Quantum arrow of time
1 September 2019
In spite if the fact that the Schrödinger equation is time reversible, measurements make quantum processes irreversible, and this irreversibility has not yet been explained exhaustively in the quantum theory. One of the approaches is the introduction of quantum entropy. R.W. Murch (the Institute of Materials Science and Engineering and the Washington University in St Luis, USA) and his colleagues performed an experiment showing the existence of quantum time arrow for an open system that experiences the backreaction of measurements. The system was a superconducting transmon qubit coupled to an electromagnetic mode in a microwave waveguide. The quantum states of the qubit were measured by the phase shift of a reflected signal, and a pulse with an opposite phase shift led to a backward evolution of the qubit state. The series of successive measurements preset the qubit quantum trajectory. 280 thousand of quantum trajectories were measured, and the entropy due to the pass probability was calculated. This allowed the arrow of time to be characterized as the direction of most probable processes, namely, prevalence of forward trajectories over backward ones. With increasing duration of the chain of measurements the irreversibility (prevalence of forward trajectories) increased, which was another evidence in favor of the existence of the time arrow. For classical and quantum irreversibility see the book “Dynamics and information” by B.B. Kadomtsev and also his reviews and papers in Phys. Usp. 46 1183 (2003),
Phys. Usp. 39 609 (1996),
Phys. Usp. 38 923 (1995),
Phys. Usp. 37 425 (1994)
Source: Phys. Rev. Lett. 123 020502 (2019)
1 September 2019
Many experiments were performed on quantum teleportation of states of particles without transporting the particles themselves. However, the experiments were only performed with two-dimensional subspaces of quantum levels represented by qubits. Y.-H. Luo (the University of Science and Technology of China and the CAS Center for Experience in Quantum Information and Quantum Physics, China) with co-authors proposed a scheme of teleportation of photon quantum states of any dimension and demonstrated it experimentally on an example of teleportation of a qutrit corresponding to a three-dimensional space. In this scheme, the sender and the receiver first exchange photons in a three-dimensional entangled state. Then the sender takes measurements inducing interference between the previously distributed state of photons, the state of teleported and auxiliary photons.The receiver receives information on the results of measurements through the classical channel and then, having performed a unitary transformation of his part of entangled state, reproduces the teleported quantum state. In the experiment, the three-dimensional states of photon pairs entangled in trajectories were obtained using lasers, splitters, and nonlinear crystals. The quantum fidelity of 0.75 was reached and the existence of three-dimensional teleportation was confirmed. Teleportation with high dimensions is more noise-resistant in the transmission line compared to qubit communication.
Source: Phys. Rev. Lett. 123 070505 (2019)
A highly selective bandpass filter
1 September 2019
Microwave bandpass filters are widely applied in communication facilities, radio measurements and other fields of radio electronics. Bandpass filters are permanently upgraded. One of the directions is elaboration of effective filters based on conducting strips of different configurations. Researchers from L.V. Kirensky Institute of Physics SB RAS and the Siberian Federal University (Krasnoyarsk) designed a filter with unique characteristics and demonstrated its operation. The filter was assembled on a dielectric substrate. A stripline conductor with a stub was placed on one side of the substrate and stripline conductors connected to a screen were on the other side. Two of the first three oscillation modes participated in the formation of a narrow passband, while the third mode formed a minimum of the transmission coefficient near the passband. B.A. Belyaev with co-authors used computer simulation to select optimal sizes and positions of stripline conductors. Then, a prototype device was made of four such filters. The device has the characteristic of a highly selective eighth-order bandpass filter with f0=0.52 GHz central frequency and a relative bandwidth of 14 %, while the attenuation band lasted up to ≈ 5f0. Thus, the new filter surpasses the available analogues in its selective properties.
Source: Technical Physics Letters 45 485 (2019)
The Askaryan effect and the search for ultra-high energy neutrinos
1 September 2019
In 1961, the outstanding Soviet physicist G.A. Askaryan predicted theoretically the effect of generation of bursts of coherent Vavilov – Cherenkov radio emission upon passage of high-energy photons through matter (JETP 14 441 (1962); Sov. Phys. Usp. 27 896 (1984)). Photons generate electromagnetic showers that ionize atoms on their way throwing out additional electrons from them in the direction of the shower. Simultaneously, positrons leave the shower through annihilation. As a result, the excess of negative charge in the shower can reach about 10 %, and the uncompensated charges generate Vavilov – Cherenkov radiation. Short-wave (compared to shower size) radiation is attenuated by interference, while long-wave radiation gives a coherent pulse. This effect was first observed experimentally on SLAC accelerator. The Askaryan effect is a promising method of cosmic ray particle registration in the high-energy range. Ultra-high energy neutrinos ν are being sought now using the Askaryan method by ARA (Askaryan Radio Array) detectors on the South Pole. The idea of using Antarctic ice for this purpose is due to V.A. Gusev, I.M. Zheleznykh, and M.A. Markov (NRI RAS). The ARA array includes five radio antennas located 200 m deep in the ice. According to calculations, ultra-high energy ν can either be generated directly in astrophysical objects or can be cosmogenic, i.e., can occur in the interaction of cosmic rays with background radiation (cosmogenic ν were predicted by V.S. Berezinskii and G.T. Zatsepin in 1969). At the Cosmic Ray Conference in Madison, ARA collaboration presented the results of the 2013-2016 search for ν-producing showers. Signals above the background level were not registered, but the limitations from above on the diffuse ν flux obtained earlier by ARA were improved by a factor of two. In its sensitivity, ARA begins competing with other neutrino telescopes at energies above 1010 GeV, and during the following three years ARA can either give the best limitations or register ultra-high energy ν. G.A. Askaryan also considered the Moon matter as a target for radio pulse generation (Phys. Usp. 55 741 (2012)).
Source: arXiv:1907.11125 [astro-ph.HE]
The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.
Permanent editor is Yu.N. Eroshenko.
It is compiled from a multitude of Internet sources.