Cryptography Reference
In-Depth Information
beam-splitting attack might be successful.
∗
Additionally, high-intensity
sources would allow longer transmission paths compared to single-photon
based systems [69,70]. Another important advantage over single-photon sys-
tems is that the photon pair source is immune to tampering by an illegitimate
party. Any manipulation at the photon source can be detected by the commu-
nicating parties and communication can be stopped.
3.2.5.1 Adopted BB84 Scheme
A very elegant implementation of the BB84 scheme utilizes polarization-
entangled photon pairs instead of the polarized single photons originally
used. This is very similar to the Ekert scheme [67] when Alice and Bob chose
different settings of their analyzers for their measurements of the entangled
photons. As opposed to the Ekert scheme, in which both Alice and Bob ran-
domly vary their analyzers between three settings, the adopted BB84 scheme
uses only two analyzer states, namely 0
◦
and 45
◦
. If they share, for example,
the entangled Bell singlet state
|
−
, Alice's and Bob's polarization measure-
ments will always give perfect anticorrelations if they measure with the same
settings, no matter whether the analyzers are both at 0
◦
or at 45
◦
. A way to
view this is to assume that Alice's measurement on her particle of the en-
tangled pair projects the photon traveling to Bob onto the orthogonal state
of the one observed by Alice. So the photons transmitted to Bob are polar-
ized in one of the four polarizations 0
◦
,45
◦
,90
◦
and 135
◦
, as with the BB84
scheme.
After a measurement run, when Alice and Bob independently collect
photons for a certain time, they communicate over an open classical channel.
By comparing a list of all detection times of photons registered by Alice and
Bob, they find out which detection events correspond to entangled photon
pairs. From these events they extract those cases in which they both had used
the same basis setting of the analyzers. Owing to the perfect anticorrelations
in these cases, Alice and Bob can build a string of bits (the sifted key) by
assigning a “0” to the
1 result of the individual
polarization measurements. In order to obtain identical sets of a random bit
sequence, one of them finally has to invert the bits.
+
1 results and a “1” to the
−
3.2.5.2 An Entanglement-Based Quantum
Cryptography Prototype System
We have recently developed a quantum cryptography prototype system in
cooperation with the Austrian Research Centers Seibersdorf (ARCS). It was
∗
This is due to the nonvanishing probability of producing more than two photons
per faint pulse. One possible attack on the security would then simply involve a
beamsplitter, which distributes one photon of a pulse to Eve and one to Bob. This
would allow Eve to gain sufficient information to reconstruct the distributed key. True
single-photon sources are needed to overcome this sufficiency [68].
