"Quantum mechanics will be expanded by biological terms." »
It is one thing to assume in theory that all the objects around us are essentially quantum, and quite another to see it and prove it experimentally. In a recent pre-print of an article published on arXiv.org, scientists from the University of Vienna led by Marcus Arndt demonstrated for the first time the quantum behaviour of a biomolecule - the natural antibiotic peptide gramicidin.
The setup that allowed scientists from the University of Vienna to create a directed beam of intact molecules of the natural peptide gramicidin and demonstrate that it can exhibit wave-like properties Source
Imagine getting into a lift on one floor and getting out on two different floors at the same time. Or simultaneously boarding a flight to Bangkok and another to Paris. You are met on arrival in Paris, however, you never make it to Bangkok. This may seem like a rather convoluted way of earning extra air-miles, but if you are a photon or an electron, then this behaviour is not unusual, because you can allow yourself to be in two places at the same time or, as physicists call it, be in a quantum superposition.
The fact that an object can be in several mutually exclusive states at the same time (the principle of superposition) seems impossible to us, however, this is one of the basic principles of quantum mechanics, with the help of which it has been possible to successfully predict the behaviour of microscopic particles – such as photons, electrons, atoms – for more than half a century. What is more, quantum properties have recently been discovered in complex molecules, including biological peptide polymers (1). Physicists from the University of Vienna have succeeded in doing this under the leadership of Markus Arndt. So, how is the quantum behaviour of peptides expressed?
One of the fundamental aspects of quantum mechanics is that its objects can behave both like particles and like waves, a feature called wave-particle duality. This idea was put forward as early as the 1920s by the French scientist Louis de Broglie, who suggested that all material objects have wave properties (2).
Louis de Broglie
Despite the extraordinary boldness of such an idea for the time, de Broglie received the Nobel Prize in Physics for his theory in 1929. The hypothesis was developed and formulated mathematically by Erwin Schrödinger, who described the behaviour of quantum objects using a system of equations, the so-called wave functions, for which he also received the Nobel Prize in Physics in 1933. Since then, 'de Broglie waves' have been detected in many particles, from electrons to molecules (1, 2-6). Using quantum optics, it has been possible to register wave properties for particles, such as interference and diffraction. A classic example illustrating the wave-like nature of particles is the two slit electron diffraction experiment (7). When a beam of electrons is passed through a screen with two slots, a characteristic interference pattern is observed at the detector behind it, that is, particles behave like waves. This picture persists even if electrons are passed through one at a time. It is as if an electron passes through both slits at the same time, that is to say the particle 'interferes with itself'. This is the quantum superposition (8). The presence of an interference pattern is thus clear evidence of the wave-like nature of objects (8).
Can the principle of superposition also apply to larger objects? The difficulty is that the de Broglie wavelength is inversely proportional to the size of the object (9). The larger it is, the shorter its de Broglie wavelength, and the more difficult it is to register and thereby prove the presence of wave properties.
The quantum physics of 'macroscopic' objects is an extremely young branch of science. Just 20 years ago, in 1999, wave properties (interference effects) were discovered by scientists from the University of Vienna in a football-shaped molecule of fullerene C60 (a molecule consisting of 60 carbon atoms) (4).
The structure of fullerene
Since then, this group of scientists, amongst others, has been able to demonstrate the quantum properties of even larger molecules, from various types of fullerenes and other natural compounds to huge synthetic molecules and molecular clusters consisting of hundreds of atoms (2,7,10-12). The research papers established that quantum interference effects that had long been observed in fundamental particles can also be detected in objects at a molecular level. So how large can such objects be? Is it possible, for example, to measure the quantum properties of the 'molecules of life' - DNA and proteins?
It was suggested in 2005 that proteins and peptides are also quantum objects, and their behaviour can be described from the perspective of string theory . The scientists proposed considering proteins 'strings', just like physical particles are in the framework of the eponymous theory (9,13). Various methods have been proposed for how to put a living organism (a virus or bacteria) in a superposition state (5.6). However, quantum experiments with large (and fragile) biomolecules have until recently remained difficult to implement in practice, as researchers have not been able to create a 'molecular beam' of sufficient strength.
In 2019, a group of scientists from the University of Vienna led by Marcus Arndt presented experimental evidence of the presence of wave properties in the short peptide gramicidin to the scientific community (1). A peptide is a natural biopolymer consisting of amino acid residues, which in the case of gramicidin is 15. This peptide was isolated from the soil bacterium Bacillus brevis, and is a natural antibiotic. Although gramicidin is almost five times smaller than the largest molecule which has shown wave properties (11, 12), it is, like other biomolecules, extremely fragile. Therefore, creating a directed beam from intact molecules is not a simple task. However, the Austrian scientists managed it. They used ultrashort ultraviolet laser pulses to vaporise the peptide molecules. Then, the evaporated molecules were carried away by a flow of argon or helium into a beam. This beam was directed through three diffraction gratings to a detector, where interference fringes were measured. The scientists were able to determine the de Broglie wavelength for gramicidin in argon: it amounted to 350 femtometers (350*10-15m), which is about 10,000 times less than the radius of the molecule itself. The delocalisation of the molecule was more than 20 times its size (1). All of this would have been impossible if gramicidin had only behaved like a particle. The researchers hypothesise is that under such conditions it is possible to study the wave characteristics of small proteins such as insulin (1). The findings open new possibilities for studying the quantum nature of other biomolecules, such as enzymes, DNA, and even simple lifeforms (1).
To us, quantum physics seems a strange and 'non-intuitive' science about some mysterious world, whose manifestations we do not see in nature. The paradoxical behaviour of quantum objects from the point of view of an observer from the macrocosm is well demonstrated by Schrödinger's famous cat, which is both alive and dead. In 1935, Erwin Schrödinger proposed an extremely popular thought experiment. A cat, a bottle of poison, a radioactive source and a radiation sensor (Geiger counter) are all placed in a closed box connected to a device that breaks the bottle when radioactive decay is detected. If the poison is spilt, the cat will die. We cannot know in principle whether the counter has detected radioactive decay or not: the laws of quantum mechanics obey the laws of probability. Thus, the cat’s condition is associated with the state of the radioactive source, and the cat is in a superposition of living and dead states: he is "neither alive nor dead" (3, 6). An object in the macrocosm being in two states at the same time contradicts our intuitive understanding. However, as recent studies have shown, the boundary between the macro- and micro- worlds is rather arbitrary. At the beginning of the year, physicists were able to demonstrate the quantum properties of a silicon nanoparticle consisting of 100 million atoms, greatly expanding the boundaries of the laws of quantum mechanics.
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