Abstract:
A boson sampling device is a specialised quantum computer that solves a problem which is strongly believed to be computationally hard for classical computers. Recently a number of small-scale implementations have been reported, all based on multi-photon interference in multimode interferometers. In the hard-to-simulate regime, even validating the device's functioning may pose a problem . In a recent paper, Gogolin et al. showed that so-called symmetric algorithms would be unable to distinguish the experimental distribution from the trivial, uniform distribution. Here we report new boson sampling experiments on larger photonic chips, and analyse the data using a scalable statistical test recently proposed by Aaronson and Arkhipov. We show the test successfully validates small experimental data samples against the hypothesis that they are uniformly distributed. We also show how to discriminate data arising from either indistinguishable or distinguishable photons. Our results pave the way towards larger boson sampling experiments whose functioning, despite being non-trivial to simulate, can be certified against alternative hypotheses.

Abstract:
In this work we proof that boson sampling with $N$ particles in $M$ modes is equivalent to short-time evolution with $N$ excitations in an XY model of $2N$ spins. This mapping is efficient whenever the boson bunching probability is small, and errors can be efficiently postselected. This mapping opens the door to boson sampling with quantum simulators or general purpose quantum computers, and highlights the complexity of time-evolution with critical spin models, even for very short times.

Abstract:
We study the step bunching process in three different 1D step flow models and obtain scaling relations for the step bunches formed in the long times limit. The first one was introduced by S.Stoyanov [Jap. J.Appl. Phys. 29, (1990) L659] as the simplest 'realistic' model of step bunching due to drift of the adatoms. Here we show that it could lead to (at least) two different types of step bunching, depending on the magnitude of the drift. The other two models are minimal models: the equations for step velocity are constructed ad hoc from two terms with opposite effects - destabilizing and, respectively, stabilizing the regular step train.

Abstract:
We formulate a new (1+1)D step model of potentially unstable vicinal growth that we call "C+ - C-" model and study the step bunching process in it. The basic assumption is that the equilibrium adatom concentrations on both sides of the step are different and this may cause destabilization of the regular step train. We deduce equations of step motion and numerically integrate them to obtain the step positions on a discrete time set. New dynamic phenomena are observed during the bunching process: 1. parts of the crystal surface undergo temporary evaporation in the course of coalescence of two bunches, the larger being the one that "goes back", 2. the minimal interstep distance appears in the beginning rather than in the middle of the bunch. We speculate that the latter may serve as a diagnostic criterion for a new universality class constructed from the time- and size-scaling exponents describing the bunching process in the diffusion-limited regime.

Abstract:
We study further the recently introduced [Ranguelov et al., Comptes Rendus de l'Acad. Bulg. des Sci. 60, 4 (2007) 389] "C+-C-" model of step flow crystal growth over wide range of model parameters. The basic assumption of the model is that the reference ("equilibrium") densities used to compute the supersaturation might be different on either side of a step. We obtain the condition for linear stability of the whole step train in the form CL/CR>1 (L/R stands for left/right in a descending from left to right step train). Further we integrate numerically the equations of step motion to monitor the bunching process in the long times limit. Thus we obtain the exact size- and time- scaling of the step bunches including the numerical prefactors. We show that in a broad range of parameters the morphology is characterized with appearance of the minimal interstep distance in the bunch in the beginning of the bunches (at the trailing edge of the bunch) and may be described by a single universality class, different from those already generated by continuum theories [Krug et al., PRB 71, 045412].

Abstract:
Today's increasing demand for wirelessly uploading a large volume of User Generated Content (UGC) is still significantly limited by the throttled backhaul of residential broadband (typically between 1 and 3Mbps). We propose BaPu, a carefully designed system with implementation for bunching WiFi access points' backhaul to achieve a high aggregated throughput. BaPu is inspired by a decade of networking design principles and techniques to enable efficient TCP over wireless links and multipath. BaPu aims to achieve two major goals:1) requires no client modification for easy incremental adoption; 2) supports not only UDP, but also TCP traffic to greatly extend its applicability to a broad class of popular applications such as HD streaming or large file transfer. We prototyped BaPu with commodity hardware. Our extensive experiments shows that despite TCP's sensitivity to typical channel factors such as high wireless packet loss, out-of-order packets arrivals due to multipath, heterogeneous backhaul capacity, and dynamic delays, BaPu achieves a backhaul aggregation up to 95% of the theoretical maximum throughput for UDP and 88% for TCP. We also empirically estimate the potential idle bandwidth that can be harnessed from residential broadband.

Abstract:
Quantum computers are expected to be more efficient in performing certain computations than any classical machine. Unfortunately, the technological challenges associated with building a full-scale quantum computer have not yet allowed the experimental verification of such an expectation. Recently, boson sampling has emerged as a problem that is suspected to be intractable on any classical computer, but efficiently implementable with a linear quantum optical setup. Therefore, boson sampling may offer an experimentally realizable challenge to the Extended Church-Turing thesis and this remarkable possibility motivated much of the interest around boson sampling, at least in relation to complexity-theoretic questions. In this work, we show that the successful development of a boson sampling apparatus would not only answer such inquiries, but also yield a practical tool for difficult molecular computations. Specifically, we show that a boson sampling device with a modified input state can be used to generate molecular vibronic spectra, including complicated effects such as Duschinsky rotations.

Abstract:
While universal quantum computers ideally solve problems such as factoring integers exponentially more efficiently than classical machines, the formidable challenges in building such devices motivate the demonstration of simpler, problem-specific algorithms that still promise a quantum speedup. We construct a quantum boson sampling machine (QBSM) to sample the output distribution resulting from the nonclassical interference of photons in an integrated photonic circuit, a problem thought to be exponentially hard to solve classically. Unlike universal quantum computation, boson sampling merely requires indistinguishable photons, linear state evolution, and detectors. We benchmark our QBSM with three and four photons and analyze sources of sampling inaccuracy. Our studies pave the way to larger devices that could offer the first definitive quantum-enhanced computation.

Abstract:
Hadron multiplicity from $W$ boson is calculated in pQCD. The agreement of our theoretical predictions with the LEP data says in favor of universality of the QCD evolution in hard processes.

Abstract:
We report recent advances on the study of universal weakly bound four-boson states from the solutions of the Faddeev-Yakubovsky equations with zero-range two-body interactions. In particular, we present the correlation between the energies of successive tetramers between two neighbor Efimov trimers and compare it to recent finite range potential model calculations. We provide further results on the large momentum structure of the tetramer wave function, where the four-body scale, introduced in the regularization procedure of the bound state equations in momentum space, is clearly manifested. The results we are presenting confirm a previous conjecture on a four-body scaling behavior, which is independent of the three-body one. We show that the correlation between the positions of two successive resonant four-boson recombination peaks are consistent with recent data, as well as with recent calculations close to the unitary limit. Systematic deviations suggest the relevance of range corrections.