Decoherence - Fluctuation Relation and Measurement Noise

We discuss fluctuations in the measurement process and how these fluctuations are related to the dissipational parameter characterising quantum damping or decoherence. On the example of the measuring current of the variable-barrier or QPC problem we show there is an extra noise or fluctuation connected with the possible different outcomes of a measurement. This noise has an enhanced short time component which could be interpreted as due to ``telegraph noise'' or ``wavefunction collapses''. Furthermore the parameter giving the the strength of this noise is related to the parameter giving the rate of damping or decoherence.Comment: 6 pages, no figures, for Okun Festschrift, Physics Report

Optimal energy quanta to current conversion

We present a microscopic discussion of a nano-sized structure which uses the quantization of energy levels and the physics of single charge Coulomb interaction to achieve an optimal conversion of heat flow to directed current. In our structure the quantization of energy levels and the Coulomb blockade lead to the transfer of quantized packets of energy from a hot source into an electric conductor to which it is capacitively coupled. The fluctuation generated transfer of a single energy quantum translates into the directed motion of a single electron. Thus in our structure the ratio of the charge current to the heat current is determined by the ratio of the charge quantum to the energy quantum. An important novel aspect of our approach is that the direction of energy flow and the direction of electron motion are decoupled.Comment: 9 pages, 6 figure

MiR-766 induces p53 accumulation and G2/M arrest by directly targeting MDM4

p53, a transcription factor that participates in multiple cellular functions, is considered the most important tumor suppressor. Previous evidence suggests that post-transcriptional deregulation of p53 by microRNAs contributes to tumorigenesis, tumor progression and therapeutic resistance. In the present study, we found that the microRNA miR-766 was aberrantly expressed in breast cancer, and that over-expression of miR-766 caused accumulation of wild-type p53 protein in multiple cancer cell lines. Supporting its role in the p53 signalling pathway, miR-766 decreased cell proliferation and colony formation in several cancer cell lines, and cell cycle analyses revealed that miR-766 causes G2 arrest. At a mechanistic level, we demonstrate that miR-766 enhances p53 signalling by directly targeting MDM4, an oncogene and negative regulator of p53. Analysis of clinical genomic data from multiple cancer types supports the relevance of miR-766 in p53 signalling. Collectively, our study demonstrates that miR-766 can function as a novel tumor suppressor by enhancing p53 signalling.Qingqing Wang, Luke A. Selth and David F. Calle

Proof of Rounding by Quenched Disorder of First Order Transitions in Low-Dimensional Quantum Systems

We prove that for quantum lattice systems in d<=2 dimensions the addition of quenched disorder rounds any first order phase transition in the corresponding conjugate order parameter, both at positive temperatures and at T=0. For systems with continuous symmetry the statement extends up to d<=4 dimensions. This establishes for quantum systems the existence of the Imry-Ma phenomenon which for classical systems was proven by Aizenman and Wehr. The extension of the proof to quantum systems is achieved by carrying out the analysis at the level of thermodynamic quantities rather than equilibrium states.Comment: This article presents the detailed derivation of results which were announced in Phys. Rev. Lett. 103 (2009) 197201 (arXiv:0907.2419). v3 incorporates many corrections and improvements resulting from referee comment

Quantum Brayton cycle with coupled systems as working substance

We explore the quantum version of Brayton cycle with a composite system as the working substance. The actual Brayton cycle consists of two adiabatic and two isobaric processes. Two pressures can be defined in our isobaric process, one corresponds to the external magnetic field (characterized by $F_x$) exerted on the system, while the other corresponds to the coupling constant between the subsystems (characterized by $F_y$). As a consequence, we can define two types of quantum Brayton cycle for the composite system. We find that the subsystem experiences a quantum Brayton cycle in one quantum Brayton cycle (characterized by $F_x$), whereas the subsystem's cycle is of quantum Otto in another Brayton cycle (characterized by $F_y$). The efficiency for the composite system equals to that for the subsystem in both cases, but the work done by the total system are usually larger than the sum of work done by the two subsystems. The other interesting finding is that for the cycle characterized by $F_y$, the subsystem can be a refrigerator while the total system is a heat engine. The result in the paper can be generalized to a quantum Brayton cycle with a general coupled system as the working substance.Comment: 7 pages, 3 figures, accepted by Phys. Rev.

Numerical renormalization group calculation of impurity internal energy and specific heat of quantum impurity models

We introduce a method to obtain the specific heat of quantum impurity models via a direct calculation of the impurity internal energy requiring only the evaluation of local quantities within a single numerical renormalization group (NRG) calculation for the total system. For the Anderson impurity model, we show that the impurity internal energy can be expressed as a sum of purely local static correlation functions and a term that involves also the impurity Green function. The temperature dependence of the latter can be neglected in many cases, thereby allowing the impurity specific heat, $C_{\rm imp}$, to be calculated accurately from local static correlation functions; specifically via $C_{\rm imp}=\frac{\partial E_{\rm ionic}}{\partial T} + 1/2\frac{\partial E_{\rm hyb}}{\partial T}$, where $E_{\rm ionic}$ and $E_{\rm hyb}$ are the energies of the (embedded) impurity and the hybridization energy, respectively. The term involving the Green function can also be evaluated in cases where its temperature dependence is non-negligible, adding an extra term to $C_{\rm imp}$. For the non-degenerate Anderson impurity model, we show by comparison with exact Bethe ansatz calculations that the results recover accurately both the Kondo induced peak in the specific heat at low temperatures as well as the high temperature peak due to the resonant level. The approach applies to multiorbital and multichannel Anderson impurity models with arbitrary local Coulomb interactions. An application to the Ohmic two state system and the anisotropic Kondo model is also given, with comparisons to Bethe ansatz calculations. The new approach could also be of interest within other impurity solvers, e.g., within quantum Monte Carlo techniques.Comment: 16 pages, 15 figures, published versio

Macroscopic entanglement of many-magnon states

We study macroscopic entanglement of various pure states of a one-dimensional N-spin system with N>>1. Here, a quantum state is said to be macroscopically entangled if it is a superposition of macroscopically distinct states. To judge whether such superposition is hidden in a general state, we use an essentially unique index p: A pure state is macroscopically entangled if p=2, whereas it may be entangled but not macroscopically if p<2. This index is directly related to the stability of the state. We calculate the index p for various states in which magnons are excited with various densities and wavenumbers. We find macroscopically entangled states (p=2) as well as states with p=1. The former states are unstable in the sense that they are unstable against some local measurements. On the other hand, the latter states are stable in the senses that they are stable against local measurements and that their decoherence rates never exceed O(N) in any weak classical noises. For comparison, we also calculate the von Neumann entropy S(N) of a subsystem composed of N/2 spins as a measure of bipartite entanglement. We find that S(N) of some states with p=1 is of the same order of magnitude as the maximum value N/2. On the other hand, S(N) of the macroscopically entangled states with p=2 is as small as O(log N)<< N/2. Therefore, larger S(N) does not mean more instability. We also point out that these results are analogous to those for interacting many bosons. Furthermore, the origin of the huge entanglement, as measured either by p or S(N), is discussed to be due to the spatial propagation of magnons.Comment: 30 pages, 5 figures. The manuscript has been shortened and typos have been fixed. Data points of figures have been made larger in order to make them clearly visibl

Instability of human societies as a result of conformity

We introduce a new model that mimics the strong and sudden effects induced by conformity in tightly interacting human societies. Such effects range from mere crowd phenomena to dramatic political turmoil. The model is a modified version of the Ising Hamiltonian. We have studied the properties of this Hamiltonian using both a Metropolis simulation and analytical derivations. Our study shows that increasing the value of the conformity parameter, results in a first order phase transition. As a result a majority of people begin honestly to support the idea that may contradict the moral principles of a normal human beings though each individual would support the moral principle without tight interaction with the society. Thus, above some critical level of conformity our society occurs to be instable with respect to ideas that might be doubtful. Our model includes, in a simplified way, human diversity with respect to loyalty to the moral principles.Comment: 5 pages, 5 figures, accepted in Int. journ of modern physics section

The Impact of Earnings on the Pricing of Credit Default Swaps

This study evaluates the impact of earnings on firm credit risk as captured by Credit Default Swaps (CDS). We find that earnings (changes) are negatively correlated with one-year swap premia (changes) after controlling for equity returns but not with longer term premia (changes). We also find that earnings surprises are significantly correlated with one-year CDS premia changes in the short window surrounding preliminary earnings dates and that absolute earnings surprises are significantly correlated with absolute one-year CDS premia changes in the short window surrounding SEC filing dates. These results suggest that high earnings convey favorable information about the short-term default risk of firms but not about the long term default risk. We further document that accruals/cash flow information conveyed by SEC filings provides information about long-term credit risk. Furthermore, the empirical results are consistent with structural and hybrid model-driven implications of CDS pricing

AFM pulling and the folding of donor-acceptor oligorotaxanes: phenomenology and interpretation

The thermodynamic driving force in the self-assembly of the secondary structure of a class of donor-acceptor oligorotaxanes is elucidated by means of molecular dynamics simulations of equilibrium isometric single-molecule force spectroscopy AFM experiments. The oligorotaxanes consist of cyclobis(paraquat-\emph{p}-phenylene) rings threaded onto an oligomer of 1,5-dioxynaphthalenes linked by polyethers. The simulations are performed in a high dielectric medium using MM3 as the force field. The resulting force vs. extension isotherms show a mechanically unstable region in which the molecule unfolds and, for selected extensions, blinks in the force measurements between a high-force and a low-force regime. From the force vs. extension data the molecular potential of mean force is reconstructed using the weighted histogram analysis method and decomposed into energetic and entropic contributions. The simulations indicate that the folding of the oligorotaxanes is energetically favored but entropically penalized, with the energetic contributions overcoming the entropy penalty and effectively driving the self-assembly. In addition, an analogy between the single-molecule folding/unfolding events driven by the AFM tip and the thermodynamic theory of first-order phase transitions is discussed and general conditions, on the molecule and the cantilever, for the emergence of mechanical instabilities and blinks in the force measurements in equilibrium isometric pulling experiments are presented. In particular, it is shown that the mechanical stability properties observed during the extension are intimately related to the fluctuations in the force measurements.Comment: 42 pages, 17 figures, accepted to the Journal of Chemical Physic