Holography ‘quantum leap’ could revolutionize imaging

A new kind of quantum holography that uses tangled photons to overcome the limitations of conventional holographic approaches can lead to improved medical imaging and the advancement of quantum information science.

A team of physicists from the University of Glasgow is the first in the world to find a way to use quantum entangled photons to encode information in a hologram. The process behind their breakthrough is set out in an article published in the magazine today Natural Physics.

Holography is known to many by its use as security images printed on credit cards and passports, but it has many other practical applications including data storage, medical imaging and defense.

Classic holography creates two-dimensional versions of three-dimensional objects with a beam of laser light divided into two paths. The orbit of one beam, known as the object beam, illuminates the subject of holography with the reflected light collected by a camera or special holographic film. The trajectory of the second beam, known as the reference beam, is jumped directly onto the collecting surface of the mirror without touching the subject.

The holograph is created by measuring the differences in the light phase where the two beams meet. The phase is the amount that the waves of the subject and the object rays mix and interfere with each other, a process made possible by a property of light known as ‘coherence’.

The Glasgow team’s new quantum holography process also uses a beam of laser light that is divided into two paths, but unlike in classic holography, the rays are never reunited. Instead, the process uses the unique properties of quantum entanglement – a process known to Einstein as ‘ghostly action at a distance’ – to gather the coherent information needed to construct a holography, even though the beams separated forever.

The process begins in the laboratory by shining a blue laser through a special non-linear crystal that divides the beam in two, creating entangled photons in the process. Entangled photons are intrinsically linked – when an agent responds to one photon, the measure is also affected, no matter how far apart. The photons in the team’s process are entangled in their direction, but also in their polarization.

The two streams of entangled photons are then sent on different paths. One photon stream – the equivalent of the object beam in classical holography – is used to determine the thickness and polarization response of a target object by measuring the retardation of the photons as they move through it. The waveform of light shifts in different degrees, it moves through the object and changes the phase of light.

Meanwhile, its entangled mate strikes a spatial light modulator, the equivalent of the reference beam. Spatial light modulators are optical devices that can partially slow down the speed of light moving through it. Once the photons pass through the modulator, they have a different phase compared to their entangled partners investigating the target object.

In standard holography, the two paths will then be placed on top of each other, and the degree of phase interference between them will be used to generate a hologram on the camera. In the most striking aspect of the team’s quantum rendition of holography, the photons never overlap after passing through their respective targets.

Instead, because the photons are entangled as a single ‘non-local’ particle, the phase shifts experienced by each photon individually are shared by both at the same time.

The perturbation phenomenon occurs remotely, and a hologram is obtained by measuring correlations between the entangled photon positions using separate megapixel digital cameras. A high-quality phase image of the object is ultimately detected by measuring four holograms measured for four different global phase shifts implemented by the spatial light modulator on one of the two photons.

In the team’s experiment, phase patterns were reconstructed from artificial objects such as the letters ‘UofG’ programmed on a liquid crystal display, but also from real objects such as a transparent band, silicone oil droplets on a microscope slide and a bird feather.

Dr Hugo Defienne, of the University of Glasgow’s School of Physics and Astronomy, is the lead author of the article. Dr Defienne said: “Classical holography does very clever things with the direction, color and polarization of light, but it has limitations, such as interference from unwanted light sources and a strong sensitivity to mechanical instability.

“The process we have developed frees us from the limitations of classical coherence and guides holography in the quantum field. The use of tangled photons offers new ways to create sharper, richer detailed holograms, offering new possibilities for practical applications of the technique. .

“One of the applications may be in medical imaging, where holography is already used in microscopy to examine details of delicate specimens that are often almost transparent. Our process allows for higher resolution, lower sound images, which can help to reveals finer details of cells and helps us to learn more about how biology functions at the cellular level. ‘

Professor Daniele Faccio of the University of Glasgow leads the group that made the breakthrough and is co-author of the article.

Prof Faccio said: ‘What’s really exciting is that we’ve found a way to integrate megapixel digital cameras into the tracking system.

“Many recent discoveries in quantum optical physics have been made over the past few years using single-pixel sensors. This has the advantage of being small, fast and affordable, but their disadvantage is that they have only very limited data on the state of the entangled photons involved in the process.It will take extraordinary time to capture the level of detail we can gather into a single image.

The CCD sensors we use give us an unprecedented amount of resolution to play with – up to 10,000 pixels per image of each entangled photon. This means we can measure the quality of their entanglement and the amount of photons in the beams with remarkable accuracy.

“The quantum computers and quantum communications networks of the future need at least the degree of detail about the entangled particles they are going to use. This puts us one step closer to enabling real step change in those rapidly evolving fields. It’s really exciting. we would like to build on this success with further refinements. ‘

The team’s article, entitled “Quantum Holography with Polarization Entanglement”, is published in Natural Physics.