Counterfactual Quantum Communication.
Quantum Communications is a strange phenomena.
One of the weirdest proposed forms is called counterfactual communication.
It is a type of communication where no particles travel between two recipients.
The scientists have been able to transfer a black and white bitmap image,
from one location to another, without sending any physical particles.
Quantum communications also known as quantum teleportation,
is also a form of information transfer.
Regular quantum teleportation is based on the principle of entanglement.
This is when two particles become inextricably linked, so that whatever happens to one,
will automatically affect the other, no matter how far they are apart.
Einstein referred to this as, ’spooky action at a distance’.
Scientist have already used it to send messages over vast distances.
This form of quantum teleportation relies on particle transmission,
in some form or the other.
The two particles need to be together, when they are entangled,
before being sent to the people, on either end of the message.
They start in one place, and need to be transmitted to another,
before communication can occur between them.
Alternatively particles can be entangled at a distance, but it usually requires another particle,
such as photons, to travel between the two.
Direct counterfactual quantum communication relies on a phenomena called,
the quantum zeno effect.
The quantum zeno effect occurs, when an unstable quantum system,
is repeatedly measured.
In the quantum world whenever you look at a system, or measure it, the system changes.
In this case, unstable particles can never decay while they are being measured.
So the quantum zeno effect creates a system,
that’s effectively frozen with a very high probablity
Counterfactual quantum communication is based on the quantum zeno effect,
and is defined as the transfer of a quantum state, from one site to another,
without any quantum or classical particles being transported between them.
This requires a quantum channel to run between the two sites.
There is a small probability that a quantum particle will cross the channel.
If that happens, the system is discarded and a new one is setup.
To setup such a complex system researchers in China, placed to single photon detectors,
in the output ports of the last of an array of beam splitters.
Because of the quantum zeno effect, the system is frozen in a certain state.
So its possible to predict which of the detectors would click,
whenever photons passed through.
A series of nested interferometers measure the state of the system,
to make sure it does not change.
It works based on the fact, in the quantum world, all light particles can be fully described,
by wave functions, rather than as particles.
So by embedding messages in light, scientist were able to transmit this message,
without ever directly sending a particle.
The basic idea of this setup came from holography technology.
In the 1940’s, a new imaging technique-holography-was developed to record,
not only light intensity, but also the phase of light.
The phase of light can be used for imaging.
Let us assume that someone wants to send a image to Alice using only light.
This light acts as a wave, and not a particle in quantum realm.
Alice transfers a single photon to a nested interferometer,
where it can be detected by three single photon detectors, D0, D1, or Df.
If D0 or D1 ‘click’, Alice can conclude a logic result of 1 or 0.
If DF clicks the result is considered as inclusive.
After the communication of all bits, scientists were able to reassemble the image.
Black pixels were defined as logic 0, while white pixels were defined as logic 1.
In the experiment the phase of light itself became the carrier of information,
and the intensity of the light was irrelevant to the experiment.
This was a big step forward in quantum communication.
The technology could also be used for imaging sensitive ancient artefacts,
that could not survive direct light shining on them.
The experiment demonstrated how bizarre and unexplored the quantum world is.