Cryptography Reference
In-Depth Information
inequality [60]. There have been proposals of some experimental techniques to
engineer entangled qunits in photons [58,61,62]. Here we will discuss a gen-
eral scheme for a quantum communication protocol based on the orbital an-
gular momentum of light [63]. This scheme has already been succesfully used
for the experimental realization of a quantum coin tossing protocol [64].
In a general communication scheme, prior to the sharing of information,
the two parties, say Alice and Bob, have to define a procedure that will assure
that the signal sent by one party is properly received by the other. Usually, this
scheme works as follows. First, Alice prepares a signal state she wants to send.
Bob will measure it and communicate the result to Alice, who will correct the
parameters of her sending device following Bob's indications. This process
will be repeated until the two parties adjust the corresponding devices. After
this step is completed, Alice can safely assume that any subsequent signal
which is sent is properly received by Bob.
Using pairs of photons entangled in orbital angular momentum, we can
prepare any qutrit state, transmit it, and measure it. The preparation is done
by projecting one of the two photons onto some desired state. This projects
the second photon nonlocally onto a corresponding state. This state may be
transmitted to Bob and finally measured by him. The measurement employs
tomographic reconstruction. This last step is usually a technically demanding
problem, inasmuch as it needs the implementation and control of arbitrary
transformations in the quantum system's Hilbert space.
The experimental setup we used is shown in Figure 3.7. A 351 nm wave-
length argon-ion laser pumps a 1.5-mm-thick BBO (
-barium-borate) crystal
cut for Type I phase matching conditions. The crystal is positioned so as to
produce down-converted pairs of equally polarized photons at a wavelength
of 702 nm emitted at an angle of 4
◦
off the pump direction. These photons are
directly entangled in the orbital angular momentum degree of freedom. Alice
can manipulate one of the down-converted photons while the other is sent
to Bob. Before being detected, Bob's photon traverses two sets of holograms.
β
Electronics
Bob
side
Alice
side
Nonlinear
cry
st
al
Transforming
holograms
Projecting
holograms
Pump beam
Figure 3.7
Experimental setup from [63]. A 351 nm wavelength laser pumps a BBO
crystal. The two generated 702 nm down-converted photons are sent to Alice's and
Bob's detectors, respectively. Before being detected, each photon propagates through
a set of holograms. Each photon was coupled into single-mode fibers and directed to
detectors based on avalanche photodiodes operating in the photon counting regime.
