Holograms used to be just a scientific curiosity. However, thanks to the rapid development of lasers, they have gradually taken center stage and appear on the security images for credit cards and Banknotes, in science fiction films – most memorable in Star Wars – and even “live” on stage when the long-dead rapper Tupac was reborn at the 2012 Coachella music festival for fans.
Holography is the photographic process in which light scattered by an object is picked up and represented in three dimensions. The discovery was invented by the Hungarian-British physicist Dennis Gabor in the early 1950s and earned him the Nobel Prize in Physics in 1971.
Alongside banknotes, passports, and controversial rappers, holography has become an indispensable tool for other practical applications, including data storage, biological microscopy, medical imaging, and medical diagnosis. In a technique called holographic microscopy, scientists create holograms to decipher biological mechanisms in tissues and living cells. For example, this technique is routinely used to analyze red blood cells to determine the presence of malaria parasites and identify sperm for IVF processes.
Now we have discovered a new type of quantum holography to overcome the limitations of traditional holographic approaches. This groundbreaking discovery could lead to improved medical imaging and accelerate the advancement of quantum information science. This is a scientific field that covers all technologies based on quantum physics, including quantum computing and quantum communication.
How holograms work
Classic holography creates two-dimensional representations of three-dimensional objects with a laser light beam divided into two paths. The path of a ray, called an object ray, illuminates the subject of the holography, with the reflected light collected by a camera or special holographic film. The path of the second beam, known as the reference beam, is reflected by a mirror directly onto the collection surface without touching the object.
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The hologram is created by measuring the differences in the phase of light in which the two rays meet. The phase is the amount by which the waves of the subject and object rays mix and interfere with each other. Similar to waves on the surface of a swimming pool, the interference phenomenon creates a complex wave pattern in space that contains both regions where the waves cancel each other out (valleys) and others where they add to each other (crests).
Interference generally requires light to be “coherent” – the same frequency everywhere. For example, the light emitted by a laser is coherent and therefore this type of light is used in most holographic systems.
Holography with entanglement
Therefore, optical coherence is critical to any holographic process. However, our new study bypasses the need for coherence in holography by taking advantage of what is known as “quantum entanglement” between particles of light called photons.
Conventional holography is essentially based on optical coherence, since firstly, light must interfere in order to create holograms and, secondly, light must be coherent in order to interfere. The second part is not entirely correct, however, as there are certain types of light that can be both incoherent and create interference. This is the case with light made up of entangled photons, which is emitted by a quantum source in the form of a stream of particles grouped in pairs – entangled photons.
These pairs carry a unique property called quantum entanglement. When two particles are entangled, they are closely related and effectively act as a single object, although they may be separate in space. As a result, any measurement made on an entangled particle affects the entire entangled system.
In our study, the two photons in each pair are separated and sent in two different directions. A photon is directed onto an object which, for example, can be a slide with a biological sample. When it hits the object, the photon will deviate slightly or slow down a little depending on the thickness of the sample material it passed through. As a quantum object, however, a photon has the surprising property of not only behaving as a particle but also as a wave at the same time.
Such a wave-particle duality property allows it to examine not only the thickness of the object at the exact point where it hits it (as a larger particle would do), but its thickness all along its length at once measure up. The thickness of the sample – and thus its three-dimensional structure – is “impressed” on the photon.
Since the photons are entangled, the projection printed on one photon is shared by both at the same time. The interference phenomenon then occurs remotely, without the need to overlap the beams, and finally a hologram is obtained by capturing the two photons with separate cameras and measuring correlations between them.
How a hologram with entangled photons is created. University of Glasgow, author provided
The most impressive aspect of this quantum holographic approach is that the interference phenomenon occurs even though the photons can never interact with each other and can be separated from each other by any distance – an aspect known as “nonlocality” – and made possible by the presence of quantum entanglement between the photons .
So the object we are measuring and the final measurements could be taken on opposite ends of the planet. In addition to this fundamental interest, the use of entanglement instead of optical coherence in a holographic system offers practical advantages such as better stability and stability to noise. This is because quantum entanglement is a property that is inherently difficult to access and control and therefore has the advantage of being less sensitive to external anomalies.
Because of these advantages, we can produce biological images of much better quality than current microscopy techniques. Soon, this quantum holographic approach could be used to decipher biological structures and mechanisms in cells that had never been observed before.
This article by Hugo Defienne, Lecturer and Marie Curie Fellow, Faculty of Physics and Astronomy, University of Glasgow is republished by The Conversation under a Creative Commons license. Read the original article.