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.
    Note that although the OTIMA matter interferometers require an amount of time proportional to the superposed mass to confirm a superposition, the proportionality constant is vastly larger than the one apparently demonstrated here.
  • In this talk we explain the elements of symplectic geometry, and sketch the proof of one of its foundational results — Gromov’s nonsqueezing theorem — using J-holomorphic curves.
    Figure 1.2. The symplectic area \omega(S) is the sum of the area of its projection pr_{12}(S) to the plane (x_1, x_2) = (p_1, q_1) given by the velocity and position in the first direction together with the area of the corresponding projection pr_{34}(S) for the two coordinates in the second direction. I have drawn the first 3 coordinates; the fourth is left to your imagination.
    This papers has some intuition for how to think about symplectic geometry, which surprisingly still isn’t understood in a transparent intuitive way.
  • We discuss the possibility of devising cosmological observables which violate Bell's inequalities. Such observables could be used to argue that cosmic scale features were produced by quantum mechanical effects in the very early universe. As a proof of principle, we propose a somewhat elaborate inflationary model where a Bell inequality violating observable can be constructed.
    Proof-of-concept inflationary model producing natural Bell experiments (in the CMB?), which would testify to the quantum origins of primordial fluctuations. First time I’ve ever felt the inclination to learn about multi-field inflationary models.
  • For more than 80 years, the counterintuitive predictions of quantum theory have stimulated debate about the nature of reality. In his seminal work, John Bell proved that no theory of nature that obeys locality and realism can reproduce all the predictions of quantum theory. Bell showed that in any local realist theory the correlations between distant measurements satisfy an inequality and, moreover, that this inequality can be violated according to quantum theory. This provided a recipe for experimental tests of the fundamental principles underlying the laws of nature. In the past decades, numerous ingenious Bell inequality tests have been reported. However, because of experimental limitations, all experiments to date required additional assumptions to obtain a contradiction with local realism, resulting in loopholes. Here we report on a Bell experiment that is free of any such additional assumption and thus directly tests the principles underlying Bell's inequality. We employ an event-ready scheme that enables the generation of high-fidelity entanglement between distant electron spins. Efficient spin readout avoids the fair sampling assumption (detection loophole), while the use of fast random basis selection and readout combined with a spatial separation of 1.3 km ensure the required locality conditions. We perform 245 trials testing the CHSH-Bell inequality S≤2 and find S=2.42±0.20. A null hypothesis test yields a probability of p=0.039 that a local-realist model for space-like separated sites produces data with a violation at least as large as observed, even when allowing for memory in the devices. This result rules out large classes of local realist theories, and paves the way for implementing device-independent quantum-secure communication and randomness certification.
    Previously announced, but included here for completeness. Also see blog post by Scott Aaronson. I’m told that several experimental groups are very close to this, which is probably why this group announced ASAP rather than embargoing the preprint before it appears in Nature. Expect to see several new loophole-free Bell experiments using different systems (photons, electrons, etc.) from other group announced in the coming year or two.
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