Ultrashort light pulses play an important role in the generation, manipulation and characterization of nonclassical light. Nonclassical light is often generated from a classical pump beam using a nonlinear optical process. One might expect that the high peak intensity of ultrashort pulses would provide the necessary driver for such processes, and this is indeed the case. Ultrashort pulses have been used to generate squeezed states, and quadrature entangled states via three and four wave mixing, photon number states, polarization and frequency or directional entangled photon pairs states via three wave mixing.
Ultrashort light pulses play an important role in the generation, manipulation and characterization of nonclassical light. Nonclassical light is often generated from a classical pump beam using a nonlinear optical process. One might expect that the high peak intensity of ultrashort pulses would provide the necessary driver for such processes, and this is indeed the case. Ultrashort pulses have been used to generate squeezed states, and quadrature entangled states via three and four wave mixing, photon number states, polarization and frequency or directional entangled photon pairs states via three wave mixing.
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.
Nonclassical states of light are central resources for quantum metrology and quantum information, yet most high performance squeezing platforms operate in narrowband or continuous-wave regimes where the quantum noise is effectively stationary. This work advances ultrafast quantum optics by demonstrating how squeezed states can be generated, measured, and engineered on femtosecond and attosecond time scales, so that quantum fluctuations become a controllable property of the optical waveform itself. In this work, broadband few-cycle coherent drivers are produced using the light field synthesizer (LFS) setup that coherently combines UV/VIS/NIR channels with independently adjustable delays and amplitudes. Ultrafast squeezed light is generated via degenerate four-wave mixing (DFWM) in fused silica using a three-beam interaction geometry, implemented as a quantum light field squeezer (QLFS) that enables quasi-collinear phase matching across a wide bandwidth. A metrology framework compatible with ultrashort pulses is developed: amplitude (intensity) uncertainty is extracted from shot-to-shot spectral statistics, while relative phase fluctuations are retrieved from spectral-interference fringes in channel-overlap regions. These observables are connected to a displaced squeezed state model by fitting variance expressions, enabling phase-space visualization through Wigner function reconstruction. Beyond static squeezing, the QLFS provides attosecond-resolved control of the generated quantum state by tuning inter-pulse delay and phase matching geometry, allowing the redistribution of uncertainty between conjugate quadratures and revealing time-dependent (sub-cycle) structure of the intensity uncertainty within the optical cycle. Finally, the work extends ultrafast quantum optics into solid-state systems by demonstrating squeezed light driven tunneling in a graphene-silicon-graphene phototransistor, where state-dependent modulation of current noise is observed alongside simultaneous optical uncertainty measurements. Together, these results establish a practical route to waveform-level quantum state engineering and motivate future ultrafast quantum metrology, strong field quantum optics control, and hybrid photonic electronic quantum technologies.
High-harmonic generation is a light up-conversion process occurring in a strong laser field, leading to coherent bursts of extreme ultrashort broadband radiation [Lewenstein , Phys. Rev. A , 2117 (1994)]. As a new perspective, we propose that ultrafast strong-field electronic or photonic processes such as high-harmonic generation can potentially generate nonclassical states of light well before the decoherence of the system occurs [Gorlach , Nat. Commun. , 4598 (2020); Stammer ., Phys. Rev. Lett. , 123603 (2022)]. This could address fundamental challenges in quantum technology such as scalability, decoherence, or the generation of massively entangled states [Lewenstein , Luca Argenti Michael Chini, 27 (2024)]. Here, we report experimental evidence of the nonclassical nature of the harmonic emission in several semiconductors excited by a femtosecond infrared laser. By investigating single- and double-beam intensity cross-correlation [Loudon, Rep. Prog. Phys. , 913 (1980)], we measure characteristic nonclassical features in the single-photon statistics. We observe two-mode squeezing in the generated harmonic radiation, which depends on the laser intensity that governs the transition from super-Poissonian to Poissonian photon statistics. The measured violation of the Cauchy-Schwarz inequality realizes a direct test of multipartite entanglement in high-harmonic generation [Wasak, Phys. Rev. A , 033616 (2014)]. This result is supported by the theory of multimodal detection and the Hamiltonian from which the effective squeezing modes of the harmonics can be derived [Gonoskov , Phys. Rev. B , 125110 (2024); Christ New J. Phys. , 033027 (2011)]. With this work, we show experimentally that high-harmonic generation is a new quantum bosonic platform that intrinsically produces nonclassical states of light with unique features such as multipartite broadband entanglement or multimode squeezing. The source operates at room temperature, using standard semiconductors and a standard commercial fiber laser, opening up new routes for the quantum industry, such as optical quantum computing, communication, and imaging. Published by the American Physical Society 2024
Ultrafast quantum light pulses with durations of 1ps and below show great promise as information carriers in quantum communication and computation. In future they may also be used to probe physical processes at ultrashort timescales with a resolution beyond the limits of Heisenberg uncertainty. This thesis focuses on the creation and manipulation of quantum light pulses with second order nonlinear optical processes in optical waveguides. In chapter 1. Introduction, we lead the reader towards this work's topic by giving a brief qualitative overview over the EPR paradox, quantum entanglement, three-wave-mixing and quantum light pulses. Chapter 2. Basic concepts familiarizes the reader with the physical concepts and mathematical tools underlying this thesis. In chapter 3. Spectral engineering, we discuss the requirements to produce separable photon pair states with group velocity matching and examine several nonlinear materials widely used for optical waveguide inscription for their suitability to group velocity matching in the telecom wavelength regime. In chapter 4. A PP-KTP waveguide as parametric downconversion source, we give the basic spontaneous parametric downconversion source setup, as well as some initial measurements to characterize the single photon detectors, to demonstrate the production of correlated photons, and to determine the phasematching properties of the PP-KTP source. In the following chapter, 5. Fiber spectrometer, we present the single-photon fiber spectrometer. We discuss the experimental setup and calibration of the device, and measure the joint spectrum of photon pairs from our PP-KTP source. Finally, we investigate the behavior of a joint spectrum measurement of a high mean photon number source with binary detectors. In 6. Two-mode squeezed vacuum source, we characterize spectral correlations of an ultrafast SPDC source with the second order correlation function g². We present the g² measurement results and show that we can control the spectral correlations to produce a two-mode squeezed vacuum state of light. We then determine the mean photon number and gain of the source. 7. Quantum pulse gate: While the previous chapters focus on the creation and characterization of ultrafast quantum pulses of light, we now propose a way to manipulate the mode structure of a given quantum light state. We discuss the concept of an active optical filter sensitive to spectral/temporal pulse shape: The quantum pulse gate. In the final chapter 8. Conclusion and outlook, we recapitulate the main results of this thesis and provide a few pointers towards possibilities for future research building on it.