82 research outputs found
Undecidability and the problem of outcomes in quantum measurements
We argue that it is fundamentally impossible to recover information about
quantum superpositions when a system has interacted with a sufficiently large
number of degrees of freedom of the environment. This is due to the fact that
gravity imposes fundamental limitations on how accurate measurements can be.
This leads to the notion of undecidability: there is no way to tell, due to
fundamental limitations, if a quantum system evolved unitarily or suffered
wavefunction collapse. This in turn provides a solution to the problem of
outcomes in quantum measurement by providing a sharp criterion for defining
when an event has taken place. We analyze in detail in examples two situations
in which in principle one could recover information about quantum coherence: a)
"revivals" of coherence in the interaction of a system with the environment and
b) the measurement of global observables of the system plus apparatus plus
environment. We show in the examples that the fundamental limitations due to
gravity and quantum mechanics in measurement prevent both revivals from
occurring and the measurement of global observables. It can therefore be argued
that the emerging picture provides a complete resolution to the measurement
problem in quantum mechanics.Comment: 14 pages, Latex, one figure, version to appear in Foundations of
Physic
A generative modeling approach for benchmarking and training shallow quantum circuits
Hybrid quantum-classical algorithms provide ways to use noisy
intermediate-scale quantum computers for practical applications. Expanding the
portfolio of such techniques, we propose a quantum circuit learning algorithm
that can be used to assist the characterization of quantum devices and to train
shallow circuits for generative tasks. The procedure leverages quantum hardware
capabilities to its fullest extent by using native gates and their qubit
connectivity. We demonstrate that our approach can learn an optimal preparation
of the Greenberger-Horne-Zeilinger states, also known as "cat states". We
further demonstrate that our approach can efficiently prepare approximate
representations of coherent thermal states, wave functions that encode
Boltzmann probabilities in their amplitudes. Finally, complementing proposals
to characterize the power or usefulness of near-term quantum devices, such as
IBM's quantum volume, we provide a new hardware-independent metric called the
qBAS score. It is based on the performance yield in a specific sampling task on
one of the canonical machine learning data sets known as Bars and Stripes. We
show how entanglement is a key ingredient in encoding the patterns of this data
set; an ideal benchmark for testing hardware starting at four qubits and up. We
provide experimental results and evaluation of this metric to probe the trade
off between several architectural circuit designs and circuit depths on an
ion-trap quantum computer.Comment: 16 pages, 9 figures. Minor revisions. As published in npj Quantum
Informatio
O SERVIÇO SOCIAL NAS MOBILIZAÇÕES POPULARES: CONTRIBUIÇÕES PARA O RESGATE DA CIDADANIA
RESUMO: Levando em consideração a atual conjuntura brasileira e o crescimento exponencial das desigualdades sociais, com ênfase no enorme déficit habitacional no paÃs, os/as assistentes sociais necessitam no exercÃcio cotidiano, proporcionar aos usuários dos serviços o fortalecimento da participação e mobilização popular em prol da permanecia e acesso ao espaço urbano. Compreendida enquanto, estratégia de enfrentamento e resistência à s expressões da questão social, permite fomentar o controle social e viabilizar de maneira concreta e palpável aos sujeitos, a participação nos referentes processos emancipatórios com vistas à autonomia e protagonismo. Dessa forma, abordaremos no presente texto, a intervenção dos/das assistentes sociais nas organizações comunitárias e mobilizações populares por direito a permanência e moradia nos grandes centros urbanos, relativas à s experiências vivenciadas na comunidade do Cristal na cidade de Porto Alegre/RS
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
Energy storage and coherence in closed and open quantum batteries
We study the role of coherence in closed and open quantum batteries. We
obtain upper bounds to the work performed or energy exchanged by both closed
and open quantum batteries in terms of coherence. Specifically, we show that
the energy storage can be bounded by the Hilbert-Schmidt coherence of the
density matrix in the spectral basis of the unitary operator that encodes the
evolution of the battery. We also show that an analogous bound can be obtained
in terms of the battery's Hamiltonian coherence in the basis of the unitary
operator by evaluating their commutator. We apply these bounds to a 4-state
quantum system and the anisotropic XY Ising model in the closed system case,
and the Spin-Boson model in the open case.Comment: 14 pages double column, 9 pages appendix; revised manuscript with
citations and new plot; results unchange
Random quantum batteries
Quantum nanodevices are fundamental systems in quantum thermodynamics that have been the subject of profound interest in recent years. Among these, quantum batteries play a very important role. In this paper we lay down a theory of random quantum batteries and provide a systematic way of computing the average work and work fluctuations in such devices by investigating their typical behavior. We show that the performance of random quantum batteries exhibits typicality and depends only on the spectral properties of the time evolving operator, the initial state, and the measuring Hamiltonian. At given revival times a random quantum battery features a quantum advantage over classical random batteries. Our method is particularly apt to be used both for exactly solvable models like the Jaynes-Cummings model or in perturbation theory, e.g., systems subject to harmonic perturbations. We also study the setting of quantum adiabatic random batteries
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
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