Past observable dynamics of a continuously monitored qubit

Monitoring a quantum observable continuously in time produces a stochastic measurement record that noisily tracks the observable. For a classical process such noise may be reduced to recover an average signal by minimizing the mean squared error between the noisy record and a smooth dynamical estimate. We show that for a monitored qubit this usual procedure returns unusual results. While the record seems centered on the expectation value of the observable during causal generation, examining the collected past record reveals that it better approximates a moving-mean Gaussian stochastic process centered at a distinct (smoothed) observable estimate. We show that this shifted mean converges to the real part of a generalized weak value in the time-continuous limit without additional postselection. We verify that this smoothed estimate minimizes the mean squared error even for individual measurement realizations. We go on to show that if a second observable is weakly monitored concurrently, then that second record is consistent with the smoothed estimate of the second observable based solely on the information contained in the first observable record. Moreover, we show that such a smoothed estimate made from incomplete information can still outperform estimates made using full knowledge of the causal quantum state.Comment: 11 pages, 4 figure

A single-world consistent interpretation of quantum mechanics from fundamental time and length uncertainties

Within ordinary ---unitary--- quantum mechanics there exist global protocols that allow to verify that no definite event ---an outcome to which a probability can be associated--- occurs. Instead, states that start in a coherent superposition over possible outcomes always remain as a superposition. We show that, when taking into account fundamental errors in measuring length and time intervals, that have been put forward as a consequence of a conjunction of quantum mechanical and general relativity arguments, there are instances in which such global protocols no longer allow to distinguish whether the state is in a superposition or not. All predictions become identical as if one of the outcomes occurs, with probability determined by the state. We use this as a criteria to define events, as put forward in the Montevideo Interpretation of Quantum Mechanics. We analyze in detail the occurrence of events in the paradigmatic case of a particle in a superposition of two different locations. We argue that our approach provides a consistent (C) single-world (S) picture of the universe, thus allowing an economical way out of the limitations imposed by a recent theorem by Frauchiger and Renner showing that having a self-consistent single-world description of the universe is incompatible with quantum theory. In fact, the main observation of this paper may be stated as follows: If quantum mechanics is extended to include gravitational effects to a QG theory, then QG, S, and C are satisfied.Comment: thoughts and comments more than welcom

The Dynamic of Belief and Unbelief as Developed in Dostoevsky Crime and Punishment and its RThe Dynamic of Belief and Unbelief as Developed in Dostoevsky Crime and Punishment and its Relevance for Contemopary Life elevance for Contemopary Life

A laudable piece of writing often generates significant movements within the reader, sometimes even altering one’s approach to reality. This thesis is a study of the Russian novel Crime and Punishment by Fyodor Dostoevsky. My overall conclusion is that the classic offers a first-hand vision of God’s presence in our everyday reality. Through the work Dostoevsky proposes a God who is present to the poor and acts in a special way through them. Even in the most miserable of contexts, God is seen as present and active in the lives of the suffering and the outcast. From this analysis I attempt to demonstrate how such quality literature, such as that exemplified in Crime and Punishment, can impact the reader in such a profound way that she or he is moved to alter their approach to God and neighbor. I create, finally, a dialogue between the novel and a concrete reality in a poor area of northern Argentina. The concepts of Mother Earth, Icon, and Silence are engaged to show that these common aspects of mestizo life in northern Argentina can inspire contemporary people to find God in the ordinary. Some Latin American liberation theologians, similarly, such as Gustavo Gutierrez, provide key insights into the role of the poor in salvation history and the invitation to participate in this redeeming process. This thesis can therefore be characterized as an adaptation of the theological method “See-Judge-Act”, my own rendition here ultimately taking shape as Read-Reflect-Apply

Plan de acción sobre empresas de mayor siniestralidad laboral en Andalucía.

De las distintas acciones que realizan las Administraciones para reducir la siniestralidad laboral, vamos a analizar los resultados obtenidos por la Junta de Andalucía como consecuencia de la ejecución del Plan de Acción sobre Empresas de Mayor Siniestralidad en Andalucía en el que se asesora a las empresas para reducir la siniestralidad mediante la mejora de las condiciones de trabajo, en una provincia representativa de la media de la accidentalidad andaluza, como es Córdoba. Sobre la base de los resultados obtenidos, y a modo de conclusión, se incide en la importancia de este tipo de campañas en la reducción de la siniestralidad

Extreme Decoherence and Quantum Chaos

We study the ultimate limits to the decoherence rate associated with dephasing processes. Fluctuating chaotic quantum systems are shown to exhibit extreme decoherence, with a rate that scales exponentially with the particle number, thus exceeding the polynomial dependence of systems with fluctuating $k$-body interactions. Our findings suggest the use of quantum chaotic systems as a natural test-bed for spontaneous wave function collapse models. We further discuss the implications on the decoherence of AdS/CFT black holes resulting from the unitarity loss associated with energy dephasing.Comment: 6+10 pp, 2+3 figures; published versio

Probing Quantumness with Joint Continuous Measurements of Non-Commuting Observables

We analyze the continuous measurement of two noncommuting observables for a qubit, and investigate whether the simultaneously observed noisy signals are consistent with the evolution of an equivalent classical system. Following the approach outlined by Leggett and Garg, we show that the readouts violate macrorealistic inequalities for arbitrarily short temporal correlations. Moreover, the derived inequalities are manifestly violated even in the absence of Hamiltonian evolution, unlike for Leggett-Garg inequalities that use a single continuous measurement. Such a violation should indicate the failure of at least one postulate of macrorealism: either physical quantities do not have well-defined values at all times or the measurement process itself disturbs what is being measured. Nevertheless, for measurements of equal strength we are able to construct a classical stochastic model for a spin that perfectly emulates both the qubit evolution and the observed noisy signals, thus emulating the violations; interestingly, this model also requires an unphysical noise to emulate the readouts, which effectively restricts the ability of an observer to learn information about the spin

Linear feedback stabilization of a dispersively monitored qubit

The state of a continuously monitored qubit evolves stochastically, exhibiting competition between coherent Hamiltonian dynamics and diffusive partial collapse dynamics that follow the measurement record. We couple these distinct types of dynamics together by linearly feeding the collected record for dispersive energy measurements directly back into a coherent Rabi drive amplitude. Such feedback turns the competition cooperative, and effectively stabilizes the qubit state near a target state. We derive the conditions for obtaining such dispersive state stabilization and verify the stabilization conditions numerically. We include common experimental nonidealities, such as energy decay, environmental dephasing, detector efficiency, and feedback delay, and show that the feedback delay has the most significant negative effect on the feedback protocol. Setting the measurement collapse timescale to be long compared to the feedback delay yields the best stabilization.Comment: 16 pages, 7 figure