Gravity is capable of causing decoherence of large quantum systems

Fig. 1. A complex molecule in quantum delocalized states simultaneously at two different heights, differing by Δx. Due to the fact that in a gravitational field, time flows differently at different altitudes, the frequency of internal vibrations are slightly different, and this becomes a new source of decoherence of the quantum state. The image of the article under discussion in Nature Physics

In the journal Nature Physics recently published a theoretical article reveals new aspects of the interaction of gravity and quantum mechanics. It shows that in the quantum system of a large number of particles, even if completely isolated from the external environment, but in a gravitational field, it is still going to happen decoherence due to the effect of gravitational time dilation. Although this mechanism does not apply to objects around us, it reveals new details of the transition from the quantum description of large systems to classical.

The transition from quantum to classical – one of the most difficult outstanding issues of fundamental physics. On the one hand, the microscopic world living under the laws of quantum mechanics, with all its counterintuitive phenomena. On the other hand, in the conventional macroscopic physics of these phenomena, we do not see – and, indeed, for this reason, they seem to us unnatural. Most importantly, in the framework of the quantum mechanics do not have any indication of the limits of its applicability of the macroscopic. She says that’s up to the limit of the quantum laws operate, and for him it is necessary to use only the classical physics. Therefore, any transition to classical mechanics must somehow be output directly from the quantum laws, or – if it is not displayed – have to admit that quantum mechanics does not cover everything. How realistic is the case, it is still unknown.

Basic quantum effect, by which conveniently illustrated and experimentally to study this problem – the phenomenon of superposition. If quantum object can be in one state and another state, it can also be in a superposition of these states: in fact in both simultaneously. For example, an atom can be either here or there, or here and there at the same time with a certain probability. This nonlocality leads to interference particles with itself – and that it is fundamentally different from the usual classical states with incomplete information (that is, when the particle is actually located somewhere in a certain place, but we do not know which). Thought experiment with Schrödinger cat – the most famous illustration of this unusual, not having daily analogue nefiksirovannosti quantum state.

In fact, when comparing the quantum and classical, there are two fundamental problems of different levels. The first – is to understand where the possibility of interference disappears large object with itself. With it, quantum mechanics, in principle, the right: this effect is responsible for the phenomenon of decoherence (about it will be discussed below). The problem was narrowed to identify the physical mechanisms of decoherence in different situations and calculate the effect. The second – is to explain how to physically collapses the quantum state of the measurement. This task is beyond the scope of ordinary quantum mechanics; for such an explanation or if you want to modify the theory itself or the proposed construction of quantum mechanics (that is called the interpretation of quantum mechanics). There is not even close to a consensus, and indeed, there is not even agreement on the formulation of the problem itself.

In the recently published in the journal Nature Physics article discusses the first task is. The article describes a new source of decoherence of the quantum state, which is responsible for gravity. No modifications of quantum mechanics, no theories of quantum gravity, nor any other exotic hypotheses is not entered. The effect is completely run by ordinary quantum mechanics to classical background and is not too strong gravitational field.

Decoherence is possible in the most general terms, illustrated by the orchestra. If the orchestra plays smoothly, each instrument plays in time with a wave of his baton, and the overall sound is obtained therefore coordinated, coherent. But if each musician had its distractions, causing unpredictable delays response, the overall sound would be like cacophony. And if the orchestra at the same time consisted of many millions of instruments, instead of contrasting sound works we have just heard some smooth hum.

Approximately disappears coherent quantum process involving many particles. Interference – contrasting alternating bands and attenuation probability – is possible when all the quantum degrees of freedom oscillate synchronously. (See, for example, a detailed story about how this biological molecules used for photosynthesis in the news mechanism of photosynthesis uses vibronic quantum coherence, “Elements “28/07/2014). Interaction with the environment that can knock synchronization and then the interference disappears. It can knock down under the blows of the molecules of the environment, or if our quantum system in a vacuum, with heat absorption and emission of photons. In order to eliminate the damaging effects of the environment requires the most cool sites and to screen it from any external influences. The task is not easy, but for the individual molecules it is quite solvable. That is why one can observe not only the interference of individual particles or atoms, but even large molecules (Fig. 1). However, for larger objects, the size of the order of microns, it is still difficult technical problems.

However, until now there was a general understanding that if a quantum system reliably screened from the environment, quantum coherence it will exist indefinitely. In the new work explains that it is not. Even in a perfectly isolated quantum systems of many particles will be decoherence caused by the effect of the general theory of relativity – the time slowdown in the gravity field.

Fig. 2. Experiment spatial splitting of quantum particles
Fig. 2. Experiment spatial splitting of the quantum particle, which flies in the gravity field from the two paths and reunification interferes with itself. Drawing from the website physicsworld.com
The essence of the effect is this. Suppose we have a complex molecule with a large number of degrees of freedom (ie possible fluctuations). We have this molecule, and a pop-up at some point translate into extended state. Now she is not in one place some of the space, and at the same time at two different heights (Fig. 2), it seems to be flying in the gravity field from two paths. When these two paths cross, we translate the molecule again in a localized state, and thus we expect to see the interference. In fact, it turns out the standard atomic interferometer, but only vertically oriented in space. An experimental sounds just fantastic; in fact it is a long time ago realized for individual atoms and Bose condensate and is even used in experiments to measure the force of gravity (gravitational constant measured by new methods, “Elements”, 22.01.2007).

Fig. 2. Experiment spatial splitting of the quantum particle, which flies in the gravity field from the two paths and reunification interferes with itself. Drawing from the website physicsworld.com

If the molecule is completely shielded from the chaotic external influences, it would seem, is no problem with interference should not be. Even within the molecule there are any hesitation, they are still the same and proceed to the top of the trajectory, and the bottom. The synchronicity of these oscillations in the cleavage and the reunification of the whole molecule should not be entirely lost. So, the conclusion is broken when you consider the effects of general relativity.

The fact is that in a gravitational field time course slightly retarded, and the stronger the field (more precisely, the deeper potential), the more this slowdown. This effect is very significant in strong gravitational fields; He became particularly famous after the recent film “Interstellar”. But generally, it always works, including in the earth’s gravitational field, and moreover, it is accounted for in navigation systems GPS. Therefore, when the molecule is split into two different flying trajectory in a gravitational field, it experiences a slightly different them along the course of time. And so, when these two molecules are again reunited incarnation, synchronism between the internal vibrations may already be shot down. If this loss of synchronization is essential, coherence is lost and the interference disappears. The molecule undergoes decoherence simply due to the fact that its internal vibrations “entangled in time.”

Why such a seemingly fundamental fact is still not paying attention? Because he is very weak. The relative difference of the time for the two systems separated in the Earth’s gravitational field to a height h, is
x = mghmc2 = rgh2R2,
where rg – is the gravitational radius of the Earth (about 1 cm), and R – the real Earth’s radius (about 6400 km). For the height of the order of microns is obtained entirely insignificant quantity: x~10-22. Therefore, the quantum particle with one degree of freedom should prokolebatsya 1022 times to this effect could be seen – and this is beyond any real experiments. However, the authors note that if we have a system with a large number (N) degrees of freedom, and they vary, then the loss of synchrony amplified N – √ times. For a macroscopic body in which the number of degrees of freedom of the order of Avogadro’s number, the rate of loss of coherence by 12 orders of magnitude faster. Since the typical time scale of vibrations – a picosecond, it turns out that the loss of coherence come very quickly, in milliseconds. And this stress for a completely insulated from external influence body!

Does this work for a new understanding of why real objects around us live in classical, not quantum laws? No, because in this case there is an active interaction with the environment that rapidly destroys the coherence. However, it reveals an important effect, which until then ignored, and which is likely to have to be taken into account when trying to implement a quantum computer and, more generally, any large quantum systems with more expected coherence time. It sets a limit even for a perfectly isolated systems – because a new effect is taken because of gravity, and it is impossible to hide from it.

And finally, from a purely fundamental point of view, this work reveals new aspects of interaction between quantum mechanics and gravity, the two theories which, in a sense, are “at odds” with each other. However, “acrimony” of these two theories related to strong fields and high energies, and under normal circumstances, they get along well (neutrons in the gravitational field of the Earth allow you to test models of dark energy and dark matter, “Elements”, 04.25.2014). But this work and the subsequent theoretical studies (C. Gooding, WG Unruh, 2015. Bootstrapping Time Dilation Decoherence) demonstrate new non-trivial aspects of that relationship. Since these effects are of fundamental interest, they, of course, want to be tested experimentally. This has not yet been done, but the authors hope that, with a reasonable extrapolation techniques to manipulate quantum objects, such tests will be able to implement in the near future.

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