tag:blogger.com,1999:blog-23101388662860176432016-09-28T23:21:55.255-07:00Quantum PhysicsSite for <a href="http://communitypeerreview.blogspot.com/">Community Peer Review</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.comBlogger10386125tag:blogger.com,1999:blog-2310138866286017643.post-6361990523857191332013-08-06T00:06:00.057-07:002013-08-06T00:06:26.903-07:000903.1960 (Horacio E. Camblong et al.)<h2 class="title"><a href="http://arxiv.org/abs/0903.1960">Conformal Tightness of Holographic Scaling in Black Hole Thermodynamics</a> [<a href="http://arxiv.org/pdf/0903.1960">PDF</a>]</h2>Horacio E. Camblong, Carlos R. Ordonez<a name='more'></a><blockquote class="abstract">The near-horizon conformal symmetry of nonextremal black holes is shown to be a mandatory ingredient for the holographic scaling of the scalar-field contribution to the black hole entropy. This conformal tightness is revealed by semiclassical first-principle scaling arguments through an analysis of the multiplicative factors in the entropy due to the radial and angular degrees of freedom associated with a scalar field. Specifically, the conformal SO(2,1) invariance of the radial degree of freedom conspires with the area proportionality of the angular momentum sums to yield a robust holographic outcome.</blockquote>View original: <a href="http://arxiv.org/abs/0903.1960">http://arxiv.org/abs/0903.1960</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-12313035100696042342013-08-06T00:06:00.055-07:002013-08-06T00:06:25.579-07:001308.0666 (M. Ndong et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0666">Time optimization and state-dependent constraints in the quantum optimal<br /> control of molecular orientation</a> [<a href="http://arxiv.org/pdf/1308.0666">PDF</a>]</h2>M. Ndong, C. Koch, D. Sugny<a name='more'></a><blockquote class="abstract">We apply two recent generalizations of monotonically convergent optimization algorithms to the control of molecular orientation by laser fields. We show how to minimize the control duration by a step-wise optimization and maximize the field-free molecular orientation using state-dependent constraints. We discuss the physical relevance of the different results.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0666">http://arxiv.org/abs/1308.0666</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-60138861853236290262013-08-06T00:06:00.053-07:002013-08-06T00:06:24.941-07:001308.0670 (Nam Mai Phan et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0670">Controllable single photon stimulation of retinal rod cells</a> [<a href="http://arxiv.org/pdf/1308.0670">PDF</a>]</h2>Nam Mai Phan, Mei Fun Cheng, Dmitri A. Bessarab, Leonid A. Krivitsky<a name='more'></a><blockquote class="abstract">Retinal rod cells are commonly assumed to be sensitive to single photons [1, 2, 3]. Light sources used in prior experiments exhibit unavoidable fluctuations in the number of emitted photons [4]. This leaves doubt about the exact number of photons used to stimulate the rod cell. In this letter, we interface rod cells of Xenopus laevis with a light source based on Spontaneous Parametric Down Conversion (SPDC) [5], which provides one photon at a time. Precise control of generation of single photons and directional delivery enables us to provide unambiguous proof of single photon sensitivity of rod cells without relying on the statistical assumptions. Quantum correlations between single photons in the SPDC enable us to determine quantum efficiency of the rod cell without pre-calibrated reference detectors [6, 7, 8]. These results provide the path for exploiting resources offered by quantum optics in generation and manipulation of light in visual studies. From a more general perspective, this method offers the ultimate tool for studying intrinsic characteristics of photoinduced biological processes at the single and discrete photon level [9].</blockquote>View original: <a href="http://arxiv.org/abs/1308.0670">http://arxiv.org/abs/1308.0670</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-57395033668171756462013-08-06T00:06:00.051-07:002013-08-06T00:06:23.934-07:001308.0681 (Krzysztof Jachymski et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0681">Analytical model of overlapping Feshbach resonances</a> [<a href="http://arxiv.org/pdf/1308.0681">PDF</a>]</h2>Krzysztof Jachymski, Paul S. Julienne<a name='more'></a><blockquote class="abstract">Feshbach resonances in ultracold collisions often result from an interplay between many collision channels. Simple two-channel models can be introduced to capture the basic features, but cannot fully reproduce the situation when several resonances from different closed channels contribute to the scattering process. Using the formalism of multichannel quantum defect theory we develop an analytical model of overlapping Feshbach resonances. We find a general formula for the variation of the scattering length with magnetic field in the vicinity of an arbitrary number of resonances, characterized by simple parameters. Our formula is in excellent agreement with numerical coupled channels calculations for several cases of overlapping resonances in the collisions of two $^7$Li atoms or two Cs atoms.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0681">http://arxiv.org/abs/1308.0681</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-2204022690361323372013-08-06T00:06:00.049-07:002013-08-06T00:06:23.183-07:001308.0707 (Tao Zhou)<h2 class="title"><a href="http://arxiv.org/abs/1308.0707">Success probabilities for universal unambiguous discriminators between<br /> unknown pure states</a> [<a href="http://arxiv.org/pdf/1308.0707">PDF</a>]</h2>Tao Zhou<a name='more'></a><blockquote class="abstract">A universal programmable discriminator can perform the discrimination between two unknown states, and the optimal solution can be approached via the discrimination between the two averages over the uniformly distributed unknown input pure states, which has been widely discussed in previous works. In this paper, we consider the success probabilities of the optimal universal programmable unambiguous discriminators when applied to the pure input states. More precisely, the analytic results of the success probabilities are derived with the representation theory of U(d) group and angular momentum theory, and we find that the success probabilities have nothing to do with the dimension d while the amounts of the copies in the two program registers are equal.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0707">http://arxiv.org/abs/1308.0707</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-9080876511246850962013-08-06T00:06:00.047-07:002013-08-06T00:06:22.229-07:001308.0712 (F. Intravaia et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0712">Quantum friction and non-equilibrium fluctuation theorems</a> [<a href="http://arxiv.org/pdf/1308.0712">PDF</a>]</h2>F. Intravaia, R. O. Behunin, D. A. R. Dalvit<a name='more'></a><blockquote class="abstract">We use general concepts of non-equilibrium statistical mechanics to compute the quantum frictional force on an atom in steady motion above a surface. We derive the frictional force using a non-equilibrium fluctuation-dissipation relation, and compare with that computed with the quantum regression theorem. We show that beyond the weak coupling limit, quantum regression fails to predict the correct stationary atom-surface interaction as given by fluctuation-dissipation, both in and out of equilibrium, mainly due to the broadband nature of fluctuation-induced interactions.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0712">http://arxiv.org/abs/1308.0712</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-81482072431038399912013-08-06T00:06:00.045-07:002013-08-06T00:06:21.293-07:001308.0739 (F. Laloë et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0739">Angular momentum conservation in measurements on spin Bose-Einstein<br /> condensates</a> [<a href="http://arxiv.org/pdf/1308.0739">PDF</a>]</h2>F. Laloë, W. J. Mullin<a name='more'></a><blockquote class="abstract">We discuss a thought experiment where two operators, Alice and Bob, perform transverse spin measurements on a quantum system; this system is initially in a double Fock spin state, which extends over a large distance in space so that the two operators are far away from each other. Standard quantum mechanics predicts that, when Alice makes a few measurements, a large transverse component of the spin angular momentum may appear in Bob's laboratory. A paradox then arises since local angular momentum conservation seems to be violated. It has been suggested that this angular momentum may be provided by the interaction with the measurement apparatuses. We show that this solution of the paradox is not appropriate, so that another explanation must be sought. The general question is the retroaction of a quantum system onto a measurement apparatus. For instance, when the measured system is entangled with another quantum system, can its reaction on a measurement apparatus be completely changed? Is angular momentum conserved only on average over several measurements, but not during one realization of the experiment?</blockquote>View original: <a href="http://arxiv.org/abs/1308.0739">http://arxiv.org/abs/1308.0739</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-12470231346590271222013-08-06T00:06:00.043-07:002013-08-06T00:06:20.346-07:001308.0750 (M. Burrello et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0750">Topological phase transitions driven by non-Abelian gauge potentials in<br /> optical square lattices</a> [<a href="http://arxiv.org/pdf/1308.0750">PDF</a>]</h2>M. Burrello, I. C. Fulga, E. Alba, L. Lepori, A. Trombettoni<a name='more'></a><blockquote class="abstract">We analyze a tight-binding model of ultracold fermions loaded in an optical square lattice and subjected to a synthetic non-Abelian gauge potential featuring both a magnetic field and a translational invariant SU(2) term. We consider in particular the effect of broken time-reversal symmetry and its role in driving non-trivial topological phase transitions. By varying the spin-orbit coupling parameters, we find both a semimetal/insulator phase transition and a topological phase transition between insulating phases with a different number of edge states. The spin is not a conserved quantity of the system and the topological phase transitions can be detected by analyzing its polarization in time of flight images, providing a clear diagnostics for the characterization of the topological phases through the partial entanglement between spin and lattice degrees of freedom.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0750">http://arxiv.org/abs/1308.0750</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-83012590001988154562013-08-06T00:06:00.041-07:002013-08-06T00:06:19.343-07:001308.0771 (Jie Cui et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0771">Elastic and inelastic collisions of $^2Σ$ molecules in a magnetic<br /> field</a> [<a href="http://arxiv.org/pdf/1308.0771">PDF</a>]</h2>Jie Cui, Roman V. Krems<a name='more'></a><blockquote class="abstract">We calculate the cross sections for elastic scattering and Zeeman relaxation in binary collisions of molecules in the ro-vibrational ground state of a $^2\Sigma$ electronic state and the Zeeman state with the electron spin projection $M_S=1/2$ on the magnetic field axis. This is the lowest-energy state of $^2\Sigma$ molecules confined in a magnetic trap. The results are averaged over calculations with multiple molecule - molecule interaction potentials, which yields the expectation intervals for the cross sections and the elastic-to-inelastic cross section ratios. We find that the elastic-to-inelastic cross section ratios under conditions corresponding to trapped molecular ensembles at $T \sim 10^{-3}$ K exceed 100 for the majority of $^2\Sigma$ molecules. The range of $^2\Sigma$ molecules expected to be collisionally unstable in magnetic traps at $T < 10^{-3}$ K is limited to molecules with the spin-rotation interaction constant $\gamma_{\rm SR} > 0.5$ cm$^{-1}$ and the rotational constant $B_e < 4$ cm$^{-1}$.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0771">http://arxiv.org/abs/1308.0771</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-62714600058451782722013-08-06T00:06:00.039-07:002013-08-06T00:06:18.374-07:001308.0793 (Michel Bauer et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0793">Real time imaging of quantum and thermal fluctuations: the case of a<br /> two-level system</a> [<a href="http://arxiv.org/pdf/1308.0793">PDF</a>]</h2>Michel Bauer, Denis Bernard<a name='more'></a><blockquote class="abstract">A quantum system in contact with a heat bath undergoes quantum transitions between energy levels upon absorption or emission of energy quanta by the bath. These transitions remain virtual until the energy of the system is measured repeatedly, even continuously in time. Isolating the two indispensible mechanisms in competition, we describe in a synthetic way the main physical features of thermally activated quantum jumps. Using classical tools of stochastic analysis, we compute in the case of a Q-bit the complete statistics of jumps and transition times in the limit when the typical measurement time is small compared to the thermal relaxation time. The emerging picture is that quantum trajectories are similar to those of a classical particle in a noisy environment, subject to transitions a la Kramers in a multi well landscape, but with a large multiplicative noise.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0793">http://arxiv.org/abs/1308.0793</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-47151086298901894282013-08-06T00:06:00.037-07:002013-08-06T00:06:17.811-07:001308.0803 (Daniel M. Reich et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0803">Cooling molecular vibrations with shaped laser pulses: Optimal control<br /> theory exploiting the timescale separation between coherent excitation and<br /> spontaneous emission</a> [<a href="http://arxiv.org/pdf/1308.0803">PDF</a>]</h2>Daniel M. Reich, Christiane P. Koch<a name='more'></a><blockquote class="abstract">Laser cooling of molecules employing broadband optical pumping involves a timescale separation between laser excitation and spontaneous emission. Here, we optimize the optical pumping step using shaped laser pulses. We derive two optimization functionals to drive population into those excited state levels that have the largest spontaneous emission rates to the target state. We show that, when using optimal control, laser cooling of molecules works even if the Franck-Condon map governing the transitions is preferential to heating rather than cooling. Our optimization functional is also applicable to the laser cooling of other degrees of freedom provided the cooling cycle consists of coherent excitation and dissipative deexcitation steps whose timescales are separated.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0803">http://arxiv.org/abs/1308.0803</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-64682968157912510912013-08-06T00:06:00.035-07:002013-08-06T00:06:17.124-07:001308.0804 (Diwaker et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0804">Multi-channel scattering problems: Analytical approach for exact<br /> solution using Green's functions</a> [<a href="http://arxiv.org/pdf/1308.0804">PDF</a>]</h2>Diwaker, Aniruddha Chakraborty<a name='more'></a><blockquote class="abstract">We have proposed an analytical approach for exact solution of multi-channel scattering problems, in presence of Dirac Delta function couplings. Our solution is quite general and is valid for any set of potentials, if the Green's functions of the uncoupled potentials are known at the crossing point. Using our model, it is possible to express N-channel problem by N independent `single' channel problems and hence one can have a realistic solution of multi-channel scattering problem. In this paper we have shown that transition probability from first diabatic potential to any other potential can be easily evaluated using a very simple analytical formula - which only require value of eigenfunction of first uncoupled potential and value of Green's function of other uncoupled potential, at the crossing.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0804">http://arxiv.org/abs/1308.0804</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-71221243052666977912013-08-06T00:06:00.033-07:002013-08-06T00:06:16.638-07:001308.0806 (Lin Chen et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0806">A comparison of old and new definitions of the geometric measure of<br /> entanglement</a> [<a href="http://arxiv.org/pdf/1308.0806">PDF</a>]</h2>Lin Chen, Martin Aulbach, Michal Hajdusek<a name='more'></a><blockquote class="abstract">Several inequivalent definitions of the geometric measure of entanglement (GM) have been introduced and studied in the past. Here we review five known definitions and introduce two new definitions, with the qualifying criterion being that for pure states the measure is a linear or logarithmic function of the maximal fidelity with product states. The entanglement axioms and properties of the measures are studied, and qualitative and quantitative comparisons are made between all definitions. Streltsov et al. [New J. Phys 12 123004 (2010)] proved the equivalence of two linear definitions of GM, whereas we show that the corresponding logarithmic definitions are distinct. Certain classes of states such as "maximally correlated states" and isotropic states are particularly valuable for this analysis. One of the newly introduced GM definition is found to be the first one to be both normalized and weakly monotonous, thus being a prime candidate for future studies of multipartite entanglement. We also find that a large class of graph states, which includes all cluster states, have a "universal" closest separable state that minimizes the quantum relative entropy, the Bures distance and the trace distance.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0806">http://arxiv.org/abs/1308.0806</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-35237473908438804552013-08-06T00:06:00.031-07:002013-08-06T00:06:15.852-07:001308.0815 (F. Caruso et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0815">Solving a two-electron quantum dot model in terms of polynomial<br /> solutions of a Biconfluent Heun Equation</a> [<a href="http://arxiv.org/pdf/1308.0815">PDF</a>]</h2>F. Caruso, J. Martins, V. Oguri<a name='more'></a><blockquote class="abstract">The effects on the non-relativistic dynamics of a system compound by two electrons interacting by a Coulomb potential and with an external harmonic oscillator potential, confined to move in a two dimensional Euclidean space, are investigated. In particular, it is shown that it is possible to determine exactly and in a closed form a finite portion of the energy spectrum and the associated eigeinfunctions for the Schr\"odinger equation describing the relative motion of the electrons.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0815">http://arxiv.org/abs/1308.0815</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-17588410276598929972013-08-06T00:06:00.029-07:002013-08-06T00:06:15.199-07:001308.0823 (H. Schenck et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0823">Vector chiral phases in frustrated 2D XY model and quantum spin chains</a> [<a href="http://arxiv.org/pdf/1308.0823">PDF</a>]</h2>H. Schenck, V. L. Pokrovsky, T. Nattermann<a name='more'></a><blockquote class="abstract">The phase diagram of the frustrated 2D XY model is calculated analytically. The chiral (Ising) transition is described by three independent critical exponents which are calculated in $d=5/2-\epsilon$ dimensions. Vortex interaction is short range on small and logarithmic on large scales, if compared with the chiral correlation length $\xi$. The vortex unbinding transitions is triggered by the increase of $\xi$ and occurs before the chiral transition takes place. In a narrow region close to the Lifshitz point a reentrant quasi-ferromagnetic phase appears. Application to antiferromagnetic quantum spin chains and multiferroics are discussed.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0823">http://arxiv.org/abs/1308.0823</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-37169526624775394602013-08-06T00:06:00.027-07:002013-08-06T00:06:14.240-07:001308.0831 (Miroslav Gavenda et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0831">Quantum noise eater for a single photonic qubit</a> [<a href="http://arxiv.org/pdf/1308.0831">PDF</a>]</h2>Miroslav Gavenda, Lucie Čelechovská, Miloslav Dušek, Radim Filip<a name='more'></a><blockquote class="abstract">We propose a quantum noise eater for a single qubit and experimentally verify its performance for recovery of a superposition carried by a dual-rail photonic qubit. We consider a case when only one of the rails (e.g., one of interferometric arms) is vulnerable to noise. A coherent but randomly arriving photon penetrating into this single rail causes a change of its state, which results in an error in a subsequent quantum information processing. We theoretically prove and experimentally demonstrate a conditional full recovery of the superposition by this quantum noise eater.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0831">http://arxiv.org/abs/1308.0831</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-90488960836715681182013-08-06T00:06:00.025-07:002013-08-06T00:06:13.219-07:001308.0833 (Maximilian Fillinger)<h2 class="title"><a href="http://arxiv.org/abs/1308.0833">Data Structures in Classical and Quantum Computing</a> [<a href="http://arxiv.org/pdf/1308.0833">PDF</a>]</h2>Maximilian Fillinger<a name='more'></a><blockquote class="abstract">This survey summarizes several results about quantum computing related to (mostly static) data structures. First, we describe classical data structures for the set membership and the predecessor search problems: Perfect Hash tables for set membership by Fredman, Koml\'{o}s and Szemer\'{e}di and a data structure by Beame and Fich for predecessor search. We also prove results about their space complexity (how many bits are required) and time complexity (how many bits have to be read to answer a query). After that, we turn our attention to classical data structures with quantum access. In the quantum access model, data is stored in classical bits, but they can be accessed in a quantum way: We may read several bits in superposition for unit cost. We give proofs for lower bounds in this setting that show that the classical data structures from the first section are, in some sense, asymptotically optimal - even in the quantum model. In fact, these proofs are simpler and give stronger results than previous proofs for the classical model of computation. The lower bound for set membership was proved by Radhakrishnan, Sen and Venkatesh and the result for the predecessor problem by Sen and Venkatesh. Finally, we examine fully quantum data structures. Instead of encoding the data in classical bits, we now encode it in qubits. We allow any unitary operation or measurement in order to answer queries. We describe one data structure by de Wolf for the set membership problem and also a general framework using fully quantum data structures in quantum walks by Jeffery, Kothari and Magniez.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0833">http://arxiv.org/abs/1308.0833</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-59181424944065392292013-08-06T00:06:00.023-07:002013-08-06T00:06:12.090-07:001308.0842 (Levon Chakhmakhchyan et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0842">A compact entanglement distillery using realistic quantum memories</a> [<a href="http://arxiv.org/pdf/1308.0842">PDF</a>]</h2>Levon Chakhmakhchyan, Stéphane Guérin, Joshua Nunn, Animesh Datta<a name='more'></a><blockquote class="abstract">We adopt the beam splitter model for losses to analyse the performance of a recent compact continuous-variable entanglement distillation protocol [Phys. Rev. Lett. 108, 060502, (2012)] implemented using realistic quantum memories. We show that the decoherence undergone by a two-mode squeezed state while stored in a quantum memory can strongly modify the results of the preparatory step of the protocol. We find that the well-known method for locally increasing entanglement, phonon subtraction, may not result in entanglement gain when losses are taken into account. Thus, we investigate the critical number m_c of phonon subtraction attempts from the matter modes of the quantum memory. If the initial state is not de-Gaussified within m_c attempts, the protocol should be restarted to obtain any entanglement increase. Moreover, the condition m_c>1 implies an additional constraint on the subtraction beam splitter interaction transmissivity, viz. it should be about 50% for a wide range of protocol parameters. Additionally, we consider the average entanglement rate, which takes into account both the unavoidable probabilistic nature of the protocol and its possible failure as a result of a large number of unsuccessful subtraction attempts. We find that a higher value of the average entanglement can be achieved by increasing the subtraction beam splitter interaction transmissivity. We conclude that the compact distillation protocol with the practical constraints coming from realistic quantum memories allows a feasible experimental realization within existing technologies.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0842">http://arxiv.org/abs/1308.0842</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-37762395240815785482013-08-06T00:06:00.021-07:002013-08-06T00:06:11.328-07:001308.0849 (H. M. Florez et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0849">Power-Broadening-Free Correlation Spectroscopy in Cold Atoms</a> [<a href="http://arxiv.org/pdf/1308.0849">PDF</a>]</h2>H. M. Florez, L. S. Cruz, M. H. G. de Miranda, R. A. de Oliveira, J. W. R. Tabosa, M. Martinelli, D. Felinto<a name='more'></a><blockquote class="abstract">We report a detailed investigation on the properties of correlation spectra for cold atoms under the condition of Electromagnetically Induced Transparency (EIT). We describe the transition in the system from correlation to anti-correlation as the intensity of the fields increases. Such transition occurs for laser frequencies around the EIT resonance, which is characterized by a correlation peak. The transition point between correlation and anti-correlation is independent of power broadening and provides directly the ground-state coherence time. We introduce a method to extract in real time the correlation spectra of the system. The experiments were done in two distinct magneto-optical traps (MOT), one for cesium and the other for rubidium atoms, employing different detection schemes. A simplified theory is introduced assuming three-level atoms in $\Lambda$ configuration interacting with a laser with stochastic phase fluctuations, providing a good agreement with the experimental observations.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0849">http://arxiv.org/abs/1308.0849</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-63980131159105328962013-08-06T00:06:00.019-07:002013-08-06T00:06:10.384-07:001308.0901 (M. Mahdian et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0901">Quantum discord in bipartite systems based on projection measurements</a> [<a href="http://arxiv.org/pdf/1308.0901">PDF</a>]</h2>M. Mahdian, F. Marahem<a name='more'></a><blockquote class="abstract">We propose a method to detect an exact quantum discord for some general bipartite quantum systems analytically. Here we show that for some density matrices, after the orthogonal projective measurement the state of the matrix will be diagonal and classic. So we can obtain the maximum amount of classical information in this case. Consequently, for these density matrices instead of using the positive operator valued measures (POVMs), projective operators can be used. Also we obtain the relation between quantum discord with relative entropy of discord for these states and show that for various examples of X-states such as on two qubit and qubit-qutrit density matrices, the relative entropy of discord is replaced by quantum discord.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0901">http://arxiv.org/abs/1308.0901</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-10518665592945347812013-08-06T00:06:00.017-07:002013-08-06T00:06:09.466-07:001308.0912 (Xavier Fernandez-Gonzalvo et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0912">Quantum frequency conversion of quantum memory compatible photons to<br /> telecommunication wavelengths</a> [<a href="http://arxiv.org/pdf/1308.0912">PDF</a>]</h2>Xavier Fernandez-Gonzalvo, Giacomo Corrielli, Boris Albrecht, Marcelli Grimau, Matteo Cristiani, Hugues de Riedmatten<a name='more'></a><blockquote class="abstract">We report an experiment demonstrating quantum frequency conversion of weak light pulses compatible with atomic quantum memories to telecommunication wavelengths. We use a PPLN nonlinear waveguide to convert weak coherent states at the single photon level with a duration of 30ns from a wavelength of 780nm to 1552nm. We measure a maximal waveguide internal (external) conversion efficiency eta_int = 0.41 (eta_ext = 0.25), and we show that the signal to noise ratio (SNR) is good enough to reduce the input photon number below 1. In addition, we show that the noise generated by the pump beam in the crystal is proportional to the spectral bandwidth of the device, suggesting that narrower filtering could significantly increase the SNR. Finally, we demonstrate that the quantum frequency converter can operate in the quantum regime by converting a time-bin qubit and measuring the qubit fidelity after conversion.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0912">http://arxiv.org/abs/1308.0912</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-89898306311063598262013-08-06T00:06:00.015-07:002013-08-06T00:06:08.410-07:001308.0927 (Z. V. Pchelkina et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0927">Ab initio investigation of the exchange interactions in<br /> Bi$_2$Fe$_4$O$_9$: The Cairo pentagonal lattice compound</a> [<a href="http://arxiv.org/pdf/1308.0927">PDF</a>]</h2>Z. V. Pchelkina, S. V. Streltsov<a name='more'></a><blockquote class="abstract">We present the \emph{ab initio} calculation of the electronic structure and magnetic properties of Bi$_2$Fe$_4$O$_9$. This compound crystallizes in the orthorhombic crystal structure with the Fe$^{3+}$ ions forming the Cairo pentagonal lattice implying strong geometric frustration. The neutron diffraction measurements reveal nearly orthogonal magnetic configuration, which at first sight is rather unexpected since it does not minimize the total energy of the pair of magnetic ions coupled by the Heisenberg exchange interaction. Here we calculate the electronic structure and exchange integrals of Bi2Fe4O9 within the LSDA+U method. We obtain three different in-plane (J3=36 K, J4=73 K, J5=23 K) and two interplane (J1=10 K, J2=12 K) exchange parameters. The derived set of exchange integrals shows that the realistic description of Bi2Fe4O9 needs a more complicated model than the ideal Cairo pentagonal lattice with only two exchange parameters in the plane. However, if one takes into account only two largest exchange integrals, then according to the ratio x\equiv J3/J4=0.49<\sqrt{2} (a critical parameter for the ideal Cairo pentagonal lattice, see. Ref.~1) the ground state should be the orthogonal magnetic configuration in agreement with experiment. The microscopic origin of different exchange interactions is also discussed.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0927">http://arxiv.org/abs/1308.0927</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-16689811483445508552013-08-06T00:06:00.013-07:002013-08-06T00:06:07.516-07:001308.0946 (Stephen M. Barnett et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0946">Gleason-Busch theorem and Bayesian quantum theory</a> [<a href="http://arxiv.org/pdf/1308.0946">PDF</a>]</h2>Stephen M. Barnett, James D. Cresser, John Jeffers, David T. Pegg<a name='more'></a><blockquote class="abstract">We show that Gleason's theorem, in the form recently generalised by Busch, may be further simplified by dropping one of the three properties from which it was derived. The result is a more general probability than that usually employed in quantum theory in that it shows that any set of positive operators can represent the probabilities for a set of possible events. Remarkably, our more general form seems to contain Bayes's rule for conditional probabilities so there is no need to add it as an additional element. There is no need, moreover, to postulate that the measurement operators sum to the identity; rather this condition follows from our more general rule when there is no prior measurement outcome information available. We show how the new and general probability law may be applied in quantum communications and in retrodictive quantum theory.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0946">http://arxiv.org/abs/1308.0946</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-15945142690684278642013-08-06T00:06:00.011-07:002013-08-06T00:06:06.747-07:001308.0952 (Kil-Chan Ha et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0952">Global geometric difference between separable and Positive partial<br /> transpose states</a> [<a href="http://arxiv.org/pdf/1308.0952">PDF</a>]</h2>Kil-Chan Ha, Seung-Hyeok Kye<a name='more'></a><blockquote class="abstract">In the convex set of all $3\ot 3$ states with positive partial transposes, we show that one can take two extreme points whose convex combinations belong to the interior of the convex set. Their convex combinations may be even in the interior of the convex set of all separable states. In general, we need at least $mn$ extreme points to get an interior point by their convex combination, for the case of the convex set of all $m\ot n$ separable states. This shows a sharp distinction between PPT states and separable states. We also consider the same questions for positive maps and decomposable maps.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0952">http://arxiv.org/abs/1308.0952</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2310138866286017643.post-75099745568848036082013-08-06T00:06:00.009-07:002013-08-06T00:06:05.867-07:001308.1007 (Gerard 't Hooft)<h2 class="title"><a href="http://arxiv.org/abs/1308.1007">The Fate of the Quantum</a> [<a href="http://arxiv.org/pdf/1308.1007">PDF</a>]</h2>Gerard 't Hooft<a name='more'></a><blockquote class="abstract">Although the suspicion that quantum mechanics is emergent has been lingering for a long time, only now we begin to understand how a bridge between classical and quantum mechanics might be squared with Bell's inequalities and other conceptual obstacles. Here, it is shown how mappings can be formulated that relate quantum systems to classical systems. By generalizing these ideas, one gets quite general models in which quantum mechanics and classical mechanics can merge. It is helpful to have some good model examples such as string theory. It is suggested that notions such as 'super determinism' and 'conspiracy' should be looked at much more carefully than in the, by now, standard arguments.</blockquote>View original: <a href="http://arxiv.org/abs/1308.1007">http://arxiv.org/abs/1308.1007</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0