Dr Shields's photon detector, however, permits cryptographers to use a phenomenon called quantum entanglement to make a repeater that does not destroy quantum states.

ECONOMIST: Quantum information technology

Their eavesdropping apparatus (which is small enough to fit in a suitcase) was designed to take advantage of a weakness in a particular sort of photon detector in Bob's receiving equipment.

ECONOMIST: Secure cryptography is only as safe as its weakest link

The device in question is a photon detector.

ECONOMIST: Quantum information technology

The system isn't perfect, however -- in this early incarnation it can only nab 40% of transmitted data before giving itself away, and more importantly, it requires the invention of a "quantum non-demolition box" before the attacker can be anywhere but the same room as the receiver, since right now both attacker and receiver need to be using the same photon detector.

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