213 research outputs found
The effect of frequency-mismatched spontaneous emission on atom-field entanglement
A Schrödinger representation approach is used to calculate the atom-field dynamics following spontaneous emission by an atom in its excited state to a superposition of its two ground-state sublevels, in the case where the frequency separation of the ground-state sublevels is large compared to the excited-state decay rate. The emitted radiation is incident on a broadband photodetector. Using a relatively simple model for the photodetector, we show how a measurement of a photo-signal leaves the atom in a coherent superposition of the two ground states. The relative phase between the two ground-state amplitudes can be interpreted in terms of the temporal phase acquired in the time interval between spontaneous emission (viewed as a quantum jump process) and detection. Alternatively, the phase can be associated with a spatial phase of the entangled atom-field system; the source atom is projected into a state containing this spatial phase when the emitted photon is detected.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98599/1/0953-4075_45_12_124020.pd
Non-Hermitian dispersion sign reversal of radiative resonances in two dimensions
In a recent publication [Wurdack et al., Nat. Comm. 14:1026 (2023)], it was
shown that in microcavities containing atomically thin semiconductors
non-Hermitian quantum mechanics can lead to negative exciton polariton masses.
We show that mass-sign reversal can occur generally in radiative resonances in
two dimensions (without cavity) and derive conditions for it (critical
dephasing threshold etc.). In monolayer transition-metal dichalcogenides, this
phenomenon is not invalidated by the strong electron-hole exchange interaction,
which is known to make the exciton massless
Spin-Photon Entanglement and Quantum Optics with Single Quantum Dots.
InAs quantum dots (QDs) can be used as optically coupled quantum storage devices for quantum information applications. The QD can be charged with a single electron, where the spin state (up or down) provides a long lived quantum bit (qubit). The QD's optically excited states are used to initialize, manipulate, and read out the electron spin state with laser pulses. However, most practical quantum information applications require many interacting qubits, forming a quantum network. Since QDs are based on semiconductor technology, and are compatible with standard nano-fabrication processing, there is promise that they can provide a solid state platform where a scalable quantum information architecture is realizable. We focus on scaling the QD system to multiple qubits using intermediate spin-photon entangled states.
In this work, experimental and theoretical techniques are developed to study the QD-light matter interaction at the single photon level. Resonance fluorescence from a single QD is experimentally realized, and the single photon nature of the scattered radiation is verified through intensity correlation experiments. Transient fluorescence measurements on resonantly excited QDs are performed using time correlated single photon counting techniques to measure the excited state lifetime. High speed electro-optic modulators are used to time gate narrow bandwidth lasers, so that a QD can be driven under step-wise excitation, allowing for the direct observation of time-dependent optical Rabi oscillations. From these measurements, we are able to extract a decoherence rate which is consistent with the lifetime limit, indicating that pure dephasing is negligible in this system.
These techniques are applied to the QD spin system to demonstrate a spin-photon entangled state, by performing correlation measurements on the spin and photon state in two bases. A lower bound on the entanglement fidelity of 0.59(4) is achieved, which exceeds the classical limit of 0.5 by more than two standard deviations. The entanglement fidelity is limited primarily by the finite timing resolution of available single photon detectors. Taking this into account, we achieve 84% of the apparatus limited fidelity. These spin-entangled photons can be used to mediate entanglement between distant QD spins, providing the basis of an optically coupled QD spin network.PhDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/99785/1/jschaibl_1.pd
Population pulsation resonances of excitons in monolayer MoSe2 with sub 1 {\mu}eV linewidth
Monolayer transition metal dichalcogenides, a new class of atomically thin
semiconductors, possess optically coupled 2D valley excitons. The nature of
exciton relaxation in these systems is currently poorly understood. Here, we
investigate exciton relaxation in monolayer MoSe2 using polarization-resolved
coherent nonlinear optical spectroscopy with high spectral resolution. We
report strikingly narrow population pulsation resonances with two different
characteristic linewidths of 1 {\mu}eV and <0.2 {\mu}eV at low-temperature.
These linewidths are more than three orders of magnitude narrower than the
photoluminescence and absorption linewidth, and indicate that a component of
the exciton relaxation dynamics occurs on timescales longer than 1 ns. The
ultra-narrow resonance (<0.2 {\mu}eV) emerges with increasing excitation
intensity, and implies the existence of a long-lived state whose lifetime
exceeds 6 ns.Comment: (PRL, in press
Single exciton trapping in an electrostatically defined 2D semiconductor quantum dot
Interlayer excitons (IXs) in 2D semiconductors have long lifetimes and
spin-valley coupled physics, with a long-standing goal of single exciton
trapping for valleytronic applications. In this work, we use a nano-patterned
graphene gate to create an electrostatic IX trap. We measure a unique
power-dependent blue-shift of IX energy, where narrow linewidth emission
exhibits discrete energy jumps. We attribute these jumps to quantized increases
of the number occupancy of IXs within the trap and compare to a theoretical
model to assign the lowest energy emission line to single IX recombination
Phonon-assisted oscillatory exciton dynamics in monolayer MoSe2
In monolayer semiconductor transition metal dichalcogenides, the
exciton-phonon interaction is expected to strongly affect the photocarrier
dynamics. Here, we report on an unusual oscillatory enhancement of the neutral
exciton photoluminescence with the excitation laser frequency in monolayer
MoSe2. The frequency of oscillation matches that of the M-point longitudinal
acoustic phonon, LA(M). Oscillatory behavior is also observed in the
steady-state emission linewidth and in timeresolved photoluminescence
excitation data, which reveals variation with excitation energy in the exciton
lifetime. These results clearly expose the key role played by phonons in the
exciton formation and relaxation dynamics of two-dimensional van der Waals
semiconductors.Comment: Published in npj 2D Materials and Applications.
https://www.nature.com/articles/s41699-017-0035-
Directional Interlayer Spin-Valley Transfer in Two-Dimensional Heterostructures
Van der Waals heterostructures formed by two different monolayer
semiconductors have emerged as a promising platform for new optoelectronic and
spin/valleytronic applications. In addition to its atomically thin nature, a
two-dimensional semiconductor heterostructure is distinct from its
three-dimensional counterparts due to the unique coupled spin-valley physics of
its constituent monolayers. Here, we report the direct observation that an
optically generated spin-valley polarization in one monolayer can be
transferred between layers of a two-dimensional MoSe2-WSe2 heterostructure.
Using nondegenerate optical circular dichroism spectroscopy, we show that
charge transfer between two monolayers conserves spin-valley polarization and
is only weakly dependent on the twist angle between layers. Our work points to
a new spin-valley pumping scheme in nanoscale devices, provides a fundamental
understanding of spin-valley transfer across the two-dimensional interface, and
shows the potential use of two-dimensional semiconductors as a spin-valley
generator in 2D spin/valleytronic devices for storing and processing
information
- …