About demons and teleportation: two technologies that bring the “quantum future” closer

    We will talk about technologies that can contribute to the wide spread of quantum machines, the quantum equivalent of Maxwell's demon and teleportation of a quantum gate .

    / photo Wikimedia PD

    About Maxwell's Paradox

    Maxwell's demon is a fictional creature that the nineteenth-century physicist James Clerk Maxwell invented to describe the paradox of the second law of thermodynamics .

    Maxwell suggested the following thought experiment. Capacity is taken and divided by a partition into two halves. Then it is filled with conventionally “cold” and “hot” gas molecules. These molecules are mixed and move at different speeds.

    A hole is made in the partition with a device that allows hot molecules to pass from left to right, and cold to right to left. This device is called Maxwell's demon. As a result, one half of the container is heated, and the second is cooled without any energy costs.

    The paradox is that after the molecules "take their places" in the tank, the entropy of the system is less than in its original state. This violates the second law of thermodynamics, according to which the entropy of an isolated system cannot decrease, but only increase or remain the same.

    At first, physicists decided that Maxwell's demon could be identified with a perpetual motion machine, since it takes energy "from nowhere." But then it was proved that the demon also spends energy on sorting molecules. Hence, energy arises from the work of the demon, and the laws of thermodynamics are not violated. Paradox allowed Leo Silard in 1929.

    In practice, they have been trying to implement the concept of Maxwell's demon for a long time. A number of researchers even managed to achieve some success. For example, in 2010, Japanese researchers developed an electromechanical model of the Sillard engine , which is considered a kind of Maxwell's demon. It used polystyrene balls (which were molecules in the original system), floating around in a buffer solution. The role of the demon was played by electrical voltage, pushing the light balls to change their direction of motion.

    Three years ago, Maxwell's demon was realized in the form of a single-electron transistor with superconducting aluminum leads. However, scientists failed to bring the concept to life with a significant number of atoms or molecules. Until recently.

    Quantum demon: what is the essence

    In September of this year, researchers from the University of Pennsylvania managed to conduct a large-scale quantum equivalent of a thought experiment. In a special way, they grouped a separate array of a large number of cesium atoms, reducing the entropy of the system.

    For this, a team of specialists used a so-called optical trap with three pairs of lasers. It allows you to capture atoms and cool them to ultra-low temperatures (only a few degrees above absolute zero).

    As part of the experiment, researchers used lasers with a wavelength of 839 nm to form an optical 3D-lattice size of 5x5x5 and place cesium atoms in it. Initially, these atoms were in a state withthe orbital quantum number (l) is equal to 4 and the magnetic quantum number (m) is equal to –4 and distributed randomly in the lattice. However, at the end of the experiment, they formed sublattices 5x5x2 or 4x4x3 in size, which reduced the system's entropy more than twofold.

    In order to move an atom along the lattice, scientists changed its state (by changing its quantum numbers) and switched the polarization of one of the light beams. As a result, atoms in different states began to “push off” and move along the lattice. When it was necessary to “fix” the position of the atom, its quantum numbers returned to their original state.

    How useful development

    Entropy reduction is a promising option for creating qubits. Using neutral atoms for quantum computing is a difficult task. They have no electric charge, so it is difficult to force them into a state of quantum entanglement , in which the states of objects depend on each other.

    The decrease in entropy in the optical trap of atoms allows one to build quantum gates with a smaller number of errors. And quantum gates are considered the basic logical elements of a quantum computer. Therefore, the proposed system allows in the future to increase the computational efficiency of a quantum machine.

    Another technology is quantum gate teleportation.

    For quantum machines to become widespread, it is necessary to organize the coordinated work of hundreds of qubits. One of the ways to achieve this is to make the system modular: to combine small quantum systems into one large one.

    / photo by Rachel Johnson CC

    To do this, you need to give quantum gates the possibility of intermodular interaction. To this end, a team of researchers from Yale University has developed a modular quantum architecture, where quantum gates teleport (transmit their state at a distance) in real time.

    Researchers teleported the logical gate CNOT (controlled negative), which implements an operation similar to “ modulo 2 addition". Taking into account the error correction codes, the reliability of the method was 79%.

    In the future, this technology will allow organizing modular quantum computers, which will simply scale.

    All this, coupled with the achievement of researchers from the University of Pennsylvania brings the moment of widespread quantum machines. It is believed that this will happen in the next ten years.

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