations of the probing light. This resulted in a general improvement of the image quality at a fixed number of photons irradiated through the object, creating a sharper image. It also enables more accurate quantitative phase estimation. “Quantum resources such as entanglement and squeezing have been proven useful to enhance a variety of sensing applications, such as imaging, interferometric phase estimation, target detection and ranging among others. Our proposal brings another contribution to this wide panorama by showing that the very well-studied TIE phase-retrieval classical scheme can be significantly boosted by using quantum correlations nowadays routinely available in laboratories, showing potential for relatively short-term applications,” the scientists observe in the ‘Newswise’ article. As they go on to describe in their paper published in the journal ‘Light: Science & Applications’, the work paves the way for applications at wavelengths beyond the visible spectrum, such as X-ray imaging, where photon dose reduction is crucial. Qu-Test (Qu-Test) ends in 2026. For more information, please see: Qu-Test project
Keywords
Qu-Test, quantum, interferometric, non-interferometric, phase imaging, phase retrieval, phase estimation, microscopy, entanglement
