The light source is polarized by a Polarized Beam Splitter and reflects off of each of the chips.

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If they are the same, the two photons will come out of the beam-splitter together.

ECONOMIST: Quantum information technology

The two parts are arranged so that when they return to the beam-splitter, the peaks of one wave are in step with the troughs of the other.

ECONOMIST: Fundamental physics

It is for this reason that you need a device, such as Dr Shields's, which can detect and count individual photons as they come out of different parts of the beam-splitter.

ECONOMIST: Quantum information technology

Since B and C have the same quantum states, that means C also has the same quantum state as A. If A and B are different, they will come out of the beam-splitter in one of three different places, depending on exactly which way they are different.

ECONOMIST: Quantum information technology

The information from the beam-splitter is then transmitted separately to the recipient, so that he knows whether to accept C unaltered as part of the message, or apply one of three mathematical transformations to it, to arrive at the right result (this does not compromise secrecy, since any eavesdropper will not know what the transformation needs to be applied to).

ECONOMIST: Quantum information technology