Cryptography Reference
In-Depth Information
Bob
Alice
PBS
45/-45 HWP
PBS
67.5/157.5
BS
HWP
HWP
BS
0
0
PBS
0/90
Quantum Channel
(Free-Space)
1
1
0
PBS
22.5/112.5
0
1
1
Timetagging
Timetagging
Classical Channel
(Public Internet)
GPS/Rb-clock
GPS/Rb-clock
VIENNA
Alice
7.8 km
Bob
Figure 3.12 Scheme of the free-space quantum communication experiment over
Vienna. The transmitter Alice, comprising the single-mode fiber coupled polarization-
entangled photon source (DC) and sending telescope, is located in the 19th-century
observatory Kuffner Sternwarte. Bob has a receiver telescope and is located on the 46th
floor of the Millennium Tower skyscraper 7.8 km away. Alice measures the photons in
mode A from each entangled pair using a four-channel detector made of a 50/50 beam
splitter (BS), a half-wave plate (HWP), and polarizing beam splitters (PBS), which mea-
sures the photon polarization on either the H/V or
1
2 (
.
She sends the other photon in mode B, after polarization compensation (Pol.), via her
telescope and free-space link to Bob. Bob's receiver telescope is equipped with a simi-
lar four-channel detector and can measure the polarizations in the same bases as Alice
or, by rotating an extra HWP, measure another pair of complementary linear polar-
ization bases. Alice and Bob are both equipped with time-tagging cards, which record
the times at which each detection event occurs. Rubidium atomic clocks provide good
relative timing stability between the local measurements. Both stations also embed a
1 pps signal from the global positioning system (GPS) into their time-tag data stream
to give a well-defined zero time offset. During accumulation, Bob transmits his time
tags in blocks over a public Internet channel to Alice. She finds the coincident photon
pairs in real time by maximizing the cross-correlation of these time tags. Which of the
four detector channels fired is also part of each time tag and allows Alice and Bob
to determine the polarization correlations between their coincident pairs. Alice uses
her polarization compensators to establish singlet-like anticorrelations between her
measurements and Bob's.
+
/
basis, where
±=
H
±
V
)
refractor telescopes. Our new designs are based on larger and higher quality
optical elements. We also relaxed our spatial filtering requirement to re-
duce sensitivity to beam wander and fluctuations. Using locally recorded
time stamps and a public Internet channel, coincident counts from correlated
Search WWH ::




Custom Search