General Relativity & Quantum Cosmology

Due to the linearity of the field equations and the resulting bilinear structure of the Hamiltonian, quantum radiation effects such as black hole evaporation or particle creation in an expanding universe are typically described as (squeezing) processes where particles are created in pairs. Here, we address the following question: given a mode (e.g., wave-packet) corresponding to a created particle (e.g., as part of Hawking radiation), what is its partner, i.e., the other particle of the pair? After a general derivation of this partner mode, we will discuss some examples such as moving mirror radiation and speculate about possible implications for the black hole information puzzle.

Can high energy physics can be simulated by low-energy, nonrelativistic, many-body systems, such as ultracold atoms? Ultracold atomic systems lack the type of symmetries and dynamical properties of high energy physics models: in particular, they do not manifest local gauge invariance nor Lorentz invariance, which are crucial properties of the quantum field theories which are the building blocks of the standard model of elementary particles. However, it will be shown that there are ways to configure atomic system to manifest both local gauge invariance and Lorentz invariance. In particular, local gauge invariance can arise either as an effective, low energy, symmetry, or as an "exact" symmetry, following from the conservation laws in atomic interactions. Such quantum simulators may lead to new type of (table-top) experiments, to test various QCD phenomena, as the confinement of dynamical quarks, phase transitions, and other effects.

I will sketch a few interesting phenomena involving ideal plasmas, including helicity conservation, frozen flux, the Blandford-Znajek mechanism, and self-confined Poynting jets, using the language of differential forms.

This year marks the 40’th anniversary of the Unruh effect as described at the first Marcel Grossmann meeting in 1975. We revisit it with emphasis on the observability issue which might be a concern at first sight, since the linear acceleration needed to reach a temperature 1 K is of order 10^20 m/s^2 . We close the talk by emphasizing that the Unruh effect does not require any verification beyond that of relativistic free field theory itself. The Unruh effect lives among us.

The quantum Zeno effect is often very controversial in the context of consciousness problems. Frequent direct measurements of a quantum system freeze its time evolution. Then what happes if an observer continuously watches a Schrodinger's cat from the start of the experiment? Naively this looks like a yes-no measurement of a unstable atom decay, which emits a gamma ray as a trigger of the cat execution. If so, the continuous cat observation may prevent the ray emission as a Zeno effect. Consequently the cat can remain alive as long as the observation is maintained. However, this is clearly incorrect. In this talk, assuming some natural conditions, we generally prove the impossibility of quantum Zeno effects generated by indirect measurements by the observer's consciousness. Naively this is a yes-no experiment of a unstable atom decay, which emits a gamma ray as a trigger of the cat execution. If so, the continuous cat observation stops the ray emission as a Zeno effect. Consequently the cat remains alive as long as the observation is maintained. However, this is clearly incorrect. In this talk, assuming some natural conditions, I generally prove the impossibility of quantum Zeno effects generated by indirect measurements by the observer's consciousness

The problem of the gravitational collapse of small mass in the higher derivative and ghost free theories of gravity is discussed. It will be demonstrated how higher derivative and non-local modifications of gravity equations regularizes static and dynamical solutions. Boosting a static solution of the linearized equations for the gravitational potential of a point mass we obtain a solution for the field of the ultra-relativistic source (gyraton). Using the latter we construct solutions for the collapsing spherical (thin and thick) null shell. By analysing the obtained solutions we demonstrate that for small enough value of the mass M an apparent horizon is not formed for the gravitational collapse of small mass in the higher-derivative and ghost free theories of gravity. We demonstrate that this “mass gap” property is connected with the presence of the UV cut-off in such theories.

In 1981 Bill discovered an analogy between the propagation of fields in the vicinity of astrophysical black holes and the that of small excitations in fluids. He postulated that this analogy allows one to test, challenge and verify, in tabletop experiments, the elusive processes of black hole mass and angular momentum loss. Indeed, 34 years later analogue gravity experiments are carried out all over the world to implement his idea. I will first present a brief overview on analogue black hole experiments, and then discuss in more detail some of my earlier (in collaboration with Bill) and more recent experimental and theoretical results on the subject.

The last decade has seen the impressive development of quantum information science, both in theory and in experiment. There are many measures that can be used to assess the achievements in the field: new algorithms, new applications and larger quantum processors, to name a few. The discovery of quantum algorithms has demonstrated the potential power of quantum information. As pointed out by Bill some years ago, to realize this potential requires the ability to overcome the imprecision and imperfection inherent in physical systems. Quantum error correction (QEC) has provided a solution, showing that errors can be corrected with a reasonable amount of resources as long as their rate is sufficiently small. Implementing QEC protocols remains one of the most important challenges in QIP. In the experimental arena, the quest to build quantum processors that could outperform their classical counterparts has led to many blueprint proposals for potential devices based on NMR, electron spin resonance, ion traps, atom traps, optics, superconducting devices and nitrogen-vacancy centres, among others. Many have demonstrated not only the possibility of controlling quantum bits, but also the ability to do so in practice, showing the progression of quantum information science from the blackboard to the laboratory. My presentation will give an overview of some of the recent results in quantum information science on the way to implement quantum error correction. I will show how noise can be characterise efficiently when our goal is to find suitable quantum error correcting codes. I will show demonstrations of control to implement some quantum error correcting codes and finally how can noise be extracted through algorithmic cooling. I will comments on some challenges that need to be solved and a path towards implementing many round of quantum error correction.

I review the contentious question, "Does a uniformly accelerated detector radiate?" As Audretsch and Muller pointed out long ago, this is partly a semantic dispute. The talk draws on recent discussions with Alex Calogeracos and George Matsas.

We evaluate the one-graviton loop contribution to the vacuum polarization on de Sitter background in a 1-parameter family of exact, de Sitter invariant gauges. Our result is computed using dimensional regularization and fully renormalized with BPHZ counterterms, which must include a noninvariant owing to the time-ordered interactions.

Because the graviton propagator engenders a physical breaking of de Sitter invariance two structure functions are needed to express the result. In addition to its relevance for the gauge issue this is the first time a covariant gauge graviton propagator has been used to compute a noncoincident loop.

Presented is a discussion of quantum field theory on curved spacetime and of microlocal analysis, with an emphasis on the way that these two areas connected for me personally through a specific problem, namely that of resolving Kay's singularity conjecture for two point functions of a linear scalar field on a globally hyperbolic spacetime. A particular case of this conjecture is presented, namely the translation invariant case on flat Minkowski spacetime, which does not require microlocal analysis. Next, the results of Duistermaat and Hoermander concerning distinguished parametrices of the Klein Gordon equation on a curved spacetime are described, since they lead to the notion of a wave front set (or microlocal) spectral condition, which could be viewed as a remnant of the spectral condition on flat spacetime. This condition on the wavefront set of the two point function has been employed by Brunetti, Koehler and Fredenhagen to develop a method of renormalization on a general curved spacetime, which has been developed further by Hollands and Wald. Other QFT-related topics to which microlocal methods may apply are: Lorentz symmetry breaking models and many body QM models (e.g., the free electron gas in a metal). In the case of vector or spinor models, the polarization set may be used to refine information about the singularities. Similarly, the principal symbol of the two point function, viewed as a Fourier integral operator, is a constant times a canonical half density on the natural Lagrangian submanifold associated with the Klein-Gordon operator, suggesting a tangent space Lorentz invariance property for the free model.