In this thesis, we examine waveguide phase matching conditions necessary for the generation and detection of squeezed states of light. In particular, the temperature dependence of the nonlinear interactions that generate squeezed light is investigated. The theoretical expressions for the conversion efficiencies of the waveguides and the possible squeezing levels are derived. Both of these expressions are dependent on the difference in phase between the interacting modes of light. The phase difference can be assumed to vary linearly with both temperature and wavelength, hence to achieve and maintain optimal squeezing conditions the temperature of the waveguides needs to be controlled for different wavelengths. To do this, a PTC10 temperature controller from Stanford Research Systems was implemented into the lab setup. The optimal operating temperatures of the waveguides were obtained by characterizing the waveguides and fitting the theoretical expressions to the obtained data points. This was done for the integer wavelengths between 1548-1552 \SI{}{\nano \meter}. The highest measured squeezing level was \SI{0.83}{dB} at \SI{1550}{\nano \meter}. The optimal temperatures were then mapped to their respective wavelength and programmed into the temperature controller. The results revealed an approximately linear relationship between the fundamental wavelength and the optimal operating temperature. Furthermore, the measured behavior agrees well with the theoretical model, demonstrating reliable wavelength dependent tuning of squeezed light generation in a single-pass waveguide platform.
In this thesis, we examine waveguide phase matching conditions necessary for the generation and detection of squeezed states of light. In particular, the temperature dependence of the nonlinear interactions that generate squeezed light is investigated. The theoretical expressions for the conversion efficiencies of the waveguides and the possible squeezing levels are derived. Both of these expressions are dependent on the difference in phase between the interacting modes of light. The phase difference can be assumed to vary linearly with both temperature and wavelength, hence to achieve and maintain optimal squeezing conditions the temperature of the waveguides needs to be controlled for different wavelengths. To do this, a PTC10 temperature controller from Stanford Research Systems was implemented into the lab setup. The optimal operating temperatures of the waveguides were obtained by characterizing the waveguides and fitting the theoretical expressions to the obtained data points. This was done for the integer wavelengths between 1548-1552 \SI{}{\nano \meter}. The highest measured squeezing level was \SI{0.83}{dB} at \SI{1550}{\nano \meter}. The optimal temperatures were then mapped to their respective wavelength and programmed into the temperature controller. The results revealed an approximately linear relationship between the fundamental wavelength and the optimal operating temperature. Furthermore, the measured behavior agrees well with the theoretical model, demonstrating reliable wavelength dependent tuning of squeezed light generation in a single-pass waveguide platform.
Quadrature-amplitude and phase squeezing are theoretically investigated in a planar waveguide geometry where the use of a linear grating fabricated on top of the waveguide reproduces a photonic bandgap structure. The introduction of a nonlinear grating, obtained with a modulation of the nonlinear susceptibility χ(2), provides an additional degree of freedom that allows, together with the linear grating, tuning of the fundamental field in a selected resonance of the transmission spectrum and, at the same time, control of the phase-matching condition between the fundamental and second-harmonic fields. The results show that quadrature-amplitude squeezing is achieved for the fundamental field, increasing the second-harmonic input intensity. The second-harmonic field is tuned in the passband of the photonic bandgap. The low nonlinear conversion efficiency, given by a suitable selection of the mismatch, gives rise to the possibility of having a fundamental field of quite the same intensity, but less noisy than at the entry.
Continuous variable encoding of quantum information requires the deterministic generation of highly correlated quantum states of light in the form of quantum networks, which, in turn, necessitates the controlled generation of a large number of squeezed modes. In this paper, we present an experimental source of multimode squeezed states of light at telecommunication wavelengths. Generation at such wavelengths is especially important as it can enable quantum information processing, communication, and sensing beyond the laboratory scale. We use a single-pass spontaneous parametric down-conversion process in a nonlinear waveguide pumped with the second harmonic of a femtosecond laser. Our measurements reveal significant squeezing in more than 21 frequency modes, with a maximum squeezing value exceeding 2.5 dB. We demonstrate multiparty entanglement by measuring the state's covariance matrix. Finally, we show the source reconfigurability by preparing few-node cluster states and measure their nullifier squeezing level. These results pave the way for a scalable implementation of continuous variable quantum information protocols at telecommunication wavelengths, particularly for multiparty, entanglement-based quantum communications. Moreover, the source is compatible with additional pulse-by-pulse multiplexing, which can be utilized to construct the necessary three-dimensional entangled structures for quantum computing protocols. Published by the American Physical Society 2024
The generation of light with amplitude-noise levels below the standard quantum limit (SQL) by constant-current operation of light-emitting and semiconductor laser diodes has been well established.1 Nearly all of this work, however, has been conducted with the light source cooled to cryogenic temperatures. At room temperature, the highest observed squeezing levels were 0.6 dB for light-emitting diodes and 0.2 dB for laser diodes.2 We presently report up to 1.8 dB of squeezing from a commercially available index-guided quantum-well laser (SDL-5410-C) operated at room temperature. Although cross-gain saturation can lead to amplitude squeezing of the total number of photons emitted by a multimode semiconductor laser in the limit of fast intraband scattering and the absence of wavelength-dependent loss,3 we found that it was necessary to suppress the >100 near-threshold longitudinal side modes of the laser to obtain squeezing at room temperature. This was accomplished either by using optical feedback from a grating in an external cavity configuration or by injection-locking the laser to a single-frequency laser. Both of these techniques allow the output wavelength of the laser to be named and thus the squeezing level as a function of wavelength to be studied.
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.
Squeezed light is expected to enable new quantum metrology and quantum computation applications, but the very high degree of experimental control required in practice places severe demands on the physical devices used to generate it. As ultrafast squeezed light sources are reaching an unprecedented degree of brightness and modal control, there is a strong interest in developing methods for their experimental characterization and theoretical understanding. In this thesis, we demonstrate both a new application and a new characterization technique for pulsed squeezed light. In the first part of the thesis, we present a new metrological application of pulsed squeezed light: quantum-enhanced label-free nonlinear microscopy. Stimulated emission microscopy (SEM) is a state-of-the-art pump-probe microscopy technique whose sensitivity is ultimately limited by shot noise in the probe beam. We propose to overcome this fundamental limitation by using a sub-Poissonian probe generated via optical parametric amplification. We demonstrate an intensity-squeezed sub-picosecond pulsed probe, and a simple stimulated emission microscope. Using this squeezed probe, sub-Poissonian statistics are observed in the SEM signal. After this proof-of-principle experiment, we aim to provide an improved design for a source of intensity-squeezed light. First, we develop a theoretical framework to simulate the photon statistics of a general sub-Poissonian source, considering pulsed and focused pump and seed fields in a unified spatio-temporal description. Next, we use this framework to suggest possible improvements to our previous squeezed probe. In the second part of this thesis, we provide new theoretical and experimental tools to analyze the high gain regime of pulsed spontaneous parametric down-conversion (SPDC). We study a waveguided source capable of generating twin-beams with tens of photons in a single spatio-temporal mode. We develop a framework to efficiently simulate SPDC for an arbitrary nonlinear gain, including the effect of parasitic nonlinearities such as self-phase modulation (SPM) of the pump and cross-phase modulation (XPM) between pump and twin-beams. We experimentally characterize our source using a new technique based on stimulated difference-frequency generation (DFG). In a type-II down-conversion process, we measure the frequency generation produced not only in the orthogonal polarization to the seeding beam, but also in its same polarization, which we interpret as cascaded difference-frequency generation. We argue that the additional information obtained in this measurement allows for a self-referenced estimation of the squeezing gain. Using these measurements, we accurately fit our theoretical model, revealing that, in the high gain regime, parasitic nonlinearities strongly affect the spectral structure of the twin-beams. We propose a simple strategy to reduce the impact of these nonlinearities on the modal structure of SPDC.
Abstract This study investigates the squeezing properties of light across a two-octave system consisting of multichannel single-mode nonlinear waveguides with evanescent coupling. The focus is on the first, second, and fourth harmonics generation. We analyze different input combinations and coupling setups to gain insight into how they impact the level of squeezing. The results show that multiple initial inputs amplify the squeezing effect, while certain initialization setups help maintain squeezing at higher harmonic levels. When the waveguides are fully coupled in the first octave, there is a large increase in squeezing at the first and second harmonic. Reduced coupling leads to a drop in the level of squeezing. At the fourth harmonic frequency, instability is observed in all configurations, which hinders the accurate detection of squeezing. By optimizing the quantity and arrangement of interconnected waveguides, it is possible to improve the nonlinear frequency conversion and thus the level of squeezing. This research may enhance the advancement of quantum optical systems by emphasizing the significance of coupling interactions across two harmonic generations.