This is the idea behind quantum imaging: create an entangled pair of photons and send one towards the object you want to image and hang on to the other.
But then what? For some time, physcists have been whisperin’ about the extraordinary potential of this technique. Some imagine that it might be possible to create images of objects that cannot otherwise be seen, objects inside black boxes, for example, or black holes.
The thinking is that the photon you hold in your hand can somehow tell you something about the object it’s entangled cousin has hit. So you can create an image of an object without ever seeing its reflection.
But pin physicists down about what they mean and they start a-mumblin’ and a-dribblin’ incoherently. Despite rumours from places like Boston University where various bods are testing the idea in the bowels of the physics department, nobody has ever provided experimental evidence that quantum illumination is anything but a hatful of hot air.
So if ever a field needed an injection of common sense, this is it. Step forward quantum theorist and all round bright spark Seth Lloyd from MIT. He’s taken the thinkin’ and given it a thorough shakin’ by the scruff of its neck.
Lloyd doesn’t give any credence to the ideas of reflection-free imaging but he’s found something almost as good. Lloyd has calculated that illuminating an object with entangled photons can
reduce increase the signal to noise ratio of the reflected signal by a factor of 2^e, where e is the number of bits of entanglement. That’s an exponential improvement.
What’s more, the improvement occurs even if the entanglement is completely destroyed during the process of reflection. So quantum illumination could help image anything that is currently hard to distinguish because of noise.
That’s impressive but Lloyd’s ideas raise quite a few questions, such as how to perform the required entanglement measurement on the returning photon. That’s for the experimentalists to sort out although there’s no easy and obvious answer.
So although a clever piece of work, it could be a while before we’re posing for quantum snapshots from Kodak.
Ref: arxiv.org/abs/0803.2022: Quantum Illumination