648 research outputs found
Probing physics students' conceptual knowledge structures through term association
Traditional tests are not effective tools for diagnosing the content and
structure of students' knowledge of physics. As a possible alternative, a set
of term-association tasks (the "ConMap" tasks) was developed to probe the
interconnections within students' store of conceptual knowledge. The tasks have
students respond spontaneously to a term or problem or topic area with a
sequence of associated terms; the response terms and timeof- entry data are
captured. The tasks were tried on introductory physics students, and
preliminary investigations show that the tasks are capable of eliciting
information about the stucture of their knowledge. Specifically, data gathered
through the tasks is similar to that produced by a hand-drawn concept map task,
has measures that correlate with inclass exam performance, and is sensitive to
learning produced by topic coverage in class. Although the results are
preliminary and only suggestive, the tasks warrant further study as
student-knowledge assessment instruments and sources of experimental data for
cognitive modeling efforts.Comment: 31 pages plus 2 tables and 8 figure
Optimal antibunching in passive photonic devices based on coupled nonlinear resonators
We propose the use of weakly nonlinear passive materials for prospective
applications in integrated quantum photonics. It is shown that strong
enhancement of native optical nonlinearities by electromagnetic field
confinement in photonic crystal resonators can lead to single-photon generation
only exploiting the quantum interference of two coupled modes and the effect of
photon blockade under resonant coherent driving. For realistic system
parameters in state of the art microcavities, the efficiency of such
single-photon source is theoretically characterized by means of the
second-order correlation function at zero time delay as the main figure of
merit, where major sources of loss and decoherence are taken into account
within a standard master equation treatment. These results could stimulate the
realization of integrated quantum photonic devices based on non-resonant
material media, fully integrable with current semiconductor technology and
matching the relevant telecom band operational wavelengths, as an alternative
to single-photon nonlinear devices based on cavity-QED with artificial atoms or
single atomic-like emitters.Comment: to appear in New J. Physic
An all-silicon single-photon source by unconventional photon blockade
The lack of suitable quantum emitters in silicon and silicon-based materials
has prevented the realization of room temperature, compact, stable, and
integrated sources of single photons in a scalable on-chip architecture, so
far. Current approaches rely on exploiting the enhanced optical nonlinearity of
silicon through light confinement or slow-light propagation, and are based on
parametric processes that typically require substantial input energy and
spatial footprint to reach a reasonable output yield. Here we propose an
alternative all-silicon device that employs a different paradigm, namely the
interplay between quantum interference and the third-order intrinsic
nonlinearity in a system of two coupled optical cavities. This unconventional
photon blockade allows to produce antibunched radiation at extremely low input
powers. We demonstrate a reliable protocol to operate this mechanism under
pulsed optical excitation, as required for device applications, thus
implementing a true single-photon source. We finally propose a state-of-art
implementation in a standard silicon-based photonic crystal integrated circuit
that outperforms existing parametric devices either in input power or footprint
area.Comment: 5 pages, 3 figures + Supplementary information (3 pages, 2 figures
Quantum theory of photonic crystal polaritons
We formulate a full quantum mechanical theory of the interaction between
electromagnetic modes in photonic crystal slabs and quantum well excitons
embedded in the photonic structure. We apply the formalism to a high index
dielectric layer with a periodic patterning suspended in air. The strong
coupling between electromagnetic modes lying above the cladding light line and
exciton center of mass eigenfunctions manifests itself with the typical
anticrossing behavior. The resulting band dispersion corresponds to the
quasi-particles coming from the mixing of electromagnetic and material
excitations, which we call photonic crystal polaritons. We compare the results
obtained by using the quantum theory to variable angle reflectance spectra
coming from a scattering matrix approach, and we find very good quantitative
agreement.Comment: Proceedings of the "8th Conference on Optics of Excitons in Confined
Systems" (OECS-8), 15-17 September 2003, Lecce (Italy
Electromechanical Quantum Simulators
Digital quantum simulators are among the most appealing applications of a
quantum computer. Here we propose a universal, scalable, and integrated quantum
computing platform based on tunable nonlinear electromechanical
nano-oscillators. It is shown that very high operational fidelities for single
and two qubits gates can be achieved in a minimal architecture, where qubits
are encoded in the anharmonic vibrational modes of mechanical nanoresonators,
whose effective coupling is mediated by virtual fluctuations of an intermediate
superconducting artificial atom. An effective scheme to induce large
single-phonon nonlinearities in nano-electromechanical devices is explicitly
discussed, thus opening the route to experimental investigation in this
direction. Finally, we explicitly show the very high fidelities that can be
reached for the digital quantum simulation of model Hamiltonians, by using
realistic experimental parameters in state-of-the art devices, and considering
the transverse field Ising model as a paradigmatic example.Comment: 14 pages, 8 figure
Long-distance radiative coupling between quantum dots in photonic crystal dimers
We study the mutual interaction between two identical quantum dots coupled to
the normal modes of two-site photonic crystal molecules in a planar waveguide
geometry, i.e. photonic crystal dimers. We find that the radiative coupling
between the two quantum emitters is maximized when they are in resonance with
either the bonding or the antibonding modes of the coupled cavity system.
Moreover, we find that such effective interdot coupling is sizable, in the meV
range, and almost independent from the cavities distance, as long as a normal
mode splitting exceeding the radiative linewidth can be established (strong
cavity-cavity coupling condition). In realistic and high quality factor
photonic crystal cavity devices, such distance can largely exceed the emission
wavelength, which is promising for long distance entanglement generation
between two qubits in an integrated nanophotonic platform. We show that these
results are robust against position disorder of the two quantum emitters within
their respective cavities.Comment: 10 pages, 6 figure
Steady-state entanglement between distant quantum dots in photonic crystal dimers
We show that two spatially separated semiconductor quantum dots under
resonant and continuous-wave excitation can be strongly entangled in the
steady-state, thanks to their radiative coupling by mutual interaction through
the normal modes of a photonic crystal dimer. We employ a quantum master
equation formalism to quantify the steady-state entanglement by calculating the
system {\it negativity}. Calculations are specified to consider realistic
semiconductor nanostructure parameters for the photonic crystal dimer-quantum
dots coupled system, determined by a guided mode expansion solution of Maxwell
equations. Negativity values of the order of 0.1 ( of the maximum value)
are shown for interdot distances that are larger than the resonant wavelength
of the system. It is shown that the amount of entanglement is almost
independent of the interdot distance, as long as the normal mode splitting of
the photonic dimer is larger than their linewidths, which becomes the only
requirement to achieve a local and individual qubit addressing. Considering
inhomogeneously broadened quantum dots, we find that the steady-state
entanglement is preserved as long as the detuning between the two quantum dot
resonances is small when compared to their decay rates. The steady-state
entanglement is shown to be robust against the effects of pure dephasing of the
quantum dot transitions. We finally study the entanglement dynamics for a
configuration in which one of the two quantum dots is initially excited and
find that the transient negativity can be enhanced by more than a factor of two
with respect to the steady-state value. These results are promising for
practical applications of entangled states at short time scales.Comment: 10 pages, 7 figure
Signatures of the super fluid-insulator phase transition in laser driven dissipative nonlinear cavity arrays
We analyze the non-equilibrium dynamics of a gas of interacting photons in an
array of coupled dissipative nonlinear cavities driven by a pulsed external
coherent field. Using a mean-field approach, we show that the system exhibits a
phase transition from a Mott-insulator-like to a superfluid regime. For a given
single-photon nonlinearity, the critical value of the photon tunneling rate at
which the phase transition occurs increases with the increasing photon loss
rate. We checked the robustness of the transition by showing its insensitivity
to the initial state prepared by the the pulsed excitation. We find that the
second-order coherence of cavity emission can be used to determine the phase
diagram of an optical many-body system without the need for thermalization.Comment: 4 pages, 4 figure
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