Preprints

Developments in the thermodynamics of small quantum systems envisage non-classical thermal machines. In this scenario, energy fluctuations play a relevant role in the description of irreversibility. We experimentally implement a quantum heat engine based on a spin-1/2 system and nuclear magnetic resonance techniques. Irreversibility at microscope scale is fully characterized by the assessment of energy fluctuations associated with the work and heat flows. We also investigate the efficiency lag related to the entropy production at finite time. The implemented heat engine operates in a regime where both thermal and quantum fluctuations (associated with transitions among the instantaneous energy eigenstates) are relevant to its description. Performing a quantum Otto cycle at maximum power, the proof-of-concept quantum heat engine is able to reach an efficiency for work extraction (η≈42%) very close to its thermodynamic limit (η=44%).

In classical mechanics, external constraints on the dynamical variables can be easily implemented within the Lagrangian formulation and form the basis for several interesting mechanical phenomena and devices. Conversely, the extension of this idea to the quantum realm, which dates back to Dirac, has proven notoriously difficult due to the non-commutativity of observables. Motivated by recent progress in the experimental control of quantum systems, we propose here an implementation of quantum constraints based on the idea of work protocols, which are dynamically engineered to enfore the constraints. As a proof of principle, we consider a quantum harmonic oscillator and show how the combination of two work protocols can be used to implement non-trivial constraints in quantum phase space which couple together the first and second moments of the quadrature operators. We find that such constraints affect the equations of motion for the system in a non-trivial way, inducing non-linear behavior and even classical chaos, although Gaussianity is preserved at all times. A discussion concerning the robustness of this approach to possible experimental errors is also presented.

In this work, we establish a connection between nonlinear electronic spectroscopy and quantum information protocols for the non-disturbance condition. The non-fulfillment of the later is a witness of nonclassicality. Our approach permits us to express the non-disturbance condition in terms of common observables in the context of electronic spectroscopy experiments, such as the induced polarization. We then provide the theoretical framework allowing one to infer nonclassicality from the detected signals in these experiments. A prominent feature of our proposal is then the model independence. In particular, for third-order nonlinear spectroscopies, such as the widely used two-dimensional electronic spectroscopy, we find that the induction of third-order polarization in systems satisfying inversion symmetry automatically implies nonclassicality.

We apply reverse-engineering to find electromagnetic pulses that allow for the control of populations in quantum systems under dephasing and thermal noises. In particular, we discuss two-level systems given their importance in the description of several molecular systems as well as quantum computing. Such an investigation naturally finds applications in a multitude of physical situations involving the control of quantum systems. We present an analytical description of the pulse which solves a constrained dynamics where the initial and final populations are fixed a priori. This constrained dynamics is sometimes impossible and we precisely spot the conditions for that. One of our main results is the presentation of analytical conditions for the establishment of steady states for finite coherence in the presence of noise. This might naturally find applications in quantum memories.