Quantum Critical Probing and Simulation of Colored Quantum Noise

We propose a protocol to simulate the evolution of a non-Markovian open quantum system by considering a collisional process with a many-body system, which plays the role of an environment. As a result of our protocol the environment spatial correlations are mapped into the time correlations of a noise that drives the dynamics of the open system. Considering the weak coupling limit the open system can also be considered as a probe of the environment properties. In this regard, when preparing the environment in its ground state, a measurement of the dynamics of the open system allows to determine the length of the environment spatial correlations and therefore its critical properties. To illustrate our proposal we simulate the full system dynamics with matrix-product-states and compare this with the reduced dynamics obtained with an approximated variational master equation

Emergent transport in a many-body open system driven by interacting quantum baths

We analyze an open many-body system that is strongly coupled at its boundaries to interacting quantum baths. We show that the two-body interactions inside the baths induce emergent phenomena in the spin transport. The system and baths are modeled as independent spin chains resulting in a global non-homogeneous XXZ model. The evolution of the system-bath state is simulated using matrix-product-states methods. We present two phase transitions induced by bath interactions. For weak bath interactions we observe ballistic and insulating phases. However, for strong bath interactions a diffusive phase emerges with a distinct power-law decay of the time-dependent spin current $Q\propto t^{-\alpha}$. Furthermore, we investigate long-lasting current oscillations arising from the non-Markovian dynamics in the homogeneous case, and find a sharp change in their frequency scaling coinciding with the triple point of the phase diagram.Comment: Significant changes to the presentation. Soon in PR

A Nonequilibrium quantum phase transition in strongly coupled spin chains

We study spin transport in a boundary driven XXZ spin chain. Driving at the chain boundaries is modeled by two additional spin chains prepared in oppositely polarized states. Emergent behavior, both in the transient dynamics and in the long-time quasi-steady state, is demonstrated. Time-dependent matrix-product-state simulations of the system-bath state show ballistic spin transport below the Heisenberg isotropic point. Indications of exponentially vanishing transport are found above the Heisenberg point for low energy initial states while the current decays asymptotically as a power law for high energy states. Precisely at the critical point, non-ballistic transport is observed. Finally, it is found that the sensitivity of the quasi-stationary state on the initial state of the chain is a good witness of the different transport phases

Physically Realizable Entanglement by Local Continuous Measurements

Quantum systems prepared in pure states evolve into mixtures under environmental action. Physically realizable ensembles are the pure state decompositions of those mixtures that can be generated in time through continuous measurements of the environment. Here, we define physically realizable entanglement as the average entanglement over realizable ensembles. We optimize the measurement strategy to maximize and minimize this quantity through local observations on the independent environments that cause two qubits to disentangle in time. We then compare it with the entanglement bounds for the unmonitored system. For some relevant noise sources the maximum realizable entanglement coincides with the upper bound, establishing the scheme as an alternative to locally protect entanglement. However, for local strategies, the lower bound of the unmonitored system is not reached.Comment: version 2; 5 pages, 1 figure; added references

Cooperativity of a few quantum emitters in a single-mode cavity

We theoretically investigate the emission properties of a single-mode cavity coupled to a mesoscopic number of incoherently pumped quantum emitters. We propose an operational measure for the medium cooperativity, valid both in the bad and in the good cavity regimes. We show that the opposite regimes of subradiance and superradiance correspond to negative and positive cooperativity, respectively. The lasing regime is shown to be characterized by nonnegative cooperativity. In the bad cavity regime we show that the cooperativity defines the transitions from subradiance to superradiance. In the good cavity regime it helps to define the lasing threshold, also providing distinguishable signatures indicating the lasing regime. Increasing the quality of the cavity mode induces a crossover from the solely superradiant to the lasing regime. Furthermore, we verify that lasing is manifested in a wide range of positive cooperative behavior, showing that stimulated emission and superradiance can coexist. The robustness of the cooperativity is studied with respect to experimental imperfections, such as inhomogeneous broadening and pure dephasing

Continuous Quantum Error Correction Through Local Operations

We propose local strategies to protect global quantum information. The protocols, which are quantum error correcting codes for dissipative systems, are based on environment measurements, direct feedback control and simple encoding of the logical qubits into physical qutrits whose decaying transitions are indistinguishable and equally probable. The simple addition of one extra level in the description of the subsystems allows for local actions to fully and deterministically protect global resources, such as entanglement. We present codes for both quantum jump and quantum state diffusion measurement strategies and test them against several sources of inefficiency. The use of qutrits in information protocols suggests further characterization of qutrit-qutrit disentanglement dynamics, which we also give together with simple local environment measurement schemes able to prevent distillability sudden death and even enhance entanglement in situations in which our feedback error correction is not possible.Comment: Accepted for publication in Phys. Rev.

State of the Art About COVID-19's Impact on Santiago University, Cape Verde

The COVID-19 pandemic become a critical challenge for the higher education sector worldwide. Under such a circumstance, the exploration of the capacity of this sector to adapt to such a state of uncertainty has become more of huge importance. In this chapter, we critically reflect on the Cape Verdean teaching experience during the early COVID-19 lockdown in this country. This is an exploratory case study based on a qualitative approach with an aim to reflect about new practices of teaching under a pandemic emergency. Based on the teaching experience of teaching in Santiago University, we explain how this university has changed from a face-to-face to an online teaching system and stress the challenges and opportunities that appear from this transition process. This chapter concludes that this strategy has become an opportunity to the university since it consistently raised the number of international students cooperating with them and also that the more adaptive and resilient approaches to online teaching were also a success.info:eu-repo/semantics/publishedVersio