Quantum imaging presents numerous advantages over classical imaging techniques, including exploiting quantum properties such as entanglement and superposition. However, it has encountered several challenges that have hindered its widespread adoption and application.
In a new study, Caltech researchers introduced quantum imaging by coincidence from entanglement (ICE), using spatially and polarization-entangled photon pairs to overcome these challenges. Entanglement allows the team to control not only the color and brightness of the light hitting a sample but also the polarization of that light.
Lihong Wang, the Bren Professor of Medical Engineering and Electrical Engineering, said, “Our new technique has the potential to pave the way for quantum imaging in many different fields, including biomedical imaging and potentially even remote space sensing.”
This new technique, called quantum imaging by coincidence from entanglement (ICE), takes advantage of entangled photon pairs to obtain higher-resolution images of biological materials, including thicker samples, and to make measurements of materials that have what scientists call birefringent properties.
When entering light, waves are split into two independent waves, each moving at a different speed and undergoing a variable degree of refraction; this phenomenon is known as birefringence and is displayed by specific materials. This effect results from the material’s anisotropic nature, which means that the direction light propagates affects the material’s optical characteristics.
A well-known example of a birefringent material is calcite crystals, which divide light entering the crystal into ordinary and extraordinary rays, each of which moves in a different direction and at a different speed. However, birefringence is not just found in minerals; it is also present in a variety of biological components. Collagen, cartilage, starch, and cellulose are some of the birefringent biological materials.
Two polarizers positioned at right angles to one another and sandwiching a birefringent sample between them can selectively block light waves of particular polarizations. However, as some of the incoming light waves pass through the material, their polarization changes because of the sample’s birefringent features. As a result, some light that the polarizers would have typically obstructed can now flow through and reach the detector.
An entangled photon pair is occasionally formed in Wang’s ICE setup, where light is first passed through a polarizer and then through two unique crystals of barium borate. Approximately one pair is produced for every million photons that travel through the crystals. After that, one of the two entangled photons will follow the idler arm of the system, which travels straight forward, while the other will follow the signal arm, which takes a longer detour and causes the photon to pass through the item of interest.
Ultimately, both photons pass through a second polarizer before arriving at two detectors, which note the photons’ arrival time. However, because the photons are entangled, a “spooky” quantum effect arises here: the idler arm detector can function as a virtual “pinhole” and “polarization selector” on the signal arm, instantly altering the polarization and location of the photon incident on the object in the signal arm.
Yide Zhang, lead author of the new paper and a postdoctoral scholar fellowship trainee in medical engineering at Caltech, said, “In the ICE setup, the signal and idler arms detectors function as ‘real’ and ‘virtual’ pinholes, respectively. This dual pinhole configuration enhances the spatial resolution of the object imaged in the signal arm. Consequently, ICE achieves higher spatial resolution than conventional imaging that utilizes a single pinhole in the signal arm.”
Xin Tong, co-author of the study and a medical and electrical engineering graduate student at Caltech, said, “Since each entangled photon pair always arrives at the detectors at the same time, we can suppress noises in the image caused by random photons.”
With a classical microscopy setup, scientists usually switch between different input states, illuminating an object separately with horizontally, vertically, and diagonally polarized light and then measuring the corresponding output states with a detector to ascertain the birefringent properties of a material. The aim is to measure how the sample’s birefringence affects the image that the detector perceives in each of those states. Scientists may obtain visuals from this data that would not be possible otherwise, and it also tells them about the sample’s structure.
Wang is already thinking about applications for his new method, such as measuring birefringence in space. Imagine a scenario in which an intriguing object, possibly an interstellar medium, is situated light years from Earth. Using the ICE technique, a satellite in orbit might be positioned to emit entangled photon pairs, with two ground stations serving as detectors.
It would be difficult to send any form of communication to the satellite to change the device’s source polarization due to its great distance. However, changing the polarization state in the idler arm would be the same as altering the polarization of the source light before the beam’s impact with the target because of entanglement.
Wang says, “Using quantum technology, nearly instantaneously, we can make changes to the polarization state of the photons no matter where they are. Quantum technologies are the future. Out of scientific curiosity, we need to explore this direction.”
Journal Reference:
- Yide Zhang, ZHe He, Xin Tong et al. Quantum imaging of biological organisms through spatial and polarization entanglement. Science Advances. DOI: 10.1126/sciadv.adk1495
