In the end QC would be vulnerable to a man-in-the-middle attack by watching for multi-photon emissions.
Not really, Quantum Cryptography has a nice side effect known as 'privacy amplification'. If you know that an eavesdropper has a chance, call it p, of successfully intercepting a small fraction of your qubits (which is always possible anyway with both the BB84 and BB92 protocols), you simply calculate and use the parity of blocks of n qubits as your key, the number of bits available to the eavesdropper then goes as p to the power n which means you get an exponential drop in the information available to an eavesdropper for only a linear reduction in channel capacity. So a well-designed quantum cryptography system could get around this limitation fairly easily.
The way the multi-photon problem is dealt with in practice is to reduce the output of the photon source so the mean number of photons per packet is less than one, this means that the probablility of a packet emerging with two or more photons is below some managable threshold. The BB92 protocol is robust to empty packets, so this is ok (again at a loss of channel capacity).
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Not really, Quantum Cryptography has a nice side effect known as 'privacy amplification'. If you know that an eavesdropper has a chance, call it p, of successfully intercepting a small fraction of your qubits (which is always possible anyway with both the BB84 and BB92 protocols), you simply calculate and use the parity of blocks of n qubits as your key, the number of bits available to the eavesdropper then goes as p to the power n which means you get an exponential drop in the information available to an eavesdropper for only a linear reduction in channel capacity. So a well-designed quantum cryptography system could get around this limitation fairly easily.
The way the multi-photon problem is dealt with in practice is to reduce the output of the photon source so the mean number of photons per packet is less than one, this means that the probablility of a packet emerging with two or more photons is below some managable threshold. The BB92 protocol is robust to empty packets, so this is ok (again at a loss of channel capacity).