The harnessing of the nonclassical properties in photon pairs generated by the process of Spontaneous Parametric Down Conversion (SPDC) to enhance two-photon interactions has long intrigued researchers. Their use in light-matter interactions has been proven to improve efficiency either by increasing the rate of generation or by suppressing noise. Namely, the processes of Entangled Sum Frequency Generation (eSFG) and Entangled Two Photon Absorption (eTPA) have been shown to produce more photons when stimulated with entangled photons than the classical versions of the effects. This has only been effective so far at very low intensities, which, alongside the inability to reproduce previous experimental results, has caused debates about the usefulness of the effect. In the presented work, I aim to shed light on the topic by employing a carefully crafted source of entangled photon pairs to improve two-photon effects. I attempt to demonstrate, through a series of experiments, that the quantum enhancement can be observed at an order of magnitude higher than previously thought possible. To solidify the results, a direct comparison of the quantum effect was conducted by investigating the performance of the phenomenon when it is stimulated by classical radiation from a laser. Our results confirm a quantum improvement in a regime previously considered impossible, and they enabled us to examine the ramifications of the effect on imaging. By doing so, we consider real-life microscopy applications, and therefore, we can set limits to the effect.
The harnessing of the nonclassical properties in photon pairs generated by the process of Spontaneous Parametric Down Conversion (SPDC) to enhance two-photon interactions has long intrigued researchers. Their use in light-matter interactions has been proven to improve efficiency either by increasing the rate of generation or by suppressing noise. Namely, the processes of Entangled Sum Frequency Generation (eSFG) and Entangled Two Photon Absorption (eTPA) have been shown to produce more photons when stimulated with entangled photons than the classical versions of the effects. This has only been effective so far at very low intensities, which, alongside the inability to reproduce previous experimental results, has caused debates about the usefulness of the effect. In the presented work, I aim to shed light on the topic by employing a carefully crafted source of entangled photon pairs to improve two-photon effects. I attempt to demonstrate, through a series of experiments, that the quantum enhancement can be observed at an order of magnitude higher than previously thought possible. To solidify the results, a direct comparison of the quantum effect was conducted by investigating the performance of the phenomenon when it is stimulated by classical radiation from a laser. Our results confirm a quantum improvement in a regime previously considered impossible, and they enabled us to examine the ramifications of the effect on imaging. By doing so, we consider real-life microscopy applications, and therefore, we can set limits to the effect.
Nonlinear optics has had extensive application into a vast array of scientific fields. One such nonlinear process, two-photon absorption (TPA), has had a wildly successful \nadoption into the field of biological imaging and microscopy. As far and as fast as this field is progressing, limitations stemming from the use of ultrafast lasers are starting to appear. \nIn this work, an alternative nonclassical light source will be motivated for the application \nof low photon-flux two-photon microscopy. The origin and properties of the chosen \nnonclassical source, spontaneous parametric downconversion (SPDC), will be discussed along with the spatial and spectral properties modelled. Nonlinear processes such as TPA and sum frequency generation (SFG) will be viewed as "ultrafast coincidence measurements" of two photons arriving at a molecule within the time window of excitation. These ultrafast coincidence measurements will be viewed in an alternative manner: in terms of the second-order coherence from a light source. This degree of second-order coherence can be subdivided into two categories arising from different combinations of correlations within and between entangled photon pairs. Of interest, the energy-time correlations within the photon pair allow for enhancements in ultrafast coincidence rates over coherent light sources. The makings of an experimental setup to demonstrate enhanced rates from ultrafast two-photon coincidences taking place in SFG in a nonlinear crystal will be discussed.
Long ago, it was predicted that stimulating two-photon processes such as two-photon absorption (TPA) and second harmonic generation (SHG) with entangled photons produced by parametric down-conversion (PDC) could result in higher efficiencies compared to classical illumination [1]. This was experimentally demonstrated for SHG [2] and TPA [3], where, however, the classical comparison was performed by adding losses to the PDC state rather than employing a true classical reference. In addition, these results considered spatially single-mode PDC radiation and were conducted at low photon fluxes, where the PDC modes are, on average, populated by less than one photon. This resulted in an extremely low SHG/TPA signal, considered of limited practical use for metrological applications (such as two-photon fluorescence microscopy or SHG imaging - see, e.g., [4]). A recent theoretical investigation [5] highlighted that a proper analysis of quantum-enhanced two-photon processes requires a fully spatiotemporal model of the nonlinear interactions. It further showed that highly multimode (in space and time) entangled PDC photons can provide a quantum advantage compared to an equivalent classical pulse, even in regimes well beyond the “one-photon-per-mode” limit, considered the boundary of quantum behavior.
The fields of quantum and nonlinear optics have given rise to a variety of nonclassical states of light that have been proven to surpass certain limitations set by classical physics. Namely, certain squeezed and entangled states have been shown to beat the shot-noise limit when making precision phase measurements in interferometry, as well as write lithographic patterns that are smaller than classically allowed by the Rayleigh diffraction limit. Additionally, single-photon sources and entangled photon pairs have given rise to provably secure quantum key distribution for cryptography. Producing these quantum states of light has proven a difficult task. Nonlinear crystals, when pumped by a laser, produce pairs of single photons via the process of spontaneous parametric down conversion (SPDC). This process is mediated by the second order nonlinear susceptibility of the material. When pumped in a high gain regime, these crystals give rise to optical parametric amplification, which is a viable source of squeezed light. The vast majority of research in this area has focused on crystals that are seeded by vacuum in their two modes. This dissertation concerns the field of quantum nonlinear optics. It is an investigation into the processes that occur when nonlinear materials interact with the electromagnetic field on the single photon level. I have focused on seeding nonlinear crystals with quantum states of light, including single photons and entangled states. This process results in various states directly applicable to interferometry, imaging, and cryptography. Another application investigated is an absolute radiance measurement via stimulated parametric down conversion resulting from non-vacuum seeding of a nonlinear crystal. Additionally, other nonlinear processes, including four-wave mixing, nonlinear magneto-optical effects and coherent population trapping in warm atomic vapor involving quantum states of light are investigated. The process of seeding third-order nonlinear interactions, such as in atomic vapors, gives rise to a variety of interesting, nonclassical phenomena such as entangled image transfer and nonlocal imaging. Strong analogies between SPDC and four-wave mixing are drawn. I also experimentally show an all optical pi-only phase shift of one light beam via another in warm Cesium vapor.
Two-photon processes are crucial in applications like microscopy and microfabrication, but their low cross section requires intense illumination and limits, e.g., the penetration depth in nonlinear microscopy. Entangled states have been proposed to enhance the efficiency of two-photon interactions and have shown effectiveness at low intensities. This quantum enhancement is generally believed to be lost at high intensities, for more than one photon per mode, raising doubts about its usefulness. We explored experimentally and theoretically two-photon processes driven by entangled photons at intensities beyond this threshold and compared the results with the classical case. We found that a quantum advantage can still be observed at nearly one order of magnitude higher intensities than previously assumed. Our findings show a potential path for exploiting quantum-enhanced two-photon processes in practical applications.
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.
We closely replicated and extended a recent experiment ("Spatial properties of entangled two-photon absorption," Phys. Rev. Lett. 129, 183601, 2022) that reportedly observed enhancement of two-photon absorption rates in molecular samples by using time-frequency-entangled photon pairs, and we found that in the low-flux regime, where such enhancement is theoretically predicted in-principle, the two-photon fluorescence signal is below detection threshold using current state-of-the-art methods. The results are important in the context of efforts to enable quantum-enhanced molecular spectroscopy and imaging at ultra-low optical flux. Using an optical parametric down-conversion photon-pair source that can be varied from the low-gain spontaneous regime to the high-gain squeezing regime, we observed two-photon-induced fluorescence in the high-gain regime but in the low-gain regime any fluorescence was below detection threshold. We supplemented the molecular fluorescence experiments with a study of nonlinear-optical sum-frequency generation, for which we are able to observe the low-to-high-gain crossover, thereby verifying our theoretical models and experimental techniques. The observed rates (or lack thereof) in both experiments are consistent with theoretical predictions and with our previous experiments, and indicate that time-frequency photon entanglement does not provide a practical means to enhance in-solution molecular two-photon fluorescence spectroscopy or imaging with current techniques.