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
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
Efficient Generation of Entangled Photon-Pairs The first experiments with correlated photons have been performed in the context of EPR-Bell experiments on the realistic and local properties of quantum mechanics. The source used there produced pairs of polarization entangled photons from a 2-photon decay of Calcium atoms. The technical requirements of these experiments were high (vacuum systems, stronge dye-lasers, etc.) whereas the efficiency of the source was quite low. An important step forward was the introduction of spontaneous parametric down conversion (SPDC), which has become the most common source in quantum optics for generating correlated or entangled photon pairs. In this process photons of an intense pump laser convert to photon pairs in an optical nonlinear crystal. Conservation of energy and momentum leads to strong correlations between the generated photons. With this kind of two-photon source it was possible to realize or improve many experiments on the foundations of quantum mechanics addressing the EPR-Paradoxon and in the new field of quantum information. But again, more advanced experiments and applications suffer from the limited ef- ficiency of the fluorescence process. Many photon pairs are lost by spatial and spectral filtering, which is necessary to achieve polarization entanglement and long coherence times. Different techniques have been implemented to increase the number of photon pairs using two-crystal arrangements, focusing techniques or periodically poled crystals. Most of these methods have the disadvantage that no entangled photons have been observed. It is the aim of this work to increase the yield and to improve the mode definition of entangled photon pairs generated by resonant enhancement of the pump mode and the fluorescence modes. As a first step a linear cavity for the pump mode was realized. Since the conversion probability is proportional to the pump power it was possible to increase the photon pair count rate by factor of 7 over the previous source. Besides the possibility of further improvement on already established pair correlation experiments, such an enhancement allows to build a compact source for photon pairs, in which an expensive argon-ion laser is replaced by a cheap diode laser. Among other applications such sources are of strong interest for quantum cryptography. 3 In many quantum information experiments optical fibers are use to carry the photons over long distance. Therefore, light from the parametric down-conversion source has to be efficiently coupled into fibers. In the second part we report on a new method to optimize collection efficiency by matching the angular distribution of the parametric fluorescence to the spatial mode of an optical fiber. By using this technique, we detected 366500 polarization-entangled photon pairs per second in the near-infrared region in single-mode optical fibers for 465 mW pump power (at 351.1 nm) with a 2 mm BBOcrystal. The entanglement of the photon pairs was verified by measuring polarization correlations of more than 96% in a HV-basis and in a ±45◦-basis. To our knowledge, such enormous count rates of highly entangled photon pairs have not been reached yet with any other technique. In the third part of this thesis we investigated the process of parametric downconversion in a cavity which is resonant to certain longitudinal down-conversion modes only. The idea of placing the parametric down-conversion source inside a cavity is not new. Such a device is usually referred to as a single or double resonant optical parametric oscillator (OPO) and is mainly used to generate squeezed quantum states. In that kind of application the system is operating close to but still under the threshold of oscillation. In our application the situation is quite different. The system is operating far below threshold so that mainly spontaneous emission occurs. In that mode correlations between single photons can still be observed. But bouncing the light back and forth inside the cavity increases the interaction length and hence enhances the signal levels of the down-conversion fields. Further, by resonating two certain modes only, the bandwidth is reduced by orders of magnitude and the coherence time is found to be inverse proportional to the bandwidth. A similar experiment has already been realized with a type-I parametric down-converter in the resonator. We have tried to realize a compact double resonant OPO far below threshold with a type-II parametric down-converter in a high-finesse cavity to realize a bright source of entangled photon pairs with extremely narrow bandwidth.
The field of quantum optics offers the intriguing possibility of developing new, potentially ground-breaking technologies based on the laws of quantum physics. Single quanta of light, called photons, are at the heart of many such quantum optical technologies. They can serve, for example, as information carriers in quantum cryptography and in quantum-information processing. If such applications are to fulfill their enormous potential, it is essential to have sources available that provide photons in the required quantum states. An established way of generating states of single photons is based on photon-pair sources. In this approach, the detection of one photon of a pair heralds the presence of the partner photon, which can then be further processed. However, to generate heralded photons in pure quantum states, specific conditions have to be met, making the design and implementation of such sources challenging. This thesis presents two new photon-pair sources that allow the heralded generation of single photons with high purity. Both sources are based on optical fiber, which allows high collection efficiencies for the generated states and opens the possibility of easy integration into modern-day fiber networks. Chapter 1 provides an introduction to quantum optical technologies and gives a short overview of current approaches to generate pure single photons. Chapter 2 presents the basic theory on which the experimental work presented in Chapters 3 and 4 is based. It explains the concept of a spectrally broad single photon and how photon purity can be tested using Hong-Ou-Mandel (HOM) interference. Subsequently, the properties of photon-pair states and the heralded generation of single photons from such states are discussed. It is shown that for a heralded photon to be pure, the original photon pair has to be in a factorable state, that is, free of any correlations. The introduced theory is then applied to spontaneous four-wave mixing (SFWM) in optical fibers, a nonlinear process that allows the generation of photon pairs. It is shown how the factorability of the created pairs depends on the fiber's dispersion properties, and the necessary conditions for the generation of factorable states are derived. The chapter concludes with an analysis of the photon statistics of the four-wave mixing process, explaining how statistics and the g(2) correlation function can be used to evaluate the factorability of the SFWM output state. Chapters 3 and 4 contain the main results of this thesis. Chapter 3 presents the first spectrally decorrelated photon-pair source bridging the visible and telecommunication wavelength ranges. The photon pairs are generated by SFWM in a photonic crystal fiber (PCF). The chapter documents the procedure of how the PCF source was designed and fabricated, as well as its characterization based on spectral measurements and photon statistics. One photon of the pair state is generated in the visible at 513.5 nm, where it can be efficiently detected with silicon-based single-photon detectors. It serves as a herald for its broadband partner photon, which is emitted in the telecom wavelength range at 1547 nm. A spectral coincidence measurement shows that the heralded photon exhibits a purity of up to 0.82. At increased pump powers, even the heralded generation of higher photon-number states in the telecom band becomes feasible. In Chapter 4, we use commercial birefringent fiber to herald high-purity photons of picosecond duration at 848 nm. The presented two-source HOM interference experiment shows that the heralded unfiltered state has a purity of at least 0.73. This value is increased to 0.84 with the application of wavelength filters suppressing low-intensity side peaks in the spectrum. The heralded photons are coupled into a single-mode fiber with an efficiency of 85%, the highest reported value to date for factorable sources. It is also shown that a commercial polarization controller can be used to fine-tune the generated wavelengths. This facilitates matching photons from multiple sources, as required for optical quantum computing schemes. Chapter 5 summarizes all results and provides an outlook toward potential future projects.
Light, which is composed of discrete quanta, or photons, is one of the most fundamental concepts in physics. Being an elementary entity, the behaviour of photons is governed by the rules of quantum mechanics. The ability to create, manipulate and measure quantum states of light is not only useful in foundational tests of quantum theory, but also in a wide range of quantum technologies – which aim to utilize non-classical properties of quantum systems to perform tasks not possible with classical resources. Only recently has it been possible to control the properties of number states of light, which have a fixed photon-number. Two-photon states are central to testing fundamental physical theories (such as locality and reality) and the implementation of quantum information technologies. The versatility of photon-pair states is en- abled by the potential entanglement properties it can posses. Thus controlling the correlations between photons is crucial to both pure and applied physics. To produce a single photon, a photon-pair state can be used. Detection of one photon indicates its twin’s existence. Many applications, such as optical quantum computation, require pure indistinguishable single photons. Heralding single pho- tons from a photon-pair will, in general, produce single photons in a mixed quantum state due to correlations within the pair. A common approach to creating photon-pairs is through the nonlinear sponta- neous four-wave mixing interaction in optical fibres. This thesis presents a theoreti- cal and experimental implementation of a scheme to tailor the spectral correlations within the pairs. Emphasis is placed on engineering the two-photon state such that they are completely uncorrelated. Spatial entanglement is naturally avoided due to the discrete nature of the optical fibre modes. Spectral correlations are eliminated by careful choice of dispersion characteristics and conditions. The purity of the photons generated by this scheme is demonstrated by means of two-photon inter- ference from independent sources. We measure a purity of (85.9 ± 1.6)% with no spectral filtering, exhibiting the usefulness of this source for quantum technologies and applications.
Single photons - discrete wavepackets of light - are one of the most fundamental entities in physics. In recent years, the ability to consistently create and manipulate both single photons and pairs of photons has facilitated everything from tests of quantum theory to the implementation of quantum-enhanced precision measurements. These activities all fall within the scope of the rapidly-growing field of quantum information - the exploitation of the properties of quantum states (and specifically their capability to exist in superpositions) to accomplish tasks that would not be possible with classical objects. One stated goal of research in quantum information is to build a device consisting of a network of quantum logic gates that can evaluate quantum algorithms. The photonic implementation of individual logic gates has already been demonstrated. However, partly due to standard methods of preparing single photons, current schemes have severe limitations in terms of scaling up from a single logic gate to multiple concatenated operations. Until now it has not been proven that single photons can be generated in pure and indistinguishable quantum states, something upon which the successful operation of optical quantum logic gates relies. This thesis presents an experimental demonstration of simultaneous generation of almost identical single photons in highly pure states from two independent sources based on parametric downconversion. This is a process of photon pair generation during the passage of a light beam through a nonlinear crystal; one photon from the resulting pair is detected to herald the other. The work herein describes, refines, and implements a technique that minimises the strong quantum correlations usually present within each pair by spectral engineering of the source. This allows the heralded single photons to be in pure states, a property that is confirmed by observing a high-visibility two-photon interference effect without spectral filtering.
The interaction of single photons with single atoms is a fundamental process in quantum optics which manifests the quantum mechanical nature of light and matter. As an experimentally controlled process it enables the implementation of quantum technological applications, such as quantum information processing and quantum metrology. In the last decades, substantial progress has been attained to experimentally control and manipulate single atoms.
For example, single or multiple ions can be stored in an ion trap and their quantum states can be controlled with great precision. At the same time, experimental techniques to generate and control single photons and entangled photon pairs have been widely developed. However, only modest progress has been attained in controlling the interaction of single atoms with sin~le photons. The aim of this thesis project was to experimentally study he absorption of single photons and entangled photons pairs by single trapped DCa· ions. To this end, a source of polarization-entangled photon pairs based on the process of spontaneous parametric down-conversion (SPDC) was designed and constructed . Generating photon pairs whose interaction with an atom can be controlled imposed stringent requirements on the bandwidth, tunability, stability and brightness of the source, making its setup much more complex than usual SPDC sources. In a parallel project, a set of two separate ion traps were built, capable of storing a single or a set of a few 4DCa· ions.
he first part of the thesis documents the design, construction , operation and characterization of the entangled photon pair source. The characterization proves all the necessary requirements for the photons to interact with either of two transitions, D312 P312 and D5i2 P312 at 850 and 854 nm respectively, in 4DCa· One of the crucial requirements was to make the source bandwidth match the bandwidth of these transitions (22MHz). This was not straightforward since the typical emission bandwidths of the SPDC process is several orders of magnitude higher (1 05MHz). The ion trap setup is also briefly described. One of its crucial features is the inclusion of two high numerical-aperture objectives very close to the ion trap, allowing to enhance the interaction efficiency of the SPDC photons with the ion, and to efficiently collect the fluorescence light they emit. In a second part of the thesis, a concise description of the schemes and techniques used to detect the absorption of single photons by a single trapped ion is given, including a heoretical treatment of the methods used to analyze the data.
he core part of the thesis describes the methods used to couple the entangled photon source to one of the ion traps, and explains the results obtained with the combined setup. First it shows how the source is configured to produce heralded single photons. It then presents experiments in which the absorption by a single ion of photons produced by the source is detected, and how these absorption events are time-correlated with the detection of the heralding photon. These experiments are performed under several conditions, with a growing level of control over the interaction process. Finally, two proof-of-principle experiments are presented. In the first one, the ion is pumped to polarization-sensitive Zeeman substates and the polarization dependence of the correlated absorption rate is observed. This is a first step toward more advanced experiments in which the polarization entanglemen of the photon pairs is transferred to the entanglement of one photon with one atom or to two distant atoms. A second experiment performs heralded single photon spectroscopy on a single ion, showing the expected frequency dependence of the absorption process.
Finally, methods to extend these experiments to implement entanglement transfer from photons to atoms are outlined. In particular, it is shown how the efficiency of the source can be improved to make these experiments feasible.
The creation and manipulation of biphotons is important for many applications in quantum optics and quantum information. Topics that benefit from efficient biphoton sources range from the most fundamental quantum science experiments to the highly applied fields of quantum communication and quantum computation. Biphoton sources have long been hailed as one of the leading methods for creating entangled photon pairs for tests of Bell's inequality, creating heralded photon pairs that are used in on-demand single-photon sources and heralded measurement techniques, and for quantum communication protocols to name a few. Specifically for quantum communication, biphoton sources are commonly used for cutting edge quantum key distribution (QKD) protocols. In the first part of the thesis, I focus on realizing an efficient biphoton source that produces high yield photon pairs. More specifically, I develop an optimized biphoton source using the nonlinear optical process of spontaneous parametric down-conversion in a second-order nonlinear crystal. I develop a formalism for predicting the two important metrics of a biphoton source: the heralding efficiency and joint count rate. I show how, from a large parameter space, one can tailor the phase matching of the nonlinear interaction to create a high quality biphoton source that produces both high heralding efficiency and high joint count rate. I achieve heralding efficiencies of 86$\\pm5\\%$ and joint count rates of 2.58$\\pm0.6kHzpermWpumppower.Ishowthatusingacollinearnondegenerategeometryallowsforheraldingefficienciesofupto99.7\\%$ assuming no loss in the system. I verify the theoretical model with experimental results and find good agreement. In the second part of the thesis I turn to manipulating the single photons born from the biphoton source for applications in creating single-photon spectrometers and time-frequency QKD systems. The security of QKD is only guaranteed if the two parties have access to a set of states called mutually unbiased states. I create a set of these states in time and their conjugate states in the frequency basis and show that I can manipulate single photon correlations in time and frequency so that an eavesdropper can be detected if she localizes a photon to a 1 ns time interval. Additionally, in these experiments, I stretch a single photon wavepacket of 5-ps-width to a wavepacket of 8.3-ns-width and subsequently recompress it to at least the resolution of the detectors (sim 300 ps). This demonstrates a stretch factor of >1600 for a single-photon pulse using a group velocity dispersive material. To my knowledge, this is the largest reported stretch factor for a single-photon wavepacket produced by a biphoton source. The ability to stretch and recompress a single photon by this amount has applications in creating high-resolution, high-efficiency, single-photon spectrometers as well as advancing time-frequency QKD systems and other temporal pulse shaping applications.