Indefiniteness of the order of quantum events
1 October 2018
In quantum mechanics, the temporal order of quantum events (for instance, an impact on the system) can be indefinite and, moreover, a quantum superposition of different system evolution histories with an opposite order of events may take place. This theoretical prediction was confirmed in the experiment performed by K. Goswami (Queensland University, Australia) with colleagues. Photons came to two interferometer arms through a polarization splitter. In the first or second arm (depending on polarization) quantum operations were performed on them using inverting prisms at different time instants. A criterion was formulated which, using the results of signal measurements at the interferometer output, suggested the conclusion concerning one or another order of events or indefiniteness of this order. Since the photon coherence length exceeded the arm length, the smearing of the photon wave function in time exceeded the time interval between quantum operations. For this reason, the forward and reverse order of operations were undistinguished. The indicated criterion was used to demonstrate this indistinguishability at the confidence level of 18 σ.
Source: Phys. Rev. Lett. 121 090503 (2018)
Creation of quantum entangled state using metamaterials
1 October 2018
Metamaterials are artificial periodic arrays of different elements. V.G. Veselago was the first to consider their unique electromagnetic properties in his paper in Sov. Phys. Usp. 10 509 (1968). The type of metamaterials — metasurfaces are two-dimensional arrays of subwave microantennas. T. Stav with colleagues from the Technion — Israel Institute of Technology used a dielectric metasurface for the first time the to generate entanglement between the spin and the orbital angular momentum of a single photon and also between the spin of one photon and the orbital angular momentum of another photon. When passing through a metasurface, the electromagnetic wave front takes the form of a spiral, and quantum entanglement appears owing to the Pancharatnam – Berry phase that provokes coupling between the spin and the orbital angular momentum of the photon. The presence of entanglement was shown by way of complete quantum tomography of photons in projections of their states onto the basis of orbital angular momenta and onto the basis of polarizations. This showed nonlocal correlations which cannot exist in the classical light wave. Metasurfaces may find wide application in quantum optics to generate entangled states of photons and to control them.
Source: Science 361 1101 (2018)
Unidirectional quantum steering
1 October 2018
The conception of Einstein – Podolsky – Rosen steering (EPR steering) was proposed by E. Schrodinger in 1935. This effect consists in steering the reduction of the wave function of a remote system by choosing the measuring basis for the near system. The effect of a one-way Einstein – Podolsky – Rosen steering, when a reverse steering is impossible, was first demonstrated by D.J. Saunders with co-authors in 2010, but the experimental methods were based on additional assumptions concerning quantum states or the process of their measurement that restricted the applicability of the obtained results in the general case. In their experiment, N. Tischler (Griffith University, Australia) et al. demonstrated for the first time a two-qubit one-way steering without additional assumptions, which makes their experiment an exhaustive demonstration of the given effect. The new experiment was based on a high-quality source of photons in two-qubit Werner states. These photons were sent between two stations along ways with steerable artificial loss. The measurement of photon states allowed one to perform a unidirectional quantum steering without limiting assumptions.
Source: Phys. Rev. Lett. 121 100401 (2018)
Radiative heat transfer by far-field radiation
1 October 2018
The theory of radiative heat transfer between bodies formulated by M. Planck limits from above the efficiency of heat transfer (the black body limit). However, this limitation only works when the radiation wavelength is much smaller than the size of the bodies and the distance between them. It has already been experimentally discovered that in the inverse relation of scales the heat exchange in the near field may exceed the indicated limit. The theoretical calculations showed that an analogous enhancement is also possible in the far field. This conclusion was first confirmed experimentally by D. Thompson (the University of Michigan, USA) with colleagues. The heat transfer was investigated in a vacuum between plane plates that had thickness much smaller than the wavelength and were in thermal contact with resistors. The alternating current passing through the resistor at the first plate heated it periodically. The heat was transferred to the second plate in a radiative way. Measured were the oscillations of current running through the second resistor and occurring owing to the thermal modulation of its resistance. A heat transfer was reached that exceeded the black body limit by two orders of magnitude. The heat transfer velocity agrees well with the calculations in the framework of fluctuation electrodynamics.
Source: Nature 561 216 (2018)
Laboratory simulation of astrophysical jets
1 October 2018
Plasma processes in space bodies are often so complicated that are inaccessible for a theoretical description. For example, an exhaustive magnetothermodynamic theory of the formation and propagation of plasma jets in the nuclei of active galaxies and in young stars has not yet been formulated. These phenomena can be clarified in laboratory experiments performed on plasma facilities. Such studies are being carried out on the “Plasma Focus” facility at Kurchatov Institute (Moscow, Russia) with participation of researchers from FIAN (P.N. Lebedev Physical Institute) and MIPT (Moscow Institute of Physics and Technology). The experiments PF-1000 (Poland) and KPF-4 (Abkhazia) have a similar aim. In these setups, plasma appears under the influence of electric discharge and is ejected as a directed beam. In Kurchatov Institute, narrow collimated jets were obtained with transverse dimensions kept at the level of several cm in propagation up to 100 cm, the plasma velocity exceeding
100 km s-1. An important role of radiative cooling was established and the plasma parameters and the magnetic field distribution in jets were measured. These data will possibly provide insight into the jet stabilization mechanisms, and owing to dimensionless parameters the experimental results can be scaled to astrophysical objects.
Source: Int. J. of Mod. Phys. D 27 1844009 (2018)
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.