Cryptography Reference
In-Depth Information
INPUT 1
OUTPUT
PBS
INPUT 2
Figure 6.4
Implementation of a parity check operation using a polarizing beam split-
ter (PBS) and a polarization-sensitive detector. As shown in the insert, the polarization-
sensitive detector consists of a second polarizing beam splitter oriented at a 45-degree
angle, followed by two ordinary single-photon detectors.
A CNOT operation can also be performed using a three-photon arrangement
[8] in which a single ancilla enters the top of the diagram and exits from be-
low, as shown in Figure 6.9. Although this arrangement is easier to implement,
the correct results are only obtained when a single photon actually exits in
each output port, which can be verified using coincidence measurements (the
so-called coincidence basis). The results from the first experimental demon-
stration [8] of a CNOT gate for photons are shown in Figure 6.10. Here mode
mismatch is responsible for most of the incorrect results.
The devices described above succeed with probabilities ranging from
1
/
4
/
to
2
. Increasing the probability of success would require the use of larger
numbers of ancilla photons [1,2]. In addition to requiring the generation of
ancilla photons in entangled states [34], the ancilla must also be detected
with high efficiency. In order to avoid these difficulties, we are currently
investigating the possibility of a hybrid approach [35] that combines linear
optical techniques with a small amount of nonlinearity. It is expected that an
approach of this kind will be able to reduce greatly the requirements for large
numbers of ancilla and high detection efficiency. In particular, we have shown
that the failure rate of devices of this kind can be reduced to zero using the
quantum Zeno effect [35].
1
