Theorists have solved quantum communication at interstellar distances

Star Wars Episode III: Revenge of the Sith / Lucasfilm, 2005

British theorists have studied in detail the influence of the interstellar and circumellar medium on the possibility of quantum communication at such a long distance and have come to the conclusion that such communication is possible. They showed that X-rays are best suited for this, and the quantum signal itself can be a good marker for an intelligent civilization. Research published in Physical examination D.

Electromagnetic waves, and light in particular, are the fastest signal carriers available to humans. Communication lines for radio and optical ranges have become the basis of modern telecommunication networks, without which it is impossible to imagine the modern Internet. This field of technology has become so advanced and widespread that new standards for quantum communication are being created with an eye on the capacity of already existing networks.

The main task for quantum communication has been the transmission of quantum information (more specifically quantum state) remotely. One of the directions in this area has been quantum cryptography based on the distribution of a quantum key. Such protocols allow the participants in a communication to detect the presence of any eavesdropping device, which makes the communication secure according to the laws of physics.

At the heart of quantum communication is the phenomenon of entanglement. It occurs between two or more objects, if the interaction between them is correctly configured, or if they are born in a pair. The second method is most popular in technologies based on entangled photons. Quantum entanglement does not directly depend on the distance over which the particles are separated after correlations have been created between them. However, it is constantly exposed to the environment, which can be interpreted as a measure of the quantum state. In this case, the collapse of the quantum state occurs and the particles or objects cease to be bound. This process is called decoherence. The greater the chance of decoherence, the longer the state of many particles exists, and the longer the quantum particles have flown.

Physicists are constantly setting records in the transmission range for entangled photons. For this, transmission is used directly over the air, via optical fiber, with the help of drones and even satellites. The latter method proved to be the most effective, as outer space is less “noisy” than air or solids. But how persistent will quantum entanglement be if we try to establish quantum communication on an interstellar scale? The answer to this question is not trivial, as the impact of specific astrophysical influences on the likelihood of decoherence, including gravity, is still extremely poorly studied.

Two theorists from the University of Edinburgh, Arjun Berera and Jaime Calderón-Figueroa, decided to answer this question. In their study, the researchers calculated the coherence effects caused by the many different interactions that photons experience when traveling interstellar. In addition, they calculated in detail how even undisturbed entangled photons captured by the receiver would resemble those transmitted by the transmitter, and evaluated the appropriate degree of coincidence (fidelity) of the states. According to the authors’ conclusions, there are no significant obstacles to quantum communication in the interstellar medium.

First of all, the authors considered the effect of gravity on the propagation of entangled photons using the concept of Wigner’s rotation. They showed that, unlike massive particles, for which curvatures would introduce parasitic entanglements between spin and momentum, massless wave packets would only get an additional phase shift. Of course, this will make it difficult to interpret a signal from an unknown source, but if the path of the photons is known, the calculation of phase corrections will help to restore the original quantum signal. Physicists have shown that such a recovery is possible for photons that have passed a significant part of the Milky Way.

Similar considerations apply to the degree of coincidence, which may decrease due to the effects of the theory of relativity, in particular due to redshift. The latter is more pronounced for X-rays than for the rays in the optical area. However, the researchers noted that the loss of correspondence is most pronounced when sending a signal from the earth’s surface to orbit, while the travel of light at a sufficient distance from massive bodies reduces the effect. As an example, they calculated the matching loss that would occur for a signal sent from the orbit of Proxima Centauri b to the orbit of Venus. With the sun as the only source of gravity, they showed that the error was only 10 percent.

In addition to gravity, the loss of photon coherence can be caused by many interactions in the interstellar and circumellar medium. In the first case, we are talking about the scattering of hydrogen, electrons and protons, as well as heavier elements. In addition, interaction with cosmic microwave background radiation through vacuum nonlinearity is also possible (this interaction was studied by physicists earlier). In the second – interactions with solar particles, galactic cosmic rays and particles from the radiation belts.

For each of these factors, physicists have estimated the average free path of photons. In most cases, this value turned out to be greater than the size of the Milky Way and even the observable universe. The most significant obstacle turned out to be the areas with ionized hydrogen, where the average free path for photons was in the order of hundreds of parsecs. Other potentially destructive factors were gas and dust with traces of heavier elements, as well as galactic magnetic fields. The authors’ estimates have shown that X-rays with a photon energy less than the resting mass of an electron multiplied by the square of the speed of light will be least receptive to them.

Finally, physicists discussed the prospects for quantum communication in connection with communication with extraterrestrial civilizations. They noted that a quantum signal must contain two components: classical and non-classical. Photons with non-classical statistics do not occur in nature, and therefore such a signal can be a reliable proof of the sender’s reasonableness. Researchers have suggested that Bell states – the simplest example of quantum entanglement – will be the standard for all advanced civilizations. But deciphering the signal due to the effects of loss of degree of coincidence will require an understanding of where the light came from and which way it traveled. Finally, for the correct interpretation of the signal, a good technical base is required, which, in the case of X-ray photons, is only developed.

It is worth emphasizing that quantum communication does not mean instant messaging faster than the speed of light. We talked about the contradiction between such technology and the known laws of physics using the example of quantum communication from the Mass Effect universe in the Mass of Effects article.

Marat Khamadeev

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