Quantum probes to experimentally assess correlations in a composite system

Abstract

We suggest and demonstrate experimentally a strategy to obtain relevant information about a composite system by only performing measurements on a small and easily accessible part of it, which we call quantum probe. We show in particular how quantitative information about the angular correlations of couples of entangled photons generated by spontaneous parametric down conversion is accessed through the study of the trace distance between two polarization states evolved from different initial conditions. After estimating the optimal polarization states to be used as quantum probe, we provide a detailed analysis of the connection between the increase of the trace distance above its initial value and the amount of angular correlations.PACS numbers: 03.65. Yz,03.65.Ta,42.50.Dv The control of quantum systems plays a basic role in the experimental investigation of the predictions of quantum theory as well as in the development of quantum technologies for applications. Indeed, great attention has been recently payed to engineering the dynamics of quantum systems in order to properly generate, manipulate and exploit significant quantum features [1][2][3][4][5][6].Consider a large quantum system whose full characterization is only partially feasible or requires complex measurement schemes. In such a case, it is crucial to develop effective strategies in order to assess relevant pieces of information about the overall system by only monitoring a small part, which then acts as a probe. A natural procedure is to control the interaction of the small subsystem with the rest of the total system in such a way that the former can encode the information of interest. Here, we provide an explicit example of this strategy in an all-optical setup, where the system under study consists of entangled couples of photons generated by spontaneous parametric down conversion (SPDC) [7][8][9][10]. By properly engineering the interaction between polarization and momentum degrees of freedom of the photons via a 1D spatial light modulator (SLM), we can access some information regarding the momentum correlations between the two photons by simply performing visibility measurements on the polarization degrees of freedom.As specific figure of merit, we exploit the trace distance between polarization states. As we shall see, an increase of the trace distance above its initial value allows to detect some information on momentum correlations, which has moved to the polarization degrees of freedom thanks to the engineered interaction. The tracedistance analysis of quantum dynamics has been recently introduced, leading to important results concerning the non-Markovianity of a quantum dynamics [11][12][13][14][15][16], the characterization of the presence of initial correlations between the quantum system and its environment [17][18][19][20], the relevance of non-local memory effects [21][22][23] and the reservoir engineering in ultracold gases [24][25][26].The paper is structured as follows. In the next Section we describe the physical system we are go...

Colloquium: Non-Markovian dynamics in open quantum systems

“…The discussion will demonstrate, in particular, that memory effects of the open system dynamics reflect characteristic properties of the environment. This fact opens a new perspective, namely to exploit a small open system as a quantum probe signifying nontrivial features of a complex environment, for example the critical point of a phase transition (Gessner et al, 2014a;Smirne et al, 2013). Another example to be discussed here is the use of non-Markovian dynamics to determine nonlocal correlations within a composite environment, carrying out only measurements on the open system functioning as quantum probe Liu et al, 2013a;Wißmann and Breuer, 2014).…”

High-order dispersion effects in two-photon interference

“…In the presence of a broadband arXiv:1607.05854v1 [quant-ph] 20 Jul 2016 source laser the quantum state of the two photons, i.e. the signal "s" and the idler "i" photons, which interfere at the BS, is given by (see, e.g., [24]):…”

Dynamical paths and universality in continuous-variable open systems

“…The main effect of the interaction with the environment is to set up a time scale τ M over which the dynamics of the system is effectively described by a coarsegrained Markovian process towards equilibrium. Conversely, for times shorter than τ M , the dynamics is more involved and the correlations with and within the environment play a major role [4][5][6][7][8][9]. In this regime, decoherence may be less detrimental and the dynamics may even induce recoherence: This is why a great deal of attention has been devoted to the study of the corresponding non-Markovian maps, e.g., in different continuous-variable systems ranging from quantum optics to mechanical oscillators and harmonic lattices [10][11][12][13][14].…”