'Two-way signaling' possible with a single quantum particle

February 26, 2018 by Lisa Zyga, Phys.org feature
For two partners to both communicate using a single quantum particle, the particle is prepared in a superposition of two locations. When each part of the particle is sent to the partner, the particle hits a unitary device, which guides the particle in such a way that both partners get the message that has been sent to them. Credit: Del Santo and Dakić. ©2018 American Physical Society

Classically, information travels in one direction only, from sender to receiver. In a new paper, however, physicists Flavio Del Santo at the University of Vienna and Borivoje Dakić at the Austrian Academy of Sciences have shown that, in the quantum world, information can travel in both directions simultaneously—a feature that is forbidden by the laws of classical physics.

In classical communication, such as email, text message, or phone call, a message is embedded in an information carrier, such as a particle or signal, that travels in only one direction at a time. In order to communicate in the other direction using the same information carrier, it is necessary to wait until the particle arrives at the receiver and then send the particle back to the sender. In other words, it is classically impossible to perform two-way communication by using the single exchange of a single particle.

However, this is exactly what Del Santo and Dakić theoretically show. To do this, they use a quantum particle that has been put in a superposition of two different locations. As the physicists explain, being in a means that the quantum particle is "simultaneously present" at each partner's location. Therefore, both partners are able to encode their messages into a single quantum particle simultaneously, a task that is essentially impossible using .

"Consider the simplest scenario, where two players, Alice and Bob, want to exchange a simple bit of information, i.e., either 0 or 1," Dakić explained to Phys.org. "They encode their respective bits (messages) at the same time, directly into the of a quantum particle. Once the information is encoded, the partners send their 'parts of ' towards each other.

Positioned halfway in between Alice and Bob is a unitary device, which may be experimentally implemented by, for example, a beam splitter.

"Conditioned on the messages that the particle carries, when the particle hits the unitary device, it bounces back either to Alice or Bob deterministically," Dakić said. "More precisely, the unitary device guides the particle a 'smart way,' such that, at the end both Alice and Bob get the bit (message) that has been sent to them. For example, if the particle ends up with Alice, she would know that the Bob's bit was just opposite from her bit, and vice versa."

So in the end, both players send and receive a message—all within the same amount of time it would take to send a one-way message using a classical particle.

These theoretical results have already been verified by a new experiment using single photons, reported by Del Santo, Dakić, and their coauthors. The experimental results further strengthen the new concept by showing that the communication is secure and anonymous. In particular, the direction of communication is hidden—an eavesdropper cannot tell who is the sender and who is the receiver. Consequently, the results may lead to improvements in that has advantages in terms of both speed and security.

Explore further: Researchers chart the 'secret' movement of quantum particles

More information: Flavio Del Santo and Borivoje Dakić. "Two-Way Communication with a Single Quantum Particle." Physical Review Letters. DOI: 10.1103/PhysRevLett.120.060503. Also at arXiv:1706.08144 [quant-ph]

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3.3 / 5 (3) Feb 26, 2018
The enforced temporal coherence of the signalling provides the second variable which allows two signals on an otherwise single quantum.
Otherwise you don't know when to measure and when a transition between bits happens.
Its effectively a quantisation of time.
You could only cheat this time quantisation and sent asynchronous bits by sending more photons to allow data clock regeneration at the receivers.
1 / 5 (1) Feb 26, 2018
This is just what we'd expect if isolated particles decohere temporally. The point at which decoherence and later coherence occur are adjacent.

See this article for more info on the concept
1 / 5 (1) Feb 27, 2018
The basic problem is that physicists do not fully comprehend electromagnetic radiation. A dipole radio antenna provides the necessary information provided by measurements. It was fist discovered in the year 1936 that there is transverse radiation, as indicated by phase differences at off-angle measurements. I have shown that radiation is primarily transverse by analyzing a dynamic wave moving back and forth across a dipole antenna. This is in contradiction to the conclusion of Einstein that electromagnetic radiation is radial.It is really both radial and transverse. Note that Einstein made this conclusion BEFORE these measurements were performed. This is an strikingly immense contradiction that physicists should study, and the measurements are easily obtained. Electronic engineers evidently didn't consider these phase measurements to be important for their applications. This conforms to the measurements of these physicists, who have further confirms the early measurements.
1 / 5 (1) Feb 27, 2018
I think this is a classical (pun intended) case of misconception between classical information and quantum information by the author of the above piece (not the authors of the paper)

Classically, information travels in one direction only, from sender to receiver. In a new paper, however, physicists Flavio Del Santo at the University of Vienna and Borivoje Daki� at the Austrian Academy of Sciences have shown that, in the quantum world, information can travel in both directions simultaneously

The information which they are talking about that can go both ways is quantum information which they state quite plainly in the introductory paragraphs of their paper (see the linked arxiv paper at the end of the above article):
In this letter we further investigate the discrepancy between
classical and quantum information processing.

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