We address measurement-based generation of quantum coherence in continuous variable systems. We consider Gaussian measurements performed on Gaussian states and focus on two scenarios: In the first one, we assume an initially correlated bipartite state shared by two parties and study how correlations may be exploited to remotely create quantum coherence via measurement back action. In particular, we focus on conditional states with zero first moments, so as to address coherence due to properties of the covariance matrix. We consider different classes of bipartite states with incoherent marginals and show that the larger the measurement squeezing, the larger the conditional coherence. Homodyne detection is thus the optimal Gaussian measurement to remotely generate coherence. We also show that for squeezed thermal states there exists a threshold value for the generated coherence which separates entangled and separable states at a fixed energy. Finally, we briefly discuss the tripartite case and the relationship between tripartite correlations and the conditional two-mode coherence. In the second scenario, we address the steady-state coherence of a system interacting with an environment which is continuously monitored. In particular, we discuss the dynamics of an optical parametric oscillator in order to investigate how the coherence of a Gaussian state may be increased by means of time-continuous Gaussian measurement on the interacting environment.
We address measurement-based generation of quantum coherence in continuous variable systems. We consider Gaussian measurements performed on Gaussian states and focus on two scenarios: In the first one, we assume an initially correlated bipartite state shared by two parties and study how correlations may be exploited to remotely create quantum coherence via measurement back action. In particular, we focus on conditional states with zero first moments, so as to address coherence due to properties of the covariance matrix. We consider different classes of bipartite states with incoherent marginals and show that the larger the measurement squeezing, the larger the conditional coherence. Homodyne detection is thus the optimal Gaussian measurement to remotely generate coherence. We also show that for squeezed thermal states there exists a threshold value for the generated coherence which separates entangled and separable states at a fixed energy. Finally, we briefly discuss the tripartite case and the relationship between tripartite correlations and the conditional two-mode coherence. In the second scenario, we address the steady-state coherence of a system interacting with an environment which is continuously monitored. In particular, we discuss the dynamics of an optical parametric oscillator in order to investigate how the coherence of a Gaussian state may be increased by means of time-continuous Gaussian measurement on the interacting environment.
This thesis develops a comprehensive framework for the use of continuous variable (CV) Gaussian states under environmental interactions and continuous measurements. Gaussian states provide an experimentally relevant class of quantum states because they are fully described by their first and second statistical moments. This thesis begins by representing Gaussian operations as symplectic transformations on the statistical moments and formulates the evolution of a Gaussian state under both unconditional and conditional open system dynamics. Stochastic methods are introduced in a physicist-accessible manner by sacrificing mathematical rigor over physical intuition. These methods are necessary to model environmental noise and continuous measurements represented by a Wiener process. From stochastic principles, a stochastic version of the Schr\"{o}dinger equation and a master equation are derived that capture the measurement-induced back-action. From these principles a conditional equation of motion for the covariance matrix of a Gaussian state is derived, leading to the Riccati equation. Unconditional evolution always leads to diffusive dynamics when the noise is modeled by a Wiener process. The key difference between unconditional and conditional equations of motion is that monitoring always leads to a reduction of uncertainty because a measurement gains information about the system. At the same time, all correlations to the part of the system being measured are lost, which in many cases can lead to an impure quantum state. At some cases, a sufficient measurement scheme can even be used to stabilize the unconditional equation at all scales. The developed formalism is applied to a system of multiple parametric flux pumps based on spontaneous parametric down-conversion to analyze entanglement generation in a complex network of modes. A simple formula relevant to all symmetric pumping architectures, also relevant for asymmetric cases, is derived with graph theoretical tools. The investigation revealed fundamental challenges in entanglement-generation based on multiple two-mode squeezed modes. Overall, the thesis provides a complete framework of Gaussian state dynamics connecting symplectic geometry, open quantum systems, quantum measurement theory, and stochastic methods. The results highlight the importance of Gaussian states in modern quantum technologies and future improvements in quantum computation and communication.
Gaussian bipartite states are basic tools for the realization of quantum information protocols with continuous variables. Their complete characterization is obtained by the reconstruction of the corresponding covariance matrix. Here we describe in detail and experimentally demonstrate a robust and reliable method to fully characterize bipartite optical Gaussian states by means of a single homodyne detector. We have successfully applied our method to the bipartite states generated by a sub-threshold type-II optical parametric oscillator which produces a pair of thermal cross-polarized entangled cw frequency degenerate beams. The method provides a reliable reconstruction of the covariance matrix and allows us to retrieve all the physical information about the state under investigation. These include observable quantities, such as energy and squeezing, as well as nonobservable ones such as purity, entropy, and entanglement. Our procedure also includes advanced tests for the Gaussianity of the state and, overall, represents a powerful tool to study the bipartite Gaussian state from the generation stage to the detection one.
Quantum properties are soon subject to decoherence once the quantum system interacts with the classical environment. In this paper we experimentally test how propagation losses, in a Gaussian channel, affect the bipartite Gaussian entangled state generated by a subthreshold type-II optical parametric oscillator. Experimental results are discussed in terms of different quantum markers, as teleportation fidelity, quantum discord, and mutual information, and continuous-variable (CV) entanglement criteria. To analyze state properties we have retrieved the composite system covariance matrix by a single homodyne detector. We experimentally found that, even in the presence of a strong decoherence, the generated state never disentangles and keeps breaking the quantum limit for the discord. This result proves that the class of CV entangled states discussed in this paper would allow, in principle, to realize quantum teleportation over an infinitely long Gaussian channel.
In recent years the paradigm based on entanglement as the unique measure of quantum correlations has been challenged by the rise of new correlation concepts, such as quantum discord, able to reveal quantum correlations that are present in separable states. It is in general difficult to compute quantum discord, because it involves a minimization over all possible local measurements in a bipartition. In the realm of continuous-variable (CV) systems, a Gaussian version of quantum discord has been put forward upon restricting to Gaussian measurements. It is natural to ask whether non-Gaussian measurements can lead to a stronger minimization than Gaussian ones. Here we focus on two relevant classes of two-mode Gaussian states: squeezed thermal states and mixed thermal states, and allow for a range of experimentally feasible non-Gaussian measurements, comparing the results with the case of Gaussian measurements. We provide evidence that Gaussian measurements are optimal for Gaussian states.
In the present PhD work, we focus on a specific class of quantum states of light: the non-Gaussian states. These states have the particularity of exhibiting Wigner functions with some negative values. This quantum feature is a necessary condition to perform some quantum computation task; furthermore it is also useful for various other applications, including quantum communication and metrology. Different strategies can be used to generate these states. Here, we start from Gaussian states produced by optical parametric oscillators in the continuous wave regime, (i.e. single-mode and two-mode squeezed vacuum states). The non-Gaussian feature can only be obtained by non-linear phenomena (over-quadratic Hamiltonian). In our case, the non-linearity is induced by photon-counting-based measurements (also called non-Gaussian measurements). This study is mainly divided into two parts. First, the generation of non-classical states associated with two types of qubit encoding: the single-photon state, used for quantum information with discrete variables, and the coherent state superposition (the so-called optical Schrodinger cat state), used for quantum information with continuous variables. These two states have then been used to perform some quantum information protocols. The first one addresses the problem of single-photon entanglement witness, and the other the generation of entanglement between the two encodings (also called hybrid entanglement).
This thesis explores the role of non-classical correlations in bipartite continuous variable quantum systems, and the approach taken is three-fold. We show that given two initially entangled atomic ensembles, it is possible to probabilistically increase the entanglement between them using a beamsplitter-like interaction formed from two quantum non-demolition (QND) interactions with auxiliary polarised light modes. We then develop an elegant method to calculate density matrix elements of non-Gaussian bipartite quantum states and use this to show that the entanglement in a two mode squeezed vacuum can be distilled using QND interactions and non-Gaussian elements. \n Secondly, we introduce a potential new measure of quantum entanglement in bipartite Gaussian states. This measure has an operational meaning in quantum cryptography and provides an upper bound on the amount of a secret key that can be distilled from a Gaussian probability distribution shared by two conspirators, Alice and Bob, given the presence of an eavesdropper, Eve. \n Finally, we go beyond the realm of quantum entanglement to explore other non-classical correlations in continuous variable systems. We provide solutions for a number of these measures on two mode Gaussian states and introduce the Gaussian Ameliorated Measurement Induced Disturbance (GAMID). The interplay between these different measures and quantum entanglement is examined. We then attempt to take small steps into the non-Gaussian regime by computing these non-classicality measures on the three-parameter continuous variable Werner states.