Cryptography Reference
In-Depth Information
follow the pattern (
E
short), Alice and Bob announce that they
have measured in the time basis. On these occasions, they each know which
of the superposed terms in Equation (10.7) was realized, and they use this
knowledge to establish a shared bit. The scheme is noise-immune because on
the phase-basis occasions, each leg of the two Mach-Zehnder interferometers
is traversed by one of the four photons. Thus the relative phase along the
two paths of each interferometer factors out and does not affect the measured
results. The scheme is passive because neither Alice nor Bob is required to
make active changes to their apparatus.
The security of the scheme derives from the fact that only the state in Equa-
tion (10.6) will produce the correlations that Alice and Bob measure. Therefore
the source can be controlled by the adversary without compromising security.
This technique can be viewed as the time-bin analog of the polarization based
entanglement distillation experiment described in Reference [18].
→
long,
L
→
10.4 Discussion
We have presented round-trip, one-way, and symmetric noise-immune QKD
schemes that can be implemented with existing technology for both polariza-
tion and time-bin qubits. The noise-immunity of the schemes makes active
compensation of interferometric drift and channel birefringence unnecessary.
The round-trip methods are the simplest, since they do not involve entangle-
ment. However, the bidirectional flow of signals leaves an opportunity for an
eavesdropper to compromise the security of the link by sending signals into
the apparatus of Alice and/or Bob and measuring the state of the reflected
signal. The one-way schemes remove this security concern at the cost of re-
quiring a multi-photon entangled state. A further advantage of the one-way
schemes presented here is that they do not require Bob to make active changes
to his apparatus. Finally, the symmetric schemes presented here achieve noise-
immunity while requiring neither Bob nor Alice to make active changes to
his/her apparatus. The cost of this simplicity is a doubling of the number of
photons involved in each run of the protocol.
It is interesting to observe that discoveries in the field of quantum in-
formation (entanglement swapping and entanglement distillation) can be
naturally related to other areas of quantum information theory (quantum er-
ror correction and decoherence-free subpaces) via the AWI, as demonstrated
in Figure 10.1. Since the central goal of quantum computation is a “folding in
time” of a classical computation, the AWI may yield insight into the mecha-
nisms behind the speed-up achieved by certain quantum computation algo-
rithms.
References
1. M.A. Nielsen and I.L. Chuang,
Quantum Computing and Quantum Information,
Cambridge University Press, Cambridge, 2000.
2. N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden,
Rev. Mod. Phys.,
74, 145, 2002.
