Beyond photon pairs
Başlık çevirisi mevcut değil.
- Tez No: 401590
- Danışmanlar: PROF. ERIC ELIEL, DR. MICHIEL DE DOOD
- Tez Türü: Doktora
- Konular: Fizik ve Fizik Mühendisliği, Physics and Physics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2014
- Dil: İngilizce
- Üniversite: Universiteit Leiden
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 115
Özet
Özet yok.
Özet (Çeviri)
In classical optics, light is described as a wave phenomenon and the famous Young's double slit experiment is often quoted as landmark demonstration of the wave nature of light. A fully quantum-mechanical description of light should involve both the wave nature of light as well as the notion of quantized light particles. Both these effects are captured by the introduction of photons as quanta of light. Among others, the field of quantum optics investigates the number distributions of and correlations between photons. These two concepts are important in the context of stimulated emission of photon pairs studied in this thesis. In particular, we investigate the number distribution of, and the correlations between photons that are created by the nonlinear process of parametric down-conversion. At low intensities of the pump laser, this nonlinear process produces single pairs via a spontaneous process. Once the pump intensity is increased the probability to emit more than one pair at a time increases i.e. a stimulated parametric down-conversion process gains importance, affecting the number distribution of the photon pairs. In experiments on stimulated down-conversion, both the number of optical modes available to the pair generation process and the number of modes collected in the detection process are important. The number of available modes in the downconversion process is controlled via the size of the pump beam and can be minimized by a careful choice of the combination of crystal length and pump beam size. The number of modes in the detection process can be reduced by using a small aperture in the optics used to collect the pairs, at the expense of a strong reduction in count rate. The experiments in this thesis that aim to observe and explore stimulated pair emission are thus carefully designed to keep the number of modes involved in the generation and the detection process small.Spontaneous and stimulated parametric down-conversion Spontaneous parametric down-conversion (SPDC) occurs in a non-linear medium (crystal) where the interaction with light induces spontaneous splitting of a pump photon into, two highly correlated twin photons. These photons are created into optical modes that can be labeled by transverse momenta !q and !q as shown in Fig.1. The physical properties of these photons are correlated via energy and momentum conservation dictated by the phase-matching conditions of the nonlinear mixing process. Figure 1: Schematic drawing of the SPDC process, where a blue pump photon is split by the nonlinear crystal into two identical (frequency degenerate) signal and idler photons. !q and !q indicate the transverse momenta of the two photons emitted by the crystal. Note that the drawing is not to scale; the opening angle a is typically a few degrees only. If the nonlinear medium is pumped by a single-frequency continuouswave laser, photon pairs are created spontaneously and all pairs are mutually independent. This independence arises because the coherence time of the down-converted photons ( 1 ps) is much shorter than the coherence time of the laser (& 0.1 ms). To achieve correlations between pairs, i.e., to lift the independence, one should make these coherence times comparable. This is rather simply achieved by using a pulsed laser as a source (coherence time 1 ps). Moreover, since the peak power of the pulsed laser is typically many orders of magnitude larger than that of a cw laser, the probability of stimulated events is also increased by a large amount. Chapter 2 characterizes a pulsed parametric down-conversion (PDC) setup based on periodically-poled KTP crystals.Measuring the contribution from stimulated emission Observing stimulated emission of pairs requires a careful design of the experiment so that the number of relevant optical modes is limited. Two aspects play a role here: first, the number of modes that the pairs from the source are emitted into, and second the number of modes that are involved in the detection process. The first aspect is addressed by a proper choice of beam waist and crystal length to limit the number of spatial modes in the generation process to values between 1 and 10. To understand the role of the number of modes involved in the detection process it is important to note that we use only two single photon detectors to detect the stimulated events. In a non-collinear down-conversion geometry these detectors are used to detect two photons emitted in the same direction (the other two auxiliary photons will give rise to similar correlations and are not detected). Because the detectors cannot distinguish between a single and a double photon we insert a beam-splitter in the optical path. A coincidence event where both detectors click simultaneously then signifies an event with two pairs that can either be in the same mode (and thus belong to a stimulated pair), or they can be in different modes (and thus belong to two simultaneously emitted independent photons emitted through the spontaneous process). This latter process is of minor interest to us and its contribution should be kept small. This can be achieved by using an aperture in front of the detectors to favor a particular spatial mode and the relative importance of stimulated pairs while reducing the absolute count rate on the detector. Chapter 3 introduces mode selection by spatial and spectral filtering as well as an experimental technique to distinguish stimulated photon pairs from the independent spontaneous pairs. The detectors measure a contribution due to photons emitted into the same spatial mode. Subtracting the measured count rates into different modes yields the amount of four-photon correlations. Figure 2 illustrates the down-conversion process that creates four-photon states and the coincidence measurement done by using a beam splitter and two detectors. A time delay applied to one of the two detectors provides a means to compare coincidence count rates for photons emitted in the same laser pulse to the coincidence count rate for photons emitted independently (spontaneously) by two distinct laser pulses. This technique is used through-Figure 2: Scheme for coincidence measurements in a non-collinear stimulated downconversion process, where two signal-idler photon pairs are created in the same pulse. An adjustable time-delay enables to distinguish coincidences in the same pulse and coincidences between subsequent pulses. out this thesis to distinguish stimulated emission of four photons from the spontaneous emission of double photon pairs. Visibility of four-photon events The ratio of stimulated and spontaneous coincidence counts defines the visibility of four-photon events where all photons are in the same quantum state. This ratio is a direct measure of the number of optical modes relevant to the down-conversion process. Chapter 4 presents the characterization of this visibility parameter in experiments using different focussing conditions, crystal lengths and spatial-temporal filtering. A universal expression for the visibility of the four-photon state is introduced that quantifies the influence of the size of the pump beam used in the creation of the stimulated PDC light. Chapter 5 focuses on a collinear parametric down conversion, which is illustrated in Fig.3. In a collinear geometry, all four photons are emitted into the same optical mode, which changes the nature of the stimulated downconversion process. In this case, all photons become indistinguishable and a special quantum state is created. The measurements in this chapter comprise a first experimental study that reveals a remarkable peak of excess coincidences in the collinear direction. This peak is due to four-photon stimulated emission, but is not predicted by the standard theory of squeezed states. We exclude asymmetries in the beam splitter and/or detectors as an explanation Figure 3: Schematic drawing of the collinear stimulated down-conversion process. Indistinguishable four photons are created into the same optical mode. for this effect. The effect can be explained if we assume that the transmission of photons through an aperture is enhanced by the number of photons in the state.
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