The birth of quantum biology

April 6, 2012 17:15

Experiments in physics laboratories promise to bring into our lives such wonders as quantum computers, high-performance solar cells and ultra reliable magnetic levitation train — but things are still separated from us by a high vacuum and ultra-low temperatures, within which alone it is safe to observe quantum-mechanical phenomena. From this perspective, it is very surprising that in a warm and chaotic world of the living matter quantum effects play an important role and may even teach us how to operate the whimsical "quanta" outside the lab.

 The ability of birds to the orientation of the magnetic field of the Earth — is basically a quantum effect. Picture: Daren Newman [1].

At first glance, the world of quantum effects and the world of living matter in no way meet. First there is only a subnanometer scale under high vacuum and at very low temperatures — in general, in a carefully controlled laboratory conditions. Life as "inhabits" macroscopic warm area filled by anything, but not controlled environment. Any physicist will tell you that this mysterious condition as quantum coherence (and, especially, quantum entanglement) can not exist, and second in a constantly moving environment of a living cell.

But what about a second opinion? Studies in recent years have hinted that the nature of the known techniques, which do not own physics: Coherent quantum processes can play a very significant role in this, the macroscopic world. Examples include — the ability of birds to navigate using Earth's magnetic field lines and the intricacies of photosynthesis — one of the most important biochemical reactions on the planet.

Gradually realizing the role of quantum effects in life processes, some researchers even talk of the birth of a new discipline — quantum biology. Physics is interested in the possible practical benefits "spying" for biology: "I would like to learn how to exploit quantum phenomena as effectively as does biology" — says Zeta Lloyd (Seth Lloyd), a physicist at the Massachusetts Institute of Technology, Cambridge (USA). — "If we could understand how very" naughty "quantum states do not collapse in living organisms, it may be, would bring us to a nebulous term, while quantum computers. Or learn to make solar cells far more efficient than it is today ". [1]

Quirks of quantum mechanics. Illustrate the mystery and antiintuitivnost phenomena of the quantum world, where the system is in each of the possible states at once (with a certain probability) may paradox called "Schrödinger's cat." In the absence of an observer, the radioactive core (above) is described by a superposition of two states: the decayed and undecayed. Does this mean that the radiation detector, releasing toxic gas and kill time, and do not kill the cat? One version of quantum mechanics — the so-called many-worlds interpretation — even posits the endless "budding" parallel universes (see Figure below), in some of which the cat dies, while in others — remains live. Image: Wikipedia.
From A to B: the most effective way

Scientists have long known that there are some in the photosynthesis quantum tricks — for luminous flux, consisting of randomly moving photons, ceases to be messy after the absorption of photons by molecules, "antenna", in countless located in photosynthetic membranes of every green leaf and in bacteria capable of using light energy. Now, the flow of energy has a strong focus — on the reaction centers, which effectively store it by converting carbon dioxide into the air which sugar.

Ever since the 1930s, it was clear that such a transfer of energy must be described by quantum mechanics, one of the main provisions of which states that the elementary particles — such as electrons — possess both properties of a wave. Photons are absorbed by chlorophyll molecules, performing the role of "antenna", generate excitement bursts transmitted in the form of excitons, quasiparticles that are traveling in the photosynthetic membrane of a "antenna" to the other, until they enter the reaction center, using their energy for other purposes. But whether they are random walks, as previously thought? Some researchers believe that the excitons can be coherent in the quantum sense, and then the excitation can be represented as a wave, which itself finds the optimal path to the reaction center. In other words, the excitons simultaneously explore all possible ways to the reaction center, automatically stopping at the most efficient.

Several years ago, received the first confirmation of this hypothesis — Two groups of scientists at Berkeley (California, USA) using laser technology have shown the presence of the coherence of excitons in the photosynthetic membranes of bacteria, even though they had to cool down to the temperature for the liquid gas [2, 3]. However, without forcing anyone to wait long, in 2010, the same researchers showed the same result, even at room temperature [4], have finally confirmed that the coherence — is not a laboratory artifact, and actually used the nature of the phenomenon. A little later, the coherence of photosynthetic membranes is not found in bacteria and the algae.

But how does the quantum coherence is not destroyed at temperatures so far from absolute zero and the conditions did not resemble the vacuum? Experimenters have never managed to construct a quantum system that exists in so close to the "normal" conditions. Calculations carried out by Lloyd and his colleagues showed quite unexpected thing: the "noise" of the environment (which is a consequence of the high temperature and general mobility of the system) increases the efficiency of energy transfer rather than reduces it. [5] "The mobile environment facilitates further transfer excitons, not letting them get stuck in a thicket of photosynthetic antenna" — explains the results of calculations Lloyd.

Photosynthesis — is not the only example of a "macroscopic" process, where the world of quantum mechanics makes itself felt. Many enzymatic reactions are carried out by means of tunneling, a particle "leaks" under the energy barrier rather than "storm" it. And one of the controversial theories of smell even claims the smell — it is sensitive to molecular vibrations generated during the tunneling between the molecule of the smell and the olfactory receptor [6].
Compass bird flight

No less astonishing explanation using quantum phenomena has recently been long surprised people the ability of birds to navigate toward the light, somehow determining the direction of the magnetic field of the Earth.

It is already known that the effect of this "compass" begins with the absorption of light by the retina of avian eyes. Apparently, for the absorption of photons should be the pairing of free radicals — reactive molecules, each carrying on unpaired electron. Each of these electrons have an intrinsic magnetic moment (spin), able to capture the external magnetic field. As radical distancing from each other, the spin of one of them remains largely under the influence of the nearby atomic nucleus, and the spin of another — the "free" — is focused in the field of the Earth. [7] This difference of magnetic fields can translate radicals in different energy states, which differ in the number and chemical properties.

How to warm his hands by the fire? In the light-harvesting complexes (SSC) — "antennas" of photosynthesis — is concentrated to 90% of the total chlorophyll. Quantum coherence of excitons to transfer of energy to the reaction centers, fundamentally improves the efficiency of the photosynthetic process. Sculpture SSC: © Julian Voss-Andreae, 2003 (see also "invisible sculptures").

The main hypothesis is that a compound is synthesized in a specific location relative to the field lines, and is not synthesized — in another. The difference of concentrations, thus, reflects the orientation of the magnetic field of the Earth. This assumption was tested on an artificial photochemical reaction with the use of magnetic fields that affect the lifetime of the radical pair [8]. It has been suggested that the two unpaired electrons in the pair generated by the absorption of a single photon in a state of quantum entanglement (or confusion), in which the spins of the two particles are linked, no matter what distance separates these particles. Quantum Entanglement — very "moody" state, it is easily destroyed by any external shocks, particularly at temperatures well above absolute zero. However, calculations showed that the bird "compass" this phenomenon is unsustainable tens of microseconds, which is much longer than what can be obtained in the laboratory for the same temperature [9].

It is possible that this "quantum magnetic sense" extended outside the migratory birds — after all, some insects and plants also observed magnetic sensitivity. However, to produce proof of that, you need to understand what molecules are involved in this process and carefully study their properties in the laboratory.
A sense that?

Photosynthetic organisms, of course, benefit from the quantum coherence in their photosystems. But will there be the ability to use this phenomenon as a result of natural selection, or engages chance, as a side effect of close packing of the molecules of antenna? Similarly it remains to be seen, but even if it is a second option, the impact of the "domestication" of quantum coherence is huge: it can effectively concentrate the sun's energy in the direction of the reaction centers, without the need for high-order transmission medium (antenna system) and independent of temperature.

Understanding how to reach the light-harvesting complexes of quantum coherence at room temperature, can affect the fate of all mankind. No one doubts that one day will inevitably have to give up burning of fossil fuels and move to the massive use of alternative energy sources. Sunlight — the most visible "for a song" energy on Earth, but humankind has not yet learned to exploit it effectively. Existing solar cells is too inefficient and too expensive to provide anything like a significant contribution to the global power generation even in the long term. If a person will be able to "tame" the quantum coherence, a new generation of photonic systems (for example, with the antennas based on quantum dots) will be the basis of future power.

These days, the quantum computer look as far away from his personal "brother." Laboratory implementation of quantum computers, while only a very decisive exotic and far from practical applications, consist of vacuum technology, lasers, ultra-low temperature settings and the like. Image: Science @ Berkley.

Another enticing prospect is quantum computing. Old dream of physicists and engineers is the ability to manipulate the data encoded in the quantum bits (qubits), which, for example, the spin state ("up" / "down") of an electron or nucleus. All of exotic quantum computing is that the qubits simultaneously take the values 0 and 1 (with a certain probability). If the qubits of a quantum computer, programmed to a quantum algorithm, "confusing" to each other, then by measuring the state of the system, we immediately obtain the solution of the problem of interest [10-12]. The problem, however, is that the quantum state is very "fragile", and even the coherence between the only two (!) Qubits can not keep for longer than a second for a paltry. At the moment, quantum algorithms are applied in practice only to such relatively modest tasks as a random number generator, or distributor of the private key (in cryptography).

However, as we have seen, biology bypassed this obstacle: photosynthetic antenna is, in fact, a quantum computer, determine the best way to transfer energy to the reaction center, and — calculated dynamically. The same thing with the "quantum compass" birds: in detail in order to understand how these systems and quantum computers will be more than just a tenuous mirage behind thick glass laboratory.

Nature — the best teacher, and this thesis is as old as the world. Only no one has come to me that nature can show us the structure of the mysterious quantum "kingdom."

Written based on an essay by Philip Ball [1].

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