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THE weird quantum property called 鈥渆ntanglement鈥� can be used to create quantum holograms. A pair of entangled photons always know the state of each other no matter how far apart they are. Quantum holograms use this spooky property to let us see 3D images of hidden objects.

In classical holography, a laser beam reflected off an object interferes with a reference beam, and the interference pattern is recorded on a photographic plate. This pattern creates a hologram that you can see by illuminating the plate with a laser.

If the object is hidden you cannot collect the reflected light, but perhaps entanglement can fill in the missing information, say Bahaa Saleh and colleagues at Boston University鈥檚 Quantum Imaging Laboratory in Massachusetts. If two photons are entangled, their physical properties are intimately connected. If you measure the properties of one, you instantly know the state of its partner.

For quantum holography, you need two beams of photons where each photon in one beam has an entangled partner in the other. You aim the first beam through a tiny hole in a hollow sphere which lets light in, but not out. The photons bounce off any object inside and hit the inner wall of the sphere, where a detector records the time each one hits.

The second beam is aimed at a photographic plate. When a photon from the first beam hits the sphere, it triggers a camera shutter to snap open and record its entangled partner on film. The angle at which each photon hits the film depends on the angle at which its partner struck the sphere, revealing where the photon hit the hidden object.

Each photon inside the sphere creates an interference pattern, and the same pattern is recreated by its entangled partner. An accumulation of these patterns forms a 3D hologram on the film.

鈥淚t鈥檚 a really cute idea,鈥� says Pieter Kok, a quantum physicist at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. 鈥淓verything follows the laws of physics.鈥�

The main limitation is that to tell one entangled photon from another, the pairs have to be generated slowly, so the detector records only one photon at a time. The object would have to remain still for hours. 鈥淏ut that鈥檚 a hazard in any holography experiment,鈥� says Saleh.

His team has started building the device. 鈥淲e want to show that quantum mechanics allows you to do things that are useful, rather than philosophical,鈥� says Saleh. You could record objects in sensitive environments such as archaeological sites, where they can鈥檛 be brightly illuminated.