1,253 research outputs found
Continuous variable qumodes as non-destructive probes of quantum systems
With the rise of quantum technologies, it is necessary to have practical and
preferably non-destructive methods to measure and read-out from such devices. A
current line of research towards this has focussed on the use of ancilla
systems which couple to the system under investigation, and through their
interaction, enable properties of the primary system to be imprinted onto and
inferred from the ancillae. We propose the use of continuous variable qumodes
as ancillary probes, and show that the interaction Hamiltonian can be fully
characterised and directly sampled from measurements of the qumode alone. We
suggest how such probes may also be used to determine thermodynamical
properties, including reconstruction of the partition function. We show that
the method is robust to realistic experimental imperfections such as
finite-sized measurement bins and squeezing, and discuss how such probes are
already feasible with current experimental setups.Comment: 8 pages, 3 figure
Quantum-enhanced multi-parameter estimation for unitary photonic systems
Precise device characterization is a fundamental requirement for a large
range of applications using photonic hardware, and constitutes a
multi-parameter estimation problem. Estimates based on measurements using
single photons or classical light have precision which is limited by
shot-noise, while quantum resources can be used to achieve sub-shot-noise
precision. However, there are many open questions with regard to the best
quantum protocols for multi-parameter estimation, including the ultimate limits
to achievable precision, as well as optimal choices for probe states and
measurements. In this paper, we develop a formalism based on Fisher information
to tackle these questions for set-ups based on linear-optical components and
photon-counting measurements. A key ingredient of our analysis is a mapping for
equivalent protocols defined for photonic and spin systems, which allows us to
draw upon results in the literature for general finite-dimensional systems.
Motivated by the protocol in X.-Q. Zhou, et al., Optica 2, 510 (2015), we
present new results for quantum-enhanced tomography of unitary processes,
including a comparison of Holland-Burnett and NOON probe states.Comment: 19 pages, 6 figure
Analog quantum simulation of partial differential equations
Quantum simulators were originally proposed for simulating one partial
differential equation (PDE) in particular - Schrodinger's equation. Can quantum
simulators also efficiently simulate other PDEs? While most computational
methods for PDEs - both classical and quantum - are digital (PDEs must be
discretised first), PDEs have continuous degrees of freedom. This suggests that
an analog representation can be more natural. While digital quantum degrees of
freedom are usually described by qubits, the analog or continuous quantum
degrees of freedom can be captured by qumodes. Based on a method called
Schrodingerisation, we show how to directly map D-dimensional linear PDEs onto
a (D+1)-qumode quantum system where analog or continuous-variable Hamiltonian
simulation on D+1 qumodes can be used. This very simple methodology does not
require one to discretise PDEs first, and it is not only applicable to linear
PDEs but also to some nonlinear PDEs and systems of nonlinear ODEs. We show
some examples using this method, including the Liouville equation, heat
equation, Fokker-Planck equation, Black-Scholes equations, wave equation and
Maxwell's equations. We also devise new protocols for linear PDEs with random
coefficients, important in uncertainty quantification, where it is clear how
the analog or continuous-variable framework is most natural. This also raises
the possibility that some PDEs may be simulated directly on analog quantum
systems by using Hamiltonians natural for those quantum systems
Young Engels Is Still Engels: A Response to Terrell Carver
Terrel Carver assumes that existing biographies of Engels unfairly seek to explain his ideas in relation to Marx and seek to reconstruct the life journey, especially the ideological journey, of young Engels. He proposes two arguments: first, Engels\u27 ideological growth had no direct purpose or direction and was full of uncertainties; but also, second, that the young Engels\u27 theoretical achievements have been unduly underestimated as they were much more profound than Marx\u27s, such that Marx was the main beneficiary of their initial meeting. One problem with Carver’s argument is that his new insights are often not that new at all. His anti-teleological Engels is a liberal, but this is a more-or-less static form of liberalism that seems innocent of any immanent mechanism through which Engels’s trajectory toward revolutionary socialism might be understood. “Engels vs. Marx” is a trite trope. The proposition of “Engels vs. Marx” functions to deny the theory and practice of Marxist revolutionaries and politicians after Engels. This is something that the left academia should be vigilant about
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