The principle of holography was discovered by a Hungarian physicist, Dennis Gabor, in 1948. It is moreover thanks to this work that he was awarded the Nobel Prize in physics in 1971. Since then, holography has been very popular. evolved and is used in many fields, including biology. Researchers have recently taken a new step, proposing a new approach to holographic imaging based on quantum physics.
Holography is a process for recording the light diffracted by an object, which makes it possible to subsequently restore a relief image of this object. This is achieved thanks to the properties of the light coming from lasers, which is said to be “coherent”, which means that the light waves remain in phase. This coherence is both spatial and temporal: at a given instant, all the points located in the same plane perpendicular to the beam are in the same phase state, and several light waves emitted successively by the same point remain in phase.
Today, holography is commonly used in biomedical research (biological microscopy or medical imaging). Holographic microscopy makes it possible to decipher the biological mechanisms in tissues and living cells. For example, it is commonly used to detect the presence of malaria parasites in red blood cells, or to identify sperm in IVF. Quantum holography could make it possible to go even further in microbiological exploration.
Bypass the need for consistency
Besides the biomedical field, holographic imaging is used in the leisure and entertainment sector (holograms of deceased celebrities, holograms of precious objects in museums, etc.), as a protective device (security holograms on banknotes bank, on bank cards, etc.), or to store information (holographic memory).
Classic holography creates 2D renderings of 3D objects with a beam of laser light divided into two paths. One of the beams (the object beam) illuminates the object, and then the reflected light is collected by a special camera or holographic film. The path of the second beam (the reference beam) is reflected from a mirror directly onto the collection surface without touching the object. The hologram is then created by measuring the phase differences of the light, where the two beams meet.
These interferences between the beams generally require that the light be “coherent”, in other words, that it have the same frequency everywhere. This is the case with the light emitted by a laser, which is why it is used in most holographic systems. But Hugo Defienne and his colleagues at the University of Glasgow, have managed to bypass this need for coherence by exploiting the quantum entanglement between photons. If conventional holography relies on optical coherence, it is because light must interfere to produce holograms, and it must be coherent to interfere. But physicists emphasize that this second condition is not completely true: some light sources are not coherent and still produce interference. This is the case with light made up of entangled photons, emitted by a quantum source.
Exploiting the properties of entangled photons
Remember that when two particles are said to be entangled, they are intrinsically connected and act as a single object, even if they can be separated in space. As a result, any measurement made on an entangled particle affects the entangled system as a whole. In the approach proposed by Defienne and his team, the two photons of each pair are separated and sent in two different directions. A photon is sent to an object, which could for example be a microscope slide containing a biological sample. When it hits this object, the photon is slightly deflected or slowed down depending on the thickness of the material it passes through.
But, as a quantum object, a photon has the property of behaving not only like a particle, but also like a wave. This wave-particle duality allows it not only to probe the thickness of the object at the exact spot where it struck it, but also to measure its thickness over its entire length in one go. Consequently, the thickness of the sample – and therefore its three-dimensional structure – is as if “imprinted” on the photon. And because the photons are entangled, the projection printed on a photon is shared simultaneously by both. The interference phenomenon then occurs at a distance, without the beams having to overlap; a hologram is finally obtained by detecting the two photons using separate cameras and measuring the correlations between them.
The authors of this study emphasize that in their quantum holographic approach, the interference phenomenon occurs even if the photons never interact with each other and no matter how far apart – this is the principle of “non-locality. »Quantum mechanics; the phenomenon is activated by the “simple” presence of a quantum entanglement between the photons.
Concretely, this means that with quantum holography, the “measurement” of the object and the final measurements could be carried out at completely opposite places on the planet! Beyond this fundamental interest, the use of entanglement instead of optical coherence in a holographic system offers practical advantages such as better stability and better resilience to noise. Indeed, quantum entanglement is a property that is intrinsically difficult to access and difficult to control, and therefore has the advantage of being less sensitive to external disturbances.
These advantages thus make it possible to produce biological images of much better quality than those obtained with current microscopy techniques. Soon, this quantum holographic approach could be used to unravel structures and biological mechanisms taking place inside cells that had never been observed before.