Abstracts for March 2017

  • Recent progress in synthetic chemistry and molecular quantum optics has enabled demonstrations of the quantum mechanical wave–particle duality for complex particles, with masses exceeding 10 kDa. Future experiments with even larger objects will require new optical preparation and manipulation methods that shall profit from the possibility to cleave a well-defined molecular tag from a larger parent molecule. Here we present the design and synthesis of two model compounds as well as evidence for the photoinduced beam depletion in high vacuum in one case.

    The technique of using “laser grating”, in place of physical grating (slits), for producing spatial interference of molecules relies on the laser’s ability to ionize the molecule. (Once ionized, standing electric fields can sweep it out of the way.) But for some molecules, especially large nanoparticles, this is ineffective. Solution: attach a molecular tag to the nanoparticle that reliably cleaves in the presence of a laser, allowing the nanoparticle to be vacuumed up. Rad.

  • This chapter discusses the asymptotics, singularities, and the reduction of theories. The reduction must involve the study of limits—asymptotics.
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Abstracts for October 2016

  • One of von Neumann's motivations for developing the theory of operator algebras and his and Murray's 1936 classification of factors was the question of possible decompositions of quantum systems into independent parts. For quantum systems with a finite number of degrees of freedom the simplest possibility, i.e. factors of type I in the terminology of Murray and von Neumann, are perfectly adequate. In relativistic quantum field theory (RQFT), on the other hand, factors of type III occur naturally. The same holds true in quantum statistical mechanics of infinite systems. In this brief review some physical consequences of the type III property of the von Neumann algebras corresponding to localized observables in RQFT and their difference from the type I case will be discussed. The cumulative effort of many people over more than 30 years has established a remarkable uniqueness result: The local algebras in RQFT are generically isomorphic to the unique, hyperfinite type III, factor in Connes' classification of 1973. Specific theories are characterized by the net structure of the collection of these isomorphic algebras for different space-time regions, i.e. the way they are embedded into each other.

    One of the key subtleties about trying to study quantum information in a field theory is that you can’t formally decompose the Hilbert space into a tensor product of spatially local subsystems.

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Abstracts for July-August 2016

  • Local dark matter searches with LISA
    Massimo Cerdonio, Roberto De Pietri, Philippe Jetzer and Mauro Sereno
    The drag-free satellites of LISA will maintain the test masses in geodesic motion over many years with residual accelerations at unprecedented small levels and time delay interferometry (TDI) will keep track of their differential positions at a level of picometers. This may allow investigations of fine details of the gravitational field in the solar system previously inaccessible. In this spirit, we present the concept of a method for measuring directly the gravitational effect of the density of diffuse local dark matter (LDM) with a constellation of a few drag-free satellites, by exploiting how peculiarly it would affect their relative motion. Using as a test-bed an idealized LISA with rigid arms, we find that the separation in time between the test masses is uniquely perturbed by the LDM, so that they acquire a differential breathing mode. Such an LDM signal is related to the LDM density within the orbits and has characteristic spectral components, with amplitudes increasing in time, at various frequencies of the dynamics of the constellation. This is the relevant result in that the LDM signal is brought to non-zero frequencies.
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Abstracts for May-June 2016

Lots of matter interference experiments this time, because they are awesome.

  • Quantum Interference of a Microsphere
    H. Pino, J. Prat-Camps, K. Sinha, B. P. Venkatesh, and O. Romero-Isart
    We propose and analyze an all-magnetic scheme to perform a Young’s double slit experiment with a micron-sized superconducting sphere of mass 10^{13} amu. We show that its center of mass could be prepared in a spatial quantum superposition state with an extent of the order of half a micrometer. The scheme is based on magnetically levitating the sphere above a superconducting chip and letting it skate through a static magnetic potential landscape where it interacts for short intervals with quantum circuits. In this way a protocol for fast quantum interferometry is passively implemented. Such a table-top earth-based quantum experiment would operate in a parameter regime where gravitational energy scales become relevant. In particular we show that the faint parameter-free gravitationally-induced decoherence collapse model, proposed by Diósi and Penrose, could be unambiguously falsified.

    An extremely exciting and ambitious proposal. I have no ability to assess the technical feasibility, and my prior is that this is too hard, but the authors are solid. Their formalism and thinking is very clean, and hence quite abstracted away from the nitty gritty of the experiment.

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Abstracts for March-April 2016

  • Unruh effect without trans-horizon entanglement
    Carlo Rovelli and Matteo Smerlak
    We estimate the transition rates of a uniformly accelerated Unruh-DeWitt detector coupled to a quantum field with reflecting conditions on a boundary plane (a “mirror”). We find that these are essentially indistinguishable from the usual Unruh rates, viz. that the Unruh effect persists in the presence of the mirror. This shows that the Unruh effect (thermality of detector rates) is not merely a consequence of the entanglement between left and right Rindler quanta in the Minkowski vacuum. Since in this setup the state of the field in the Rindler wedge is pure, we argue furthermore that the relevant entropy in the Unruh effect cannot be the von Neumann entanglement entropy. We suggest, an alternative, that it is the Shannon entropy associated with Heisenberg uncertainty.

    See also the related works by Gooding and Unruh, which connect to Pikovski et al. (blogged here).

  • What is the Entropy in Entropic Gravity?
    Sean M. Carroll and Grant N. Remmen
    We investigate theories in which gravity arises as a consequence of entropy. We distinguish between two approaches to this idea: holographic gravity, in which Einstein's equation arises from keeping entropy stationary in equilibrium under variations of the geometry and quantum state of a small region, and thermodynamic gravity, in which Einstein's equation emerges as a local equation of state from constraints on the area of a dynamical lightsheet in a fixed spacetime background.
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Abstracts for February 2016

  • Non-Markovianity hinders Quantum Darwinism
    Fernando Galve, Roberta Zambrini, and Sabrina Maniscalco
    We investigate Quantum Darwinism and the emergence of a classical world from the quantum one in connection with the spectral properties of the environment. We use a microscopic model of quantum environment in which, by changing a simple system parameter, we can modify the information back flow from environment into the system, and therefore its non-Markovian character. We show that the presence of memory effects hinders the emergence of classical objective reality, linking these two apparently unrelated concepts via a unique dynamical feature related to decoherence factors.

    Galve and collaborators recognize that the recent Nat. Comm. by Brandao et al is not as universal as it is sometimes interpretted, because the records that are proved to exist can be trivial (no info). So Galve et al. correctly emphasize that Darwinism is dependent on the particular dynamics found in our universe, and the effectiveness of record production is in principle an open question.

    Their main model is a harmonic oscillator in an oscillator bath (with bilinear spatial couplings, as usual) and with a spectral density that is concentrated as a hump in some finite window. (See black line with grey shading in Fig 3.) They then vary the system’s frequency with respect to this window.

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Abstracts for January 2016

  • We study the inflationary quantum-to-classical transition for the adiabatic curvature perturbation \zeta due to quantum decoherence, focusing on the role played by squeezed-limit mode couplings. We evolve the quantum state \Psi in the Schrodinger picture, for a generic cubic coupling to additional environment degrees of freedom. Focusing on the case of minimal gravitational interactions, we find the evolution of the reduced density matrix for a given long-wavelength fluctuation by tracing out the other (mostly shorter wavelength) modes of \zeta as an environment. We show that inflation produces phase oscillations in the wave functional \Psi[\zeta(x)], which suppress off-diagonal components of the reduced density matrix, leaving a diagonal mixture of different classical configurations. Gravitational nonlinearities thus provide a minimal mechanism for generating classical stochastic perturbations from inflation. We identify the time when decoherence occurs, which is delayed after horizon crossing due to the weak coupling, and find that Hubble-scale modes act as the decohering environment. We also comment on the observational relevance of decoherence and its relation to the squeezing of the quantum state.
  • The fluctuation-dissipation relation is usually formulated for a system interacting with a heat bath at finite temperature, and often in the context of linear response theory, where only small deviations from the mean are considered.
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Abstracts for October-November 2015

  • High energy particle colliders have been in the forefront of particle physics for more than three decades. At present the near term US, European and international strategies of the particle physics community are centered on full exploitation of the physics potential of the Large Hadron Collider (LHC) through its high-luminosity upgrade (HL-LHC). A number of the next generation collider facilities have been proposed and are currently under consideration for the medium and far-future of accelerator-based high energy physics. In this paper we offer a uniform approach to evaluation of various accelerators based on the feasibility of their energy reach, performance potential and cost range.
    (H/t Sabine.) The contraction has been happening for quite some time:

    maximum c.o.m. energy has drastically slowed down since the early 1990’s and the lepton colliders even went backwards in energy to study rare processes…Moreover, the number of the colliding beam facilities in operation has dropped from 9 two decades ago to 5 now…

  • Conditions for Quantum Violation of Macroscopic Realism
    Johannes Kofler and Časlav Brukner
    Why do we not experience a violation of macroscopic realism in everyday life. Normally, no violation can be seen either because of decoherence or the restriction of coarse-grained measurements, transforming the time evolution of any quantum state into a classical time evolution of a statistical mixture.
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Abstracts for September 2015

I’m trying out a new type of post: a selection of abstracts I thought were particularly interesting this month (though not necessarily released this month). Some papers I’ll have read in detail, some not. I would be particularly interested in hearing commentary on them.

  • Experiments testing macroscopic quantum superpositions must be slow
    Andrea Mari, Giacomo De Palma, Vittorio Giovannetti
    We consider a thought experiment where the preparation of a macroscopically massive or charged particle in a quantum superposition and the associated dynamics of a distant test particle apparently allow for superluminal communication. We give a solution to the paradox which is based on the following fundamental principle: any local experiment, discriminating a coherent superposition from an incoherent statistical mixture, necessarily requires a minimum time proportional to the mass (or charge) of the system. For a charged particle, we consider two examples of such experiments, and show that they are both consistent with the previous limitation. In the first, the measurement requires to accelerate the charge, that can entangle with the emitted photons. In the second, the limitation can be ascribed to the quantum vacuum fluctuations of the electromagnetic field. On the other hand, when applied to massive particles our result provides an indirect evidence for the existence of gravitational vacuum fluctuations and for the possibility of entangling a particle with quantum gravitational radiation.
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