Entanglement is a rare resource only a select few people worldwide can create. Its fundamentally non-classical behavior seeds a tree of ideas from which transformative technologies in information processing, communication, and imaging grow. Particularly interesting is entanglement between photons in a pair made by a quantum dot, the most competitive contender among quantum photonic devices. \n \nA strength of semiconductor quantum dots is their deterministic reliability and potential for production at scale. The associated price is anchored in their inherent flaw: sensitivity to imperfections in their shape puts the entanglement they generate in motion. Thus, unless a specific entangled photon pair is caught at just the right moment, it will appear as though it is in fact not entangled at all. \n \nIngenious methods for making quantum dots perfect again have been devised,almost exclusively built around reshaping the dot by physical, electric, or magnetic means. One completely different proposal stands out, which instead stops the entanglement's motion once it has already been created. As a purely optical technique it can be used for any quantum dot and is completely non-intrusive. This is the method of the rotating half-wave plate. \n \nThe entanglement between photons in a pair revolves up to a few billion times per second and halting it requires a correction one-half that rate. Physically spinning a crystal waveplate at that tremendous speed is impossible, but if the properties of a crystal are spun in an identical way such quickly varying entanglement can be restored to its starting point with ease. This is the principle of electro-optic modulation. \n \nThis thesis shows the very first demonstration of slowing the frequency of single photons in a stream by using an electro-optical rotating half-wave plate prototype. Initial results show a reduction in frequency by 127.75 million oscillations per second. Any quantum dot whose entanglement precesses no more quickly than 255.5 MHz can thus directly make use of the technique contained herein.
Entanglement is a rare resource only a select few people worldwide can create. Its fundamentally non-classical behavior seeds a tree of ideas from which transformative technologies in information processing, communication, and imaging grow. Particularly interesting is entanglement between photons in a pair made by a quantum dot, the most competitive contender among quantum photonic devices. \n \nA strength of semiconductor quantum dots is their deterministic reliability and potential for production at scale. The associated price is anchored in their inherent flaw: sensitivity to imperfections in their shape puts the entanglement they generate in motion. Thus, unless a specific entangled photon pair is caught at just the right moment, it will appear as though it is in fact not entangled at all. \n \nIngenious methods for making quantum dots perfect again have been devised,almost exclusively built around reshaping the dot by physical, electric, or magnetic means. One completely different proposal stands out, which instead stops the entanglement's motion once it has already been created. As a purely optical technique it can be used for any quantum dot and is completely non-intrusive. This is the method of the rotating half-wave plate. \n \nThe entanglement between photons in a pair revolves up to a few billion times per second and halting it requires a correction one-half that rate. Physically spinning a crystal waveplate at that tremendous speed is impossible, but if the properties of a crystal are spun in an identical way such quickly varying entanglement can be restored to its starting point with ease. This is the principle of electro-optic modulation. \n \nThis thesis shows the very first demonstration of slowing the frequency of single photons in a stream by using an electro-optical rotating half-wave plate prototype. Initial results show a reduction in frequency by 127.75 million oscillations per second. Any quantum dot whose entanglement precesses no more quickly than 255.5 MHz can thus directly make use of the technique contained herein.
Entangled photons are an important resource for quantum optics. Quantum dots are a source of on-demand and highly entangled photon pairs at a high repetition rate. However, fine structure splitting (FSS) in the biexciton-exciton cascade causes the photons to be emitted in a time-dependant state instead of an ideal Bell state. Current techniques to remove the FSS include applying a strain, electric, or magnetic field and require post-processing of the quantum dots which reduces device yield. We use a novel all-optical approach implemented by emulating a fast-rotating half-wave plate in a Lithium Niobate waveguide using a electro-optic modulator. This method allows us to frequency shift single photons and produce a time-independant entangled photon.
One 01 the main objectives of quantum optiCS is to prepare quantum light sources 'Nith the characteristics required lor various quantum optica ppllcallons. Pure single photons and entangled paired photons are two of the most 'Nidely used types of quantum tight. Single photons In a pure, well ellned quantum state may prove to be the building blocks lor future quantum technologies. These technologies promise enormous improvements ove heir ctasslcal counterparts In terms of speed or security. For eICample, in quantum inlormation processing, single photons may be used in quanturT ogic gales, playing the role of electrons in current electric circuits. Although light is all around us In all possible colors, polarizations and intensities, it I ICtremely difficult to get a single photon In a pure quantum state. On the other hand, 11 is relallvely easy to produce pairs of photons, and pure singl horons can be obtained from a two-photon source in a heralded way. Thus, generatlng paired photons with desired photonic properties becomes o'nE pf the cruciattasks in the research area 01 quantum optics and related applications.
rrhe process of spontaneous parametric down-conversion (SPDC) is one 01 the most convenient ways to produce pairs 01 photons. In SPDC, a strons ump laser beam Is shone on a nonlinear crystal. Occasionally, a pump photon disappears and two new photons with lower energy are create imultaneously. The production of photon paIrs In the nonlinear crystal is probabilistic and the fact that the two photons are produced simultaneously I n advantage. The two photons can be taken apart and one 01 them can be used as a trigger 10 indicate (herald) the presence of the olher, thereb elllng us when a photon is ready to be used. These pairs of photons have special types 01 correlation In their color (frequency), in theIr spatial shape ndIor In Iheir polarization. For many applications, such as quantum cryptography, these types of correlation may be useful, but at the moment 0 enerating pure Single photons they are detrimental since they diminish the purity of the single photon.
In this thesis, we are interested in the frequency properties 01 SPDC pairs of photons. and our main goal Is 10 demonstrate new methods 10 contre hese properties. The frequency properties ollhe photon pairs are described by its joint spectrum, which contains the information of the bandwidth, th wavalorm and the type of frequency correlation possessed by the two photons.
n the first part of our work, two new toolklts are added to the current toolbox to manipulate the joint spectrum of photon pairs. One tool is called pulse ronHill' technique. This technique is based on the appropriate Introduction of angular dispersion into the interacting waves. The feasibitity of thi method is demonstrated by observing a sevenfold increase of the band;dth of photon pairs. The corresponding experiment has been accomplished b dding angular dispersion into a coItinear SPDC geometry, where the interacting waves propagate at the same direction. Also, we discuss Ih mplicalion 01 this method lor the generation 01 narrow temporal Fourier transform limited btphotons, and paired photons 'Nith high degree 0 ntangtement. The second tool is called spatial-to·spectral' mapping technique. This technique takes eHec! in a noncoUinear geomelry, in which caSE he pump beam and the down-<:onverted paired photons propagate at different directions. With this situatton, the spatial features of the pump beam 'Nil ~ Imprinted inlo the lrequency distribution 01 the new created photon pairs. We eICperimenlalty prove this method by shaping the waveform, and tunin he types ollrequency correlation 01 the dO'M'l-converted two photons.
n addition, we propose a new scheme to fully control the Joint spectrum ollhe pholon pairs by combining the above two discussed techniques. In thi proposal, angular dispersion is introduced into a noncollinear SPDC geometry. This opens a new way to obtain pure single photons 'Nith tunabl band'Nidth.
Another topic discussed in this thesis is the exploration of more compact quantum light sources 'Nith versatile characteristics. We describe an SPDC !WO-photon source by using Brag9 reflection wavegulde (BAW) in AI .Gah. As material as down-converting material. Here, the nonlinear proces happens between a pump beam in 8 Bragg refleclion wavegulde mode, and down-converted photon pairs in total internal rellection (TtA) mode. Th ~aveguide dispersion properties can be largely modified by implementing different ridge sizes. and this allows the described monolithic source 10 oHe Ignificant control over the process band'Nidth. We numerically demonstrate the possibility 01 tuning the bandwidth 01 the photon pairs between 1 n~ nd 450 nm with the same waler structure. The results presented in this part have been obtained as a result 01 the collaboration ;th Prol. Amr S.
Helmy and his PhD student Payam Abolghasem from University of Toronto. In particular, the programs used lor the presented simulations have bee developed by the University of Toronto leam.
o summarize, In this thesis, we add new elements to the current ways 01 engineering quantum light sources. Two readily be used methods to enginee he spectrum of the photon pairs are presented. Particularly, new routes are provided to get pure heralded single photons and narrow tempora biphoton. In addil!on, a compact monolithic two-photon source ;th tunable bandwidth control Is Investigated, which could be very promIsing for furthe pOrlormance of various quantum applications towards reality.
Uoc I data Barcelona, 2009·11-16 Signature
Semiconductor Quantum dots are potentially an attractive source of non-classical light. They can confine bound electron-hole pairs, excitons, in all three spatial dimensions giving them discrete optical emission energies, much like the electronic states in atomic systems. The semiconductor nature of the system allows quantum dots to be integrated into complicated device structures including optical cavities [1] and electrical injection [2]. The radiative exciton emission from a semiconductor quantum dot can provide the simplest form of quantum light, a single photon source [2]. The finite re-excitation time following the radiative decay of an exciton in a quantum dot prevents the dot from emitting more than one photon if excited with a short optical or electronic pulse. In a similar fashion excitation of the biexciton state, consisting of two electrons and holes, in the quantum dot results in the emission of a pair of photons at the energy of the biexciton and exciton transitions. The biexciton state was, in fact, proposed as a possible source of polarisation-entangled photons in 2000 by Benson et al. [3] as it consists of two polarisation-correlated decay paths which could potentially be indistinguishable. The condition of indistinguishability is fundamental to the generation of entangled photons and is, in general, not true of the two decay paths from the biexciton state in a quantum dot; physical anisotropy causes the intermediate exciton states to hybridise and separate in energy providing ‘which-path’ information about the route taken through the biexciton decay. We demonstrate how the splitting between the intermediate exciton states in a single InAs quantum dot can be controlled by growth [4], rapid thermal annealing [5] and the application of an external magnetic field [6]. Each of these three techniques can be used to make the intermediate exciton states degenerate allowing the biexciton decay to produce singe pairs of entangled photons triggered by short laser pulses [7,8]. Entangled photon pair sources are essential for applications such as entanglement based quantum key distribution, and optical quantum computing. For these applications it is important that no more than one entangled photon pair is generated in a cycle, a requirement met by our quantum dot source which is not possible with traditional methods for generating entangled pairs of photons. Entanglement also enables quantum interferometry and metrology, the basis of image resolution beyond that possible with equivalent classical light. Biphoton interferometry using entangled emission from the biexciton decay reveals fringes with half the period of classical light [9], and less that that of the laser. In addition, interferometry suggests that this type of entangled light source is not limited by decoherence, we fine the biphoton is surprising robust against dephasing, in stark contrast to the devastating effect of decoherence upon single photons emitted by similar structures.
Generation and manipulation of the quantum state of a single photon is at the heart of many quantum information protocols. There has been growing interest in using phase modulators as quantum optics devices that preserve coherence. In this Rapid Communication, we have used an electro-optic phase modulator to shape the state vector of single photons emitted by a quantum dot to generate new frequency components (modes) and explicitly demonstrate that the phase modulation process agrees with the theoretical prediction at a single-photon level. Through two-photon interference measurements we show that for an output consisting of three modes (the original mode and two sidebands), the indistinguishability of the mode engineered photon, measured through the second-order intensity correlation [g2(0)] is preserved. This work demonstrates a robust means to generate a photonic qubit or more complex state (e.g., a qutrit) for quantum communication applications by encoding information in the sidebands without the loss of coherence.
The generation of entangled photon pairs is a fundamental cornerstone of quantum optics, and an essential technology in the high profile field of quantum communication and computing. Currently the most widely used techniques for generating entangled photon pairs are nonlinear optical processes, such as parametric down conversion, which produce a probabilistic number of pairs per excitation cycle. Such a source is of limited use in quantum information/processing applications where a regular stream of single entangled photon pairs is usually required. We have produced such a triggered source from a semiconductor device for the first time, using a two-photon cascade in a single quantum dots, demonstrating efficiencies up to 70%, close to the level required for useful applications. Single quantum dots could prove to be the first robust and compact triggered source of entangled photons.
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.