If ALICE wants BOB to have an exact replica of her particle, they first
share an entangled pair of particles. Entanglement means that neither
particle has properties of its own, they only have common properties.
E.g. for the state : "particle
2 is orthogonal to particle 3".
ALICE now combines particles 1 and 2 and, by a
Bell-state measurement (BSM), projects them onto an entangled state,
e.g. onto . Thus, she gains
the information about the specific quantum states of particle 1 or 2
before the measurement. However, she knows, that, whatever the states
have been before, they are orthogonal to each other.
Now, if the state of particle 1 is orthogonal to the state of
particle 2, and if the state of particle 3 is orthogonal to the one of
2, then the state of particle 3 must be equal to the initial state of
However, ALICE could have obtained any of the other three Bell-states
with the same probability. She tells BOB which result she obtained via a
classical channel. Then BOB turns particle 3 with one of four unitary
transformations (U) into an identical replica of particle 1.
Polarization analysis of the teleported photon:
The data shows the polarisation of the teleported photon as a
function of the delay between the arrival of photon 1 and 2 at ALICE's
beamsplitter (when translating the retroflecting UV-mirror). Only around
zero delay interference occurs and allows registration of a Bell-state.
The polarization of photon 3 is analyzed if detector p has indicated
that there is a photon to be teleported and if ALICE's Bell-state
analyzer has registered the state
Photon 3 shows the polarization defined by the polarizer acting on
The polarization, without any background subtraction, is 70%±3%. The
results for the two non-orthogonal states (45° and 90°) proves
teleportation of the quantum state of a single photon.
You find more data plots and figures on our
Zamanda yolculuk © 2005 Cetin BAL - GSM:+90 05366063183 -Turkey/Denizli
A pulse of UV-light passing through a nonlinear crystal creates the
entangled pair of photons 2 and 3. After retroflection during its second
passage through the crystal the UV-pulse creates another pair of photons.
One of these will be the teleported photon 1. It can be prepared to have any
polarization. Photon 4, deteced by detector p, serves as a trigger to
indicate that photon 1 is under way.
ALICE then looks for coincidences behind a beam splitter BS2
where photon 1 and the anciliaries are superposed. BOB, after receiving the
clasical information that ALICE obtained a coincidence count in detectors f1
identifying the Bell state
, knows that his
photon 3 is in the initial state of photon 1. This he can check using
polarization analysis with the polarizing beam splitter PBS and the
detectors d1 and d2.
Thus, photon 3 will turn out to have exactly the properties of photon 1
as defined by the polarizer.
||If one overlaps two particles at a beamsplitter,
interference effects determine the probabilities to find the two
particles incident one each from a and b either both in one of the two
outputs c and d or to find one in each output.
Only if two photons are in the state
they will leave the beamsplitter into different output arms. If one
puts detectors there, a click in each of them, i.e. a coincidence, means
the projection of the two photons onto the state
|For the other three Bell states both photons will exit
together through one of the two output arms. Here polarisation analysis
can separate the state
from the states
. (This feature
of interferometric Bell-state analysis was used in the experiment
Quantum Dense Coding.)
The identification of all 4 Bell-states will be possible if one
couples the two particles by some interaction. This should be soon
possible for various systems (photons,
or trapped ions)
which will also form the first 2-qubit quantum logic gates.
Inside a nonlinear crystal (BBO) an ultraviolet photon
(wavelength 490nm) may spontaneously split into two infrared photons (780
nm). The down-conversion photons (A and B) have orthogonal polarizations
(H or V). In the two beams along the intersections of the cones we observe
a polarization-entangled two-photon state.
For the experimental realisation of quantum teleportation it was
necessary to use pulsed down-conversion. Only if the pulse width of the
UV-light, and thus the time of generating photon pairs is shorter than the
coherence time of the down-converted photons, interferometric Bell-state
analysis can be performed. In the experiment the pulses from a mode-locked
Ti:Saphire laser have been frequency doubled to give pulses of 200fs
duration. The interfering light was observed after passage through
IR-filters of 4nm bandwidth giving a coherence time of about 520fs. (A
photograph of down-conversion light can be seen on our group home page.).
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