A team of physicists from the University of Glasgow are the first in the world to find a way to use quantum-entangled photons to encode information in a hologram.
Classical holography creates two-dimensional renderings of three-dimensional objects with a beam of laser light split into two paths. The path of one beam, known as the object beam, illuminates the holograph’s subject, with the reflected light collected by a camera or special holographic film. The path of the second beam, known as the reference beam, is bounced from a mirror directly onto the collection surface without touching the subject.
The holograph is created by measuring the differences in the light’s phase where the two beams meet. The phase is the amount that the waves of the subject and object beams mingle and interfere with each other, a process enabled by a property of light known as ‘coherence‘.
The team’s new quantum holography process also uses a beam of laser light split into two paths, but, unlike in classical holography, the beams are never reunited. Instead, the process harnesses the unique properties of quantum entanglement—a process Einstein famously called ‘spooky action at a distance’ – to gather the coherence information required to construct a holograph even though the beams are forever parted.
Their process begins in the lab by shining a blue laser through a special nonlinear crystal which splits the beam into two, creating entangled photons in the process. Entangled photons are intrinsically linked—when an agent acts on one photon, it’s partner is also affected, no matter how far apart they are. The photons in the team’s process are entangled in both in their direction of travel but also in their polarisation.
The two streams of entangled photons are then sent along different paths. One photon stream—the equivalent of the object beam in classical holography—is used to probe the thickness and polarisation response of a target object by measuring the deceleration of the photons as they pass through it. The waveform of the light shifts to different degrees it passes through the object, changing the phase of the light.
Meanwhile, its entangled partner hits a spatial light modulator, the equivalent of the reference beam. Spatial light modulators are optical devices which can fractionally slow the speed of light which passes through them. Once the photons pass through the modulator, they have a different phase compared to their entangled partners which have probed the target object.
Instead, because the photons are entangled as a single ‘non-local’ particle, the phase shifts experienced by each photon individually are simultaneously shared by both.
The interference 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 finally retrieved by combining four holograms measured for four different global phase shifts implemented by the spatial light modulator on one of the two photons.
The paper has been published in the journal Nature Physics.