Professor of physics at the Niels Bohr Institute in Copenhagen, one of the pioneers of quantum teleportation, Eugene Polzik, explained to RIA Novosti where the border is between the "real" and "quantum" worlds, why a person cannot be teleported and how he managed to create matter with "negative mass".
Five years ago, his team first implemented an experiment to teleport not a single atom or particle of light, but a macroscopic object.
He recently chaired the international advisory board of the Russian Quantum Center (RQC), replacing Mikhail Lukin, the creator of one of the largest quantum computers in the world and the world leader in quantum computing. According to Professor Polzik, he will focus on developing and realizing the intellectual potential of young Russian scientists and strengthening international participation in the work of the RCC.
“Eugene, will humanity ever be able to teleport more than single particles or a collection of atoms or other macroscopic objects?
- You have no idea how often I am asked this question - thank you for not asking me if it is possible to teleport a person. In very general terms, the situation is as follows.
The universe is a gigantic object, entangled at the quantum level. The problem is that we are not able to "see" all the degrees of freedom of this object. If we take a large object in such a system and try to consider it, then the interactions of this object with other parts of the world will give rise to what is called a "mixed state" in which there is no entanglement.
The so-called principle of monogamy operates in the quantum world. It is expressed in the fact that if we have two ideally entangled objects, then both of them cannot have as strong "invisible connections" with any other objects of the surrounding world as with each other.
Eugene Polzik, professor at the Niels Bohr Institute in Copenhagen and head of the international advisory board of the RCC. Photo: RCC.
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Returning to the question of quantum teleportation, this means that, in principle, nothing prevents us from confusing and teleporting an object the size of at least the entire Universe, but in practice, it will prevent us from seeing all these connections at the same time. Therefore, we have to isolate macro objects from the rest of the world when we conduct such experiments, and allow them to interact only with the "necessary" objects.
For example, in our experiments it was possible to accomplish this for a cloud containing a trillion atoms, due to the fact that they were in a vacuum and held in a special trap that isolated them from the outside world. These cameras, by the way, were developed in Russia - in the laboratory of Mikhail Balabas at St. Petersburg State University.
Later we moved on to experiments on larger objects that can be seen with the naked eye. And now we are conducting an experiment on the teleportation of vibrations arising in thin membranes made of dielectric materials measuring millimeter by millimeter.
Now, on the other hand, I am personally more interested in other areas of quantum physics, in which, it seems to me, real breakthroughs will take place in the near future. They will definitely surprise everyone.
Where exactly?
- We all know well that quantum mechanics does not allow us to know everything that happens in the world around us. Due to the Heisenberg uncertainty principle, we cannot simultaneously measure all properties of objects with the highest possible accuracy. And in this case, teleportation turns into a tool that allows us to bypass this limitation, transferring not partial information about the state of the object, but the entire object itself.
The same laws of the quantum world prevent us from accurately measuring the trajectory of motion of atoms, electrons and other particles, since it is possible to find out either the exact speed of their movement or their position. In practice, this means that the accuracy of all kinds of pressure, motion, and acceleration sensors is strictly limited by quantum mechanics.
Recently, we realized that this is not always the case: it all depends on what we mean by "speed" and "position." For example, if during such measurements we use not classical coordinate systems, but their quantum counterparts, then these problems will disappear.
In other words, in the classical system, we are trying to determine the position of a particular particle relative to, roughly speaking, a table, chair or some other point of reference. In a quantum coordinate system, the zero will be another quantum object with which the system of interest to us interacts.
It turned out that quantum mechanics makes it possible to measure both parameters - both the speed of movement and the trajectory - with infinitely high accuracy for a certain combination of properties of the reference point. What is this combination? A cloud of atoms serving as the zero of the quantum coordinate system must have an effective negative mass.
In fact, of course, these atoms do not have "weight problems", but they behave as if they had negative mass, due to the fact that they are located in a special way relative to each other and were inside a special magnetic field. In our case, this leads to the fact that the acceleration of the particle decreases, but does not increase its energy, which is absurd from the point of view of classical nuclear physics.
This helps us to get rid of the random changes in the position of particles or their speed of movement that occur when we measure their properties with lasers or other photon sources. If we place a cloud of atoms with "negative mass" in the path of this ray, then it will first interact with them, then it will fly through the object under study, these random disturbances eliminate each other, and we will be able to measure all parameters with infinitely high accuracy.
All this is far from theory - a few months ago we already tested these ideas experimentally and published the result in the journal Nature.
Are there any practical uses for this?
- A year ago, I already said, speaking in Moscow, that a similar principle of "removing" quantum uncertainty can be used to improve the accuracy of the work of LIGO and other gravitational observatories.
Then it was just an idea, but now it has begun to take shape. We are working on its implementation together with one of the pioneers of quantum measurements and a participant in the LIGO project, Professor Farid Khalili from RCC and Moscow State University.
Of course, we are not talking about installing such a system on the detector itself - this is a very complicated and time-consuming process, and LIGO itself has plans that we simply cannot get into. On the other hand, they are already interested in our ideas and are ready to listen to us further.
In any case, you first need to create a working prototype of such an installation, which will show that we can really step over the border in measurement accuracy imposed by the Heisenberg uncertainty principle and other laws of the quantum world.
We will conduct the first experiments of this kind on a ten-meter interferometer in Hanover, a smaller copy of LIGO. We are now assembling all the necessary components for this system, including a stand, light sources and a cloud of atoms. If we succeed, then I am sure that our American colleagues will listen to us - there are no other ways to get around the quantum limit yet.
Will the proponents of deterministic quantum theories, who believe that chances do not exist in the quantum world, consider such experiments as proof of the correctness of their ideas?
- To be honest, I do not know what they think about it. Next year we are organizing a conference in Copenhagen on the boundaries between classical and quantum physics and similar philosophical issues, and they can attend if they want to present their vision of this problem.
I myself adhere to the classical Copenhagen interpretation of quantum mechanics, and I admit that wave functions are not limited in size. So far we do not see any signs that its provisions are being violated somewhere or at odds with practice.
Laboratory of Quantum Optics at the Russian Quantum Center. Photo: RCC.
In recent years, physicists have performed countless tests of Bell's inequalities and the Einstein-Podolski-Rosen paradox, which completely rule out the possibility that the behavior of objects at the quantum level can be controlled by some hidden variables or other things outside the scope of classical quantum theory.
For example, a few months ago there was another experiment that closed all possible "holes" in Bell's equations used by proponents of the theory of hidden variables. All that remains for us is, to paraphrase Niels Bohr and Richard Feynman, "shut up and experiment": it seems to me that we should only ask ourselves those questions that can be answered through experiments.
If we go back to quantum teleportation - given the problems that you described: will it find application in quantum computers, communication satellites and other systems?
- I am sure that quantum technologies will more and more penetrate communication systems, and they will quickly enter our daily life. How exactly is not yet clear - information, for example, can be transmitted both through teleportation and through ordinary fiber-optic lines using quantum key distribution systems.
Quantum memory, in turn, I believe, will also become a reality after a while. At a minimum, it will be needed to create repeaters for quantum signals and systems. On the other hand, it is difficult to predict how and when all this will be implemented.
Sooner or later, quantum teleportation will become not exotic, but an everyday thing that everyone can use. Of course, we are unlikely to see this process, but the results of its work, including secure data transmission networks and satellite communication systems, will play a huge role in our lives.
How far will quantum technologies penetrate into other spheres of science and life that do not relate to IT or physics?
- This is a good question, which is even more difficult to answer. When the first transistors appeared, many scientists believed that they would find use only in hearing aids. This is what happened, although now only a very small proportion of semiconductor devices are used in this way.
Nevertheless, it seems to me that a quantum breakthrough will indeed occur, but not everywhere. For example, any gadgets and devices that interact with the environment and somehow measure its properties will inevitably reach the quantum limit, which we have already discussed. And our technologies will help them bypass this limit, or at least minimize interference.
Moreover, we have already solved one of these problems using the same “negative mass” approach, improving quantum magnetic field sensors. Such devices can find very specific biomedical applications - they can be used to monitor the work of the heart and brain, assessing the chances of getting a heart attack and other problems.
My colleagues from the RCC are doing something similar. Now we are discussing together what we have achieved, trying to combine our approaches and get something more interesting.