Measurement of qubit states
1 November 2018
R. McDermott (University of Wisconsin - Madison, USA) with colleagues demonstrated a new method of measuring superconducting qubit states. In their experiment, along with the resonance cavity of the qubit itself they employed a supplementary resonator tuned to the frequency of the excited state of the qubit. A microwave photon counter was used to readily distinguish between the ground and excited states. The quantum fidelity of the new method reaches 92 %. In its capability it surpasses the heterodyne measurements normally exploited for these purposes. The area of their application may be the error correction algorithms and the interfaces between quantum and classical systems. M.D. Lukin (Harvard University, USA) with colleagues demonstrated another measuring method. In their experiment they examined two SiV centers (silicon vacancies) embedded in a nanophotonic cavity and coherently interacting with each other through its electromagnetic modes. The quality of interaction between the SiV centers was improved by about an order of magnitude compared to that attained in other similar experiments. The magnetic field and radio pulses controlled the spin states of SiV centers and the spectral transfer function was measured and employed to determine the states of the system.
Sources: Science 361 1239 (2018), Science 362 662 (2018)
Vortex pairs in a ferromagnetic superconductor
1 November 2018
A group of Russian researchers from MIPT, ISSP, MISIS, MSU, and Kazan Federal University and their colleagues from Japan, France, China, and Great Britain investigated the compound EuFe2(As0.79P0.21)2 in which superconductivity and ferromagnetism coexist at a temperature below 19 K. Ferromagnetism typically destroys superconductivity, but it is not the case here because of the weak exchange field produced by the subsystem of # atoms. A magnetic power microscope was used to construct 3D distribution of magnetic fields at the sample surface at different temperatures. Below the Curie point, a new phase was revealed, namely Meissner domains, which is due to the magnetic Eu subsystem screening. As the temperature fell below 17 K this phase became transformed into “vortex domains” through the first-order phase transition. Under this transition, Abrikosov vortex-antivortex pairs were spontaneously generated in Meissner domains and the domain size increased. V.S. Stolyarov with colleagues formulated a quantitative theory successfully explaining these phenomena.
Source: Science Advances 4 eaat1061 (2018)
Pomeranchuk instability in Fermi liquids
1 November 2018
At the present time, several substances with an electronic nematic order are known for which the rotational symmetry is violated. The understanding of some important features of the nematic order is associated with the Pomeranchuk instability effect. K. Lee (the Ohio University, USA) with colleagues calculated the parameters Fl of a complex Fermi liquid. They considered the case of half-filled Landau levels when the nematic order borders on the non-Abelian quantum Hall state. For three lower Landau levels n=0, 1 and 2 - Fl were found numerically. Monte Carlo calculations with Coulomb potential cutoff on a small scale and with the wave function defined on a torus were carried out. For n=0, Pomeranchuk instabilities were not revealed. For n=1 and 2, the calculations showed Pomeranchuk instability in the nematic (l=2) channel for a wide range of cutoff parameter values. The theoretical result obtained was the first clear demonstration of the fact that the nematic Pomeranchuk instability may appear to be responsible for the nematic quantum Hall states with isotropic screening of Coulomb interactions.
Source: Phys. Rev. Lett. 121 147601 (2018)
Information recording on a single atom
1 November 2018
Researchers from Nijmegen University (Netherland) demonstrated a new magnetic mechanism of information storage on a single cobalt atom placed on a black phosphorus surface. The atom was observed and its state controlled with a scanning tunnel microscope. The atom was found to have stable states at different distances from the surface. The density functional calculations showed that transition from one state to another is due to redistribution of 3s- and 3d-orbital populations, the magnetic anisotropy playing a great role. A high energy barrier between the states makes them applicable for information recording. Information was recorded earlier on the spin state of anindividual atom and a long storage time was reached, but there remained a problem of fluctuations under a spin-sensitive data readout. In the case of a cobalt atom, all the manipulations were performed by an electric field, spin-sensitive measurements being unnecessary. The experiment was conducted at a surface temperature of 4.4 K, but one may hope that in future the method will be realized at room temperature, too.
Source: Nature Communications 9 3904 (2018)
New limit on primordial black holes
1 November 2018
In 1967, Ya.B. Zeldovich and I.D. Novikov predicted the possibility of primordial black hole (PBH) formation in the early Universe(for PBH see the UFN reviews Phys. Usp.28 213 (1985) and Phys. Usp. 61 115 (2018)). The gravitational-wave signals from black-hole coalescence registered by LIGO/Virgo detectors aroused more interest in the PBH model, as PBH, together with stellar-origin black holes, might account for these signals. In particular, the question was put again whether the entire dark matter can consist of PBH. M. Zumalacarregui and U. Seljak (the University of California) were searching for gravitational lensing of distant supernova light by black holes that occasionally found themselves on the line of sight. A PBH would focus the light by its gravitational field, thus forming characteristic singularities in the supernova outburst profile. After processing the data on 740 type Ia supernovae, from the lack of lensing signs it became clear that PBH with masses ≥0.01M☉ amount to no more than ≈ 37 % of the entire dark matter. Thus, the overwhelming part of dark matter must consist of something different because other effects restrict the number of PBH with masses less than <0.01M☉. The new elementary particles not yet registered by detectors are now being considered as one of the most probable candidates to the role of dark matter.
Source: Phys. Rev. Lett. 121 141101 (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.