In the past, holograms were just a scientific curiosity. But thanks to the rapid development of lasers, they have gradually moved to the center of the stage, appearing in security images for credit cards and banknotes, in science fiction movies – the most memorable Star Wars – and even “living” on the scene when rapper Tupac died a long time ago. for fans at the 2012 Coachella Music Festival.
Holography is the photographic process of capturing light scattered by an object and displaying it in a three-dimensional way. The discovery, invented by Hungarian-British physicist Dennis Gabor in the early 1950s, later earned him the 1971 Nobel Prize in Physics.
In addition to banknotes, passports, and controversial benchmarks, holography has become a staple for other practical applications, including data storage, biological microscopy, medical imaging, and medical diagnosis. In a technique called holographic microscopy, scientists are making 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 kind of quantum holography to overcome the limitations of conventional holographic approaches. This revolutionary discovery could lead to an improvement in the medical picture and accelerate the progress of quantum information science. This is a scientific field that covers all technologies based on quantum physics, including quantum computing and quantum communications.
How holograms work
Classical holography creates two-dimensional representations of three-dimensional objects by a beam of laser light divided into two paths. The path of a single beam, known as the beam of an object, illuminates the subject of the holography with reflected light collected by a camera or a special holographic film. The path of the second beam, known as the reference beam, bounces off the mirror directly onto the collection surface without touching the object.
The hologram is created by measuring the differences in the light phase, where two rays meet. Phase is the amount in which the waves of the beam of the subject and the object mix and interfere with each other. Slightly resembling waves on the surface of a pool, the phenomenon of interference creates a complex wave pattern in space that contains both areas where the waves cancel each other out (troughs) and other places where they add up (ridges).
Interference usually requires light to be “coherent” – it has the same frequency everywhere. For example, the light emitted by a laser is coherent and that is why this type of light is used in most holographic systems.
Holography with entanglement
Thus, optical coherence is vital for any holographic process. But our new study bypasses the need for coherence in holography by exploiting something called “quantum entanglement” between light particles called photons.
Conventional holography basically relies on optical coherence, because first, light must interfere with hologram production, and second, light must be coherent to interfere. However, the second part is not completely accurate because there are certain types of light that can be incoherent and create 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.
These pairs carry a unique property called quantum entanglement. When two particles are entangled, they are essentially connected and act as one object, although they may be separated in space. As a result, each measurement performed on a single entangled particle affects the entangled system as a whole.
In our study, two photons of each pair were separated and sent in two different directions. One photon is sent towards the object, which can be, for example, microscope slides with a biological sample on it. When it hits an object, the photon will deviate a little or slow down a little, depending on the thickness of the sample of material it has passed through. But, as a quantum object, a photon has the surprising property of behaving not only as a particle, but at the same time as a wave.
Such a property of the duality of wave particles enables it not only to probe the thickness of an object at the exact place where it hit it (as a larger particle would do), but also to suddenly measure its thickness along its entire length. The thickness of the sample – and thus its three-dimensional structure – becomes “imprinted” on the photon.
Because the photons are entangled, the projection imprinted on one photon is shared by both at the same time. The occurrence of interference then occurs remotely, without the need for air overlap, and the hologram is finally obtained by detecting two photons using separate cameras and measuring the correlations between them.
The most striking aspect of this quantum holographic approach is that interference occurs even though photons never interact and can be separated by any distance – an aspect called “nonlocality” – and is made possible by the presence of quantum intertwining between photons.
Thus, the object we are measuring and the final measurements can be performed at opposite ends of the planet. Beyond this fundamental interest, the use of entanglement instead of optical coherence in a holographic system provides practical advantages such as better stability and noise resistance. This is because quantum interweaving is a property that is basically difficult to access and control, and therefore has the advantage of being less sensitive to external deviations.
These advantages mean that we can create biological images of much better quality than those obtained by current microscopy techniques. Soon, this quantum holographic approach could be used to uncover biological structures and mechanisms within cells that have never been observed before.