Dynamics and Stability of Low-Reynolds-Number Swimming Near a Wall

The locomotion of microorganisms and tiny artificial swimmers is governed by low-Reynolds-number hydrodynamics, where viscous effects dominate and inertial effects are negligible. While the theory of low-Reynolds-number locomotion is well studied for unbounded fluid domains, the presence of a boundary has a significant influence on the swimmer’s trajectories and poses problems of dynamic stability of its motion. In this paper we consider a simple theoretical model of a microswimmer near a wall, study its dynamics, and analyze the stability of its motion. We highlight the underlying geometric structure of the dynamics, and establish a relation between the reversing symmetry of the system and existence and stability of periodic and steady solutions of motion near the wall. The results are demonstrated by numerical simulations and validated by motion experiments with macroscale robotic swimmer prototypes

Progenitor constraints on the Type-Ia supernova SN2011fe from pre-explosion Hubble Space Telescope HeII narrow-band observations

We present Hubble Space Telescope (HST) imaging observations of the site of the Type-Ia supernova SN2011fe in the nearby galaxy M101, obtained about one year prior to the event, in a narrow band centred on the HeII 4686 \AA{} emission line. In a "single-degenerate" progenitor scenario, the hard photon flux from an accreting white dwarf (WD), burning hydrogen on its surface over $\sim1$ Myr should, in principle, create a HeIII Str\"{o}mgren sphere or shell surrounding the WD. Depending on the WD luminosity, the interstellar density, and the velocity of an outflow from the WD, the HeIII region could appear unresolved, extended, or as a ring, with a range of possible surface brightnesses. We find no trace of HeII 4686 \AA{} line emission in the HST data. Using simulations, we set $2\sigma$ upper limits on the HeII 4686 \AA{} luminosity of $L_{\rm HeII} < 3.4 \times 10^{34}$ erg s$^{-1}$ for a point source, corresponding to an emission region of radius $r < 1.8$ pc. The upper limit for an extended source is $L_{\rm HeII} < 1.7 \times 10^{35}$ erg s$^{-1}$, corresponding to an extended region with $r\sim11$ pc. The largest detectable shell, given an interstellar-medium density of 1 cm$^{-3}$, has a radius of $\sim6$ pc. Our results argue against the presence, within the $\sim10^5$ yr prior to the explosion, of a supersoft X-ray source of luminosity $L_{\rm bol} \ge 3 \times 10^{37}$ erg s$^{-1}$, or of a super-Eddington accreting WD that produces an outflowing wind capable of producing cavities with radii of 2-6 pc.Comment: Accepted by MNRAS Letters; revised version following referee report and readers' comment

Locking classical information

It is known that the maximum classical mutual information that can be achieved between measurements on a pair of quantum systems can drastically underestimate the quantum mutual information between those systems. In this article, we quantify this distinction between classical and quantum information by demonstrating that after removing a logarithmic-sized quantum system from one half of a pair of perfectly correlated bitstrings, even the most sensitive pair of measurements might only yield outcomes essentially independent of each other. This effect is a form of information locking but the definition we use is strictly stronger than those used previously. Moreover, we find that this property is generic, in the sense that it occurs when removing a random subsystem. As such, the effect might be relevant to statistical mechanics or black hole physics. Previous work on information locking had always assumed a uniform message. In this article, we assume only a min-entropy bound on the message and also explore the effect of entanglement. We find that classical information is strongly locked almost until it can be completely decoded. As a cryptographic application of these results, we exhibit a quantum key distribution protocol that is "secure" if the eavesdropper's information about the secret key is measured using the accessible information but in which leakage of even a logarithmic number of key bits compromises the secrecy of all the others.Comment: 32 pages, 2 figure

Dilemma that cannot be resolved by biased quantum coin flipping

We show that a biased quantum coin flip (QCF) cannot provide the performance of a black-boxed biased coin flip, if it satisfies some fidelity conditions. Although such a QCF satisfies the security conditions of a biased coin flip, it does not realize the ideal functionality, and therefore, does not fulfill the demands for universally composable security. Moreover, through a comparison within a small restricted bias range, we show that an arbitrary QCF is distinguishable from a black-boxed coin flip unless it is unbiased on both sides of parties against insensitive cheating. We also point out the difficulty in developing cheat-sensitive quantum bit commitment in terms of the uncomposability of a QCF.Comment: 5 pages and 1 figure. Accepted versio