May 2017

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Author:
Javier Cerrillo
Body:

The possibility to extract full counting statistics (FCS) of transport processes in experimental settings exposes the necessity to upgrade existing simulation methods to gain access to environmental degrees of freedom. The techniques stemming from the theory of generating functions make it possible to encode all cumulants of the outcome of a specific measurement scheme in the form of a generalized density matrix. With this spirit we have developed a counting-field-resolved hierarchy of equations of motion (FCS-HEOM) which extends this ability to the case of strong-coupling, non-Markovian open quantum systems [1]. Exploiting the flexibility to define the underlying measurement scheme, we show that the comparison of two of them reveals transport coefficients which are the non-equilibrium generalization of energy or particle conductances.
An alternative approach to the observation of environmental dynamics comes from the field of driven open systems. An analytical solution of the dynamics of both the system and the environment for a large class of systems is possible and can be interpreted as the effect of a static Hamiltonian on a continuous class of operators [2]. This novel perspective on the Floquet theory allows us to explore transient polaron dynamics in a straight-forward fashion.
Both FCS results and Floquet simulation benefit strongly from the application of the transfer tensor method (TTM) [3], which is an approach to propose an optimized propagation alternative based on short time evolution samples. This extends the simulation power of existing exact approaches, like the chain-mapping DMRG-based simulation method known as TEDOPA [4]. This proposal departs from the traditional bottom-up approach of simulation method designs based on microscopic principles, and attempts to take a top-down perspective in placing the dynamical map as the central (and the only experimentally accessible) object.

[1] J. Cerrillo, M. Buser, T. Brandes, Physical Review B 94, 214308 (2016).
[2] S. Restrepo, J. Cerrillo, V.M. Bastidas, D.G. Angelakis, T. Brandes, Physical Review Letters 117, 250401 (2016).
[3] J. Cerrillo, J. Cao, Physical Review Letters 112, 110401 (2014).
[4] R. Rosenbach, J. Cerrillo, S.F. Huelga, J. Cao, M.B. Plenio, New Journal of  Physics 18, 023035 (2016).
 

 
 
 
 
 
 
 
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Author:
Lorenzo Maccone
Body:

Quantum Metrology calculates the ultimate precision of all estimation

strategies, measuring what is their root mean-square error (RMSE) and

their Fisher information. Here, instead, we ask how many bits of the

parameter we can recover, namely we derive an information-theoretic

quantum metrology. In this setting we redefine "Heisenberg bound" and

"standard quantum limit" (the usual benchmarks in quantum estimation

theory), and show that the former can be attained only by sequential

strategies or parallel strategies that employ entanglement among

probes, whereas parallel-separable strategies are limited by the

latter.  We highlight the differences between this setting and the

RMSE-based one.

 
Author:
Eric Lutz
Body:

We review the miniaturization of heat engines towards the nanoscale. We

discuss in detail the experimental realization of a single atom engine

using an ultracold trapped ion coupled to engineered reservoirs. We

further address the question of how to enter the quantum regime and

exploit quantum effects to enhance the performance of the machine.

 
Author:
Ravi Kunjwal
Body:

Recent work (Kunjwal and Spekkens, Phys. Rev. Lett. 115, 110403 (2015)) has shown how operational noncontextuality inequalities robust to noise can be obtained from Kochen-Specker uncolourable (or KS-uncolourable) hypergraphs without assuming that measurement outcomes are fixed deterministically by the ontic state of the system in an underlying ontological model. This result was obtained by an explicit numerical enumeration of all the extremal points of the polytope of (measurement) noncontextual assignments of probabilities to the KS-uncolourable hypergraph. I’ll focus on an analytical approach to deriving such noncontextuality inequalities that relies on constraints arising directly from the structure of the hypergraph without necessarily enumerating all the extremal probabilistic models on it. This cleanly identifies operational quantities that one can expect to be constrained (and why) by the assumption of noncontextuality instead of having to guess these quantities or obtaining them from brute-force numerical methods (such as Fourier-Motzkin elimination) without any guiding principles to identify them. This approach relies on giving such noncontextuality inequalities and their upper bounds an interpretation in terms of the hypergraph structure. I’ll demonstrate noncontextuality inequalities robust to noise for a family of KS-uncolourable hypergraphs including many known KS constructions, using this method. Indeed, for this family, the problem of obtaining noncontextuality inequalities turns out to be intimately connected to edge covers of graphs.

 
 
 
 
 
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