Using polarization to improve quantum imaging

Quantum imaging is a growing field that is counter-intuitive andScaryThe ability of light particles, or photons, to become bound or entangled in special situations. If the position of one of the two entangled photons is changed, the other will change, no matter how far apart the two photons are.

Caltech researchers demonstrated this last May How such complexity can double the resolution of classical light microscopes while preventing the imaging system's light from damaging fragile biological samples. Now the same team has developed a technique that can quantum image whole organelles and even tiny organisms.

Led by Lihong Wang, a Bren professor in the Department of Medical Engineering and Electrical Engineering, the new work uses the problem — once famously described by Albert Einstein.Scary action in the distance– To control not only the color and brightness of light hitting a sample, but also the polarization of that light.

Our new technique has the potential to pave the way for quantum imaging in a variety of fields, including biomedical imaging and remote space sensing.Wang says, The Andrew and Becky Tserng Medical Engineering Chair and Chief Executive Officer for Medical Engineering.

Like wavelength and intensity, polarization is a fundamental property of light and refers to the direction in which the electric component of a light wave is oriented relative to the wave's general direction of travel. Most light, including sunlight, is unpolarized, meaning its electromagnetic waves travel in all directions. However, filters called polarizers can be used to produce light beams with a specific polarization. A vertical polarizer, for example, allows only photons with vertical polarization to pass through. Those with horizontal polarization (meaning that the electric component of the light wave is horizontal with respect to the direction of travel) are blocked. Any light with other polarization angles (between vertical and horizontal), will partially pass through. The result is a stream of vertically polarized light.

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Polarized sunglasses reduce glare. Vertically polarized chemical coatings are used to block horizontally polarized sunlight from reflecting off a horizontal surface such as a lake or snow field. This means the wearer only observes vertically polarized light.

When changes in light intensity or color are insufficient to provide scientists with quality images of certain objects, controlling the polarization of light in an imaging system can sometimes provide additional information about the sample and provide another way to distinguish one sample from another. Background. Detecting polarization changes caused by certain samples can provide researchers with information about the internal structure and behavior of those materials.

Wang's new microscopy technique, coincidentally called quantum imaging, uses pairs of entangled photons to obtain high-resolution images of biological materials, including thick samples, and to make measurements of what scientists call birefringent properties.

Instead of bending incoming light waves continuously, as most materials do, dihedral materials bend those waves to varying degrees depending on the polarization of the light and the direction it is traveling. The most common birefringent materials studied by scientists are calcite crystals. But biological materials, cellulose, starch, and many types of animal tissue, including collagen and cartilage, are also divalent.

If a sample with dipole properties is placed between two polarizers at a 90-degree angle, some of the light passing through the sample will be changed in its polarization and sent to the detector, despite everything else. The incoming light must be blocked by two polarizers. The detected light can then provide information about the structure of the sample. In materials science, for example, scientists use dipole measurements to better understand the areas where mechanical stress develops in plastics.

In Wang's ICE system, light is first passed through a polarizer and then through a pair of special barium borate crystals, occasionally creating a entangled photon pair; About one pair is produced for every million photons passing through the crystals. From there, the two entangled photons separate and follow one of the system's two arms: one that travels straight ahead, called the idler arm, and the other that traces a more circuitous path, called the signal arm, that causes the photon. Go through the object of interest. Finally, the two photons pass through an additional polarizer before reaching two detectors, which record the time of arrival of the detected photons. However, here one occursScary Quantum effect due to entanglement of photons: detector in passive arm can act like a virtualNeedle hole and a „polarization selector” in the signal arm that instantly affects the location and polarization of the photon incident on the material in the signal arm.

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„In an ICE system, the detectors in the signal and passive weapons act as 'real' and 'virtual' pinholes, respectively.” says Yit Zhang, lead author of the new paper and postdoctoral fellow He was a Scholar Fellowship Trainee in Medical Engineering at Caltech. „This dual pinhole configuration improves the spatial resolution of the imaged object in the signal arm. As a result, ICE achieves higher spatial resolution than conventional imaging that uses a single pinhole in the signal arm..

„Because each entangled photon pair always arrives at the detectors at the same time, we can suppress noise in the image caused by random photons,” says Jin Dong, co-author of the study and a graduate student in medicine and electrical engineering at Caltech. .

To determine the dichroic properties of an object with a classical microscope setup, scientists typically illuminate an object separately with horizontally, vertically, and diagonally polarized light by alternating different input levels, and then measure the corresponding output levels with a detector. The goal is to measure how the dichroism of the sample changes the image the detector receives in each of those states. This information informs scientists about the structure of the sample and can provide images that would otherwise not be possible.

Because quantum entanglement allows pairs of photons to connect no matter how far apart, Wang is already imagining how his new system could make measurements of birefringence in space. Consider a situation where something of interest, perhaps a galactic medium, is located light-years from Earth. A satellite in space can be positioned to emit entangled photon pairs using the ICE technique, with two ground stations acting as detectors. Due to the large distance to the satellite, it is impractical to send any kind of signal to correct the source polarization of the device. However, due to complexity, changing the polarization state in the passive arm is equivalent to changing the polarization of the source light before the beam hits the object.Using quantum technology, we can make changes to the polarization state of photons wherever they are, almost instantaneously.Wang says.Quantum technologies are the future. Out of scientific interest, we need to explore this direction.”

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Job description paper„Spatial and polarization entanglement with quantum imaging of biological organisms,” appears in the March 8 issue of Scientific advances. In addition to Wang, Zhang and Dong, the paper's co-authors are medical engineering graduate student David Garrett, postdoctoral scholar research associate Rui Gao, and former postdoctoral scholar research associate Xie He, now at the Shandong Institute of Advanced Technology. This work was supported by funding from Caltech's Center for Intelligence and the National Institutes of Health.

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