Entropic energy-time uncertainty relation
1 April 2019
The quantum time-energy uncertainty relation has several versions and is interpreted differently. For example, the width of the energy level is related to its lifetime before the decay. In 1945, L.I. Mandelstam and I.E. Tamm used the Schrodinger equationto establish relation between the time of transition of a system from one state to another and the energy difference of these states. They obtained the expression that also had the form of uncertainty relation. The difference in the interpretations is partly due to the fact that the time-energy uncertainty relation cannot be written as a Robertson inequality because in the general case no Hermitian operator corresponds to time. Attempts have already been made to formulate uncertainty relations in the entropic form, where instead of quantities themselves there appear entropy of states or transition probabilities. This approach was successfully realized for coordinate-momentum variables, but for time and energy the entropy relation was only written for almost periodic processes. P.J. Coles (Los Alamos National Laboratory, USA) with co-authors obtained theoretically the time-energy entropic uncertainty relation for the general case of time-independent Hamiltonian describing the system. The new relation has the form of inequality in which the sum of conditional entropies related to the energy states is larger than or equal to the time measure logarithm (discrete or continuous). The entropic time-energy uncertainty relation can find application in quantum cryptography.
Source: Phys. Rev. Lett. 122 100401 (2019)
Quantum measurement cooling
1 April 2019
The classical cooling of a system can be realized by an external force (as in a usual freezer) or by molecule sorting (Maxwell’s demon). In the latter case, a feedback loop is needed through which the information on the molecule velocity is transmitted. M. Campisi (University of Florence, Italy) and his colleagues showed theoretically that a system can be cooled by performing quantum measurements on it even without a feedback loop. A scheme with two qubits coupled with heat reservoirs was investigated. The cooling went in two stages. First, the qubit states were measured and then the energy exchange between the qubits and reservoirs was realized. The authors showed that measurements can be taken in such a manner that the energy would go from the cold reservoir to the qubits and at the same time from the qubits to the hot reservoir. That is, we deal with quantum cooling. Such a cooling engine will be possible to realize in practice using solid-state superconducting qubits.
Source: Phys. Rev. Lett. 122 070603 (2019)
Second sound in graphite
1 April 2019
The second sound (a wave-like heat transport by phonons), whose possibility had been predicted by L. Tiszaand and L.D. Landau, was observed in liquid helium and in some solid substances. Calculations showed that second sound is also possible in graphene and graphite in a rather wide temperature range. A group of researchers from Massachusetts Institute of Technology discovered for the first time the second sound in polycrystalline graphite with natural isotopic composition at temperatures of ∼ 85-150 K. Short laser pulses induced sample heating and formed a spatially sinusoidal distribution of its temperature owing to interference of light. The heat propagation was monitored by continuous laser light diffraction on sample surface oscillations with high time resolution. The heating region propagated quickly along the sample without changing the width. This implies that heat was transferred not in a usual diffusive but in a wave-likemanner, i.e., by means of second sound. The experimental data agree well with ab initio calculations (the solution of Boltzmann equations). In particular, the second sound velocity in graphite was confirmed to lie between the velocities of the slow and fast transverse sound waves. Second sound may play an important role for cooling microelectronic devices.
Source: Science, online publication of 14 March 2019.
Coherent absorption in a disordered medium
1 April 2019
The so-called “anti-laser” in which a perfect coherent light absorption was observed was already realized experimentally in 2011. However, the anti-laser was constructed on the basis of a regular medium, namely, a sapphire single crystal. K. Pichler (the Institutefor Theoretical Physics, Vienna University of Technology, Austria) with colleagues was the first to design an “anti-laser” in a disordered medium that operated in the microwave range. The medium of cylindrical teflon elements was placed in a rectangular metallic waveguide. These chaotic cylinders scattered electromagnetic waves. The incoming microwave signal was formed by a set of antennas at the input of the waveguide and at the output the transmitted radiationwas registered. A signal-absorbing monopole antenna (a metal rod) was placed in the center of the waveguide. To obtain a perfect absorption we need not know the position of all inhomogeneities of the medium, but it suffices only to find the components of the scattering matrix whose dimension corresponds in this experiment to eight waveguide channels. The information on the scattering matrix obtained through preliminary measurements allows configuring the incoming wavefront by phases and amplitudes so that the Umov-Pointing vector be aligned with the lines that finally enter the central antenna with the result that almost all the energy is absorbed by this antenna. The absorption efficiency in the experiment was 99.78 %. For the history of the theoretical study of “anti-lasers” see Physics-Uspekhi 60 818 (2017).
Source: Nature 567 351 (2019)
Isotropy of Universe expansion
1 April 2019
The isotropy of early Universe is tested most exactly through observation of relic radiation that decoupled from the matter in early cosmological epochs. Of interest is also the question of Universe isotropy in later time (z≤1), when dark energy began to dominate in density. J. Soltis (the University of Michigan, USA) and co-authors worked out a new non-parametric test of statistical isotropy of Universe expansion and applied it to about a thousand type Ia supernovae. Used for each supernova was the information on its stellar magnitude depending on the red shift with a peculiar velocity correction. From the variations of supernova distributions about the celestial sphere one can judge about the Universe expansion isotropy. It was obtained that the rms spatial variation of the Hubble parameter does not exceed 1 % for # at the confidence level of 99.7 %, that is, the contemporary Universe is expanding isotropically with a high degree of accuracy.
Source: arXiv:1902.07189 [astro-ph.CO]
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.
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