Once, Holograms were just a scientific curiosity. But thanks to the rapid development of lasers, they have gradually moved center stage, appearing on the security imagery for credit cards and banknotes, in science fiction movies—most memorably Star Wars—and even “live” on stage when long-dead rapper Tupac reincarnated for fans at the Coachella music festival in 2012.
Holography is the photographic process of recording light that is scattered by an object and presenting it in a three-dimensional way. Invented in the early 1950s by the Hungarian-British physicist Dennis Gabor, the discovery later earned him the Nobel Prize in Physics in 1971.
Beyond banknotes, passports, and controversial rappers, holography has become an essential tool for other practical applications including data storage, biological microscopy, medical imaging, and medical diagnosis. In a technique called holographic microscopy, scientists make holograms to decipher biological mechanisms in tissues and living cells. For example, this technique is routinely used to analyze red blood cells to detect the presence of malaria parasites and to identify sperm cells for IVF processes.
But now we have discovered a new type of quantum holography to overcome the limitations of conventional holographic approaches. This groundbreaking discovery could lead to improved medical imaging and speed up the advance of quantum information science. This is a scientific field that covers all technologies based on quantum physics, including quantum computing and quantum communications.
The hologram is created by measuring the differences in the light’s phase, where the two beams meet. The phase is the amount the waves of the subject and object beams mingle and interfere with each other. A bit like waves at 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 (troughs), and others where they add (crests).
Conventional holography fundamentally relies on optical coherence because, firstly, light must interfere to produce holograms, and secondly, light must be coherent to interfere. However, the second part is not entirely true because there are certain types of light that can be both incoherent and produce interference. This is the case for light made of entangled photons, emitted by a quantum source in the form of a flow of particles grouped in pairs—entangled photons.
Such wave-particle duality property enables it to not only probe the thickness of the object at the precise location it hit it (as a larger particle would do) but to measure its thickness along its entire length all at once. The thickness of the sample—and therefore its three-dimensional structure—becomes “imprinted” onto the photon.
Because the photons are entangled, the projection imprinted on one photon is simultaneously shared by both. The interference phenomenon then occurs remotely, without the need to overlap the beams, and a hologram is finally obtained by detecting the two photons using separate cameras and measuring correlations between them.
The most impressive aspect of this quantum holographic approach is that the interference phenomenon occurs even though the photons never interact with each other and can be separated by any distance—an aspect that is called “non-locality”—and is enabled by the presence of quantum entanglement between the photons.
These advantages mean we can produce biological images of much better quality than those obtained with current microscopy techniques. Soon this quantum holographic approach could be used to unravel biological structures and mechanisms inside cells that had never been observed before.
