29 research outputs found
Valence holes as Luttinger spinor based qubits in quantum dots
We present a theory of valence holes as Luttinger spinor based qubits in
p-doped self-assembled quantum dots within the 4-band formalism. The
two qubit levels are identified with the two chiralities of the doubly
degenerate ground state. We show that single qubit operations can be
implemented with static magnetic field applied along the and
directions, acting analogously to the and
operators in the qubit subspace respectively. The coupling of two dots and
hence the double qubit operations are shown to be sensitive to the orientation
of the two quantum dots. For vertical qubit arrays, there exists an optimal
qubit separation suitable for the voltage control of qubit-qubit interactions
Measurement Accuracy in Silicon Photonic Ring Resonator Thermometers: Identifying and Mitigating Intrinsic Impairments
Silicon photonic ring resonator thermometers have been shown to provide
temperature measurements with a 10 mK accuracy. In this work we identify and
quantify the intrinsic on-chip impairments that may limit further improvement
in temperature measurement accuracy. The impairments arise from optically
induced changes in the waveguide effective index, and from back-reflections and
scattering at defects and interfaces inside the ring cavity and along the path
between light source and detector. These impairments are characterized for 220
x 500 nm Si waveguide rings by experimental measurement in a calibrated
temperature bath and by phenomenological models of ring response. At different
optical power levels both positive and negative light induced resonance shifts
are observed. For a ring with L = 100 um cavity length, the self-heating
induced resonance red shift can alter the temperature reading by 200 mK at 1 mW
incident power, while a small blue shift is observed below 100 uW. The effect
of self-heating is shown to be effectively suppressed by choosing longer ring
cavities. Scattering and back-reflections often produce split and distorted
resonance line shapes. Although these distortions can vary with resonance
order, they are almost completely invariant with temperature for a given
resonance and do not lead to measurement errors in themselves. The effect of
line shape distortions can largely be mitigated by tracking only selected
resonance orders with negligible shape distortion, and by measuring the
resonance minimum wavelength directly, rather than attempting to fit the entire
resonance line shape. The results demonstrate the temperature error due to
these impairments can be limited to below the 3 mK level through appropriate
design choices and measurement procedures
Electrostatic Control of Single InAs Quantum Dots Using InP Nanotemplates
This thesis focuses on pioneering a scalable route to fabricate quantum information devices based upon single InAs/InP quantum dots emitting in the telecommunications wavelength band around 1550 nm. Using metallic gates in combination with nanotemplate, site-selective epitaxy techniques, arrays of single quantum dots are produced and electrostatically tuned with a high degree of control over the electrical and optical properties of each individual quantum dot. Using metallic gates to apply local electric fields, the number of electrons within each quantum dot can be tuned and the nature of the optical recombination process controlled. Four electrostatic gates mounted along the sides of a square-based, pyramidal nanotemplate in combination with a flat metallic gate on the back of the InP substrate allow the application of electric fields in any direction across a single quantum dot. Using lateral fields provided by the metallic gates on the sidewalls of the pyramid and a vertical electric field able to control the charge state of the quantum dot, the exchange splitting of the exciton, trion and biexciton are measured as a function of gate voltage. A quadrupole electric field configuration is predicted to symmetrize the product of electron and hole wavefunctions within the dot, producing two degenerate exciton states from the two possible optical decay pathways of the biexciton. Building upon these capabilities, the anisotropic exchange splitting between the exciton states within the biexciton cascade is shown to be reversibly tuned through zero for the first time. We show direct control over the electron and hole wavefunction symmetry, thus enabling the entanglement of emitted photon pairs in asymmetric quantum dots. Optical spectroscopy of single InAs/InP quantum dots atop pyramidal nanotemplates in magnetic fields up to 28T is used to examine the dispersion of the s, p and d shell states. The g-factor and diamagnetic shift of the exciton and charged exciton states from over thirty single quantum dots are calculated from the spectra. The g-factor shows a generally linear dependence on dot emission energy, in agreement with previous work on this subject. A positive linear correlation between diamagnetic coefficient and g-factor is observed
Fibre Fabry-P\'erot Astrophotonic Correlation Spectroscopy for Remote Gas Identification and Radial Velocity Measurements
We present a novel remote gas detection and identification technique based on
correlation spectroscopy with a piezoelectric tunable fibre-optic Fabry-P\'erot
filter. We show that the spectral correlation amplitude between the filter
transmission window and gas absorption features is related to the gas
absorption optical depth, and that different gases can be distinguished from
one another using their correlation signal phase. Using an observed
telluric-corrected, high-resolution near-infrared spectrum of Venus, we show
via simulation that the Doppler shift of gases lines can be extracted from the
phase of the lock-in signal using low-cost, compact, and lightweight
fibre-optic components with lock-in amplification to improve the
signal-to-noise ratio. This correlation spectroscopy technique has applications
in the detection and radial velocity determination of faint spectral features
in astronomy and remote sensing. We experimentally demonstrate remote CO2
detection system using a lock-in amplifier, fibre-optic Fabry-P\'erot filter,
and single channel photodiode
Two-photon photocurrent in InGaN/GaN nanowire intermediate band solar cells
Intermediate band solar cells have the ability to reach efficiencies similar to multijunction cells using a single semiconductor junction. Here, enhanced two-photon carrier generation is demonstrated on a silicon substrate in an InGaN/GaN quantum dot-in-nanowire heterostructure intermediate band solar cell
Deterministic emitter-cavity coupling using a single-site controlled quantum dot
Site-selective epitaxy is used to deterministically control the nucleation site of a single quantum dot. A photonic crystal cavity is fabricated at the dot site for a true single quantum dot-cavity system which, by design, contains no background emitters. Cavity tuning at fixed temperature is used to measure the dot-cavity coupling over a large (>15 meV) detuning range using nonresonant excitation. The low-excitation spectra are modeled using a master equation model based on incoherent excitation. We find that pure phonon dephasing alone does not account for the observed nonresonant cavity emission and an additional cavity feeding mechanism, consistent with phonon-assisted dot-cavity coupling, must be included to reproduce the experimental spectra.Peer reviewed: YesNRC publication: Ye