Resonance fluorescence from an artificial atom in squeezed vacuum

We present an experimental realization of resonance fluorescence in squeezed vacuum. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detect the resulting fluorescence with high resolution enabled by a broadband traveling-wave parametric amplifier. We investigate the fluorescence spectra in the weak and strong driving regimes, observing up to 3.1 dB of reduction of the fluorescence linewidth below the ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum on the relative phase of the driving and squeezed vacuum fields. Our results are in excellent agreement with predictions for spectra produced by a two-level atom in squeezed vacuum [Phys. Rev. Lett. \textbf{58}, 2539-2542 (1987)], demonstrating that resonance fluorescence offers a resource-efficient means to characterize squeezing in cryogenic environments

Femtosecond phase-resolved microscopy of plasmon dynamics in individual gold nanospheres

The selective optical detection of individual metallic nanoparticles (NPs) with high spatial and temporal resolution is a challenging endeavour, yet is key to the understanding of their optical response and their exploitation in applications from miniaturised optoelectronics and sensors to medical diagnostics and therapeutics. However, only few reports on ultrafast pump-probe spectroscopy on single small metallic NPs are available to date. Here, we demonstrate a novel phase-sensitive four-wave mixing (FWM) microscopy in heterodyne detection to resolve for the first time the ultrafast changes of real and imaginary part of the dielectric function of single small (<40nm) spherical gold NPs. The results are quantitatively described via the transient electron temperature and density in gold considering both intraband and interband transitions at the surface plasmon resonance. This novel microscopy technique enables background-free detection of the complex susceptibility change even in highly scattering environments and can be readily applied to any metal nanostructure

Reaching the quantum limit of sensitivity in electron spin resonance

We report pulsed electron-spin resonance (ESR) measurements on an ensemble of Bismuth donors in Silicon cooled at 10mK in a dilution refrigerator. Using a Josephson parametric microwave amplifier combined with high-quality factor superconducting micro-resonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly 4 orders of magnitude. We demonstrate the detection of 1700 bismuth donor spins in silicon within a single Hahn echo with unit signal-to-noise (SNR) ratio, reduced to just 150 spins by averaging a single Carr-Purcell-Meiboom-Gill sequence. This unprecedented sensitivity reaches the limit set by quantum fluctuations of the electromagnetic field instead of thermal or technical noise, which constitutes a novel regime for magnetic resonance.Comment: Main text : 10 pages, 4 figures. Supplementary text : 16 pages, 8 figure

Atom chip based generation of entanglement for quantum metrology

Atom chips provide a versatile `quantum laboratory on a microchip' for experiments with ultracold atomic gases. They have been used in experiments on diverse topics such as low-dimensional quantum gases, cavity quantum electrodynamics, atom-surface interactions, and chip-based atomic clocks and interferometers. A severe limitation of atom chips, however, is that techniques to control atomic interactions and to generate entanglement have not been experimentally available so far. Such techniques enable chip-based studies of entangled many-body systems and are a key prerequisite for atom chip applications in quantum simulations, quantum information processing, and quantum metrology. Here we report experiments where we generate multi-particle entanglement on an atom chip by controlling elastic collisional interactions with a state-dependent potential. We employ this technique to generate spin-squeezed states of a two-component Bose-Einstein condensate and show that they are useful for quantum metrology. The observed 3.7 dB reduction in spin noise combined with the spin coherence imply four-partite entanglement between the condensate atoms and could be used to improve an interferometric measurement by 2.5 dB over the standard quantum limit. Our data show good agreement with a dynamical multi-mode simulation and allow us to reconstruct the Wigner function of the spin-squeezed condensate. The techniques demonstrated here could be directly applied in chip-based atomic clocks which are currently being set up

Direct amplification of femtosecond pulses

Laser systems with pulse energies of a few millijoule are based on laser amplifiers to increase the energy of laser pulses produced by their primary sources, modelocked laser oscillators. Due to the high peak powers of the produced pulses and the resulting high intensities in the amplifier, state of the art laser systems implement chirped-pulse amplification. This reduces the peak intensity during the amplification process but requires stretching and compression units before and after the amplifier, which evokes significant disadvantages. The increased complexity of the system and the long freely propagating beams compromise the stability of the system. Furthermore, high average power laser amplifiers need to implement reflective gratings, which are particularly expensive, large in size and, despite the technological advances, a source for power loss in the compression unit. The resulting complexity of chirped-pulse amplifiers constitutes a main limitation for high sensitivity measurements and impedes data acquisition over long time scales. Building laser sources that satisfy the high demands on long-term stability and reproducibility of performance requires conceptual simplification of the architecture. Therefore, this thesis discusses the first steps in implementation of amplification concepts avoiding chirped-pulse amplification and paves the way towards more compact, cost-effective, and user-friendly multi-millijoule high-average power light sources. The developed Yb:YAG thin-disk regenerative amplifier achieves a pulse energy of 2 mJ at a repetition-rate of 100 kHz with 200W average power. The integration of non-linear optical processes leads to an exceptional pulse duration of 210 fs, which is 5-fold shorter than similar Yb:YAG systems that rely on chirped-pulse amplification. The concept of direct amplification of femtosecond pulses is scaled in pulse energy to 6.6 mJ at 5 kHz or 10 kHz with a novel hybrid-amplifier. Furthermore, the output pulses are shortened with a non-linear compression stage based on an argon filled multi-pass cell. The compression stage builds a waveguide that, however, is not implemented with an optical fiber. Wave-guiding is realized with an optical imaging system that creates comparable conditions. The involved non-linear optical processes were understood, which results in a pulse duration of 37 fs at 100 kHz repetition-rate, 1.9 mJ pulse energy, 35GW peak power and more than 190W average power. The developed technology builds the foundation for ultrafast metrology with high repetition-rate in the 100 kHz range. For the first time, Yb:YAG laser amplifiers embody high average power light sources applicable as direct drivers for ultrafast physics.Lasersysteme mit Millijoule Pulsenergie basieren auf Laserverstärkern, welche die Energie von bereit gestellten Primärquellen, modengekoppelten Oszillatoren, erhöhen. Aufgrund der hohen Spitzenleistungen der erzeugten Laserpulse und den daraus resultierenden hohen Intensitäten, implementieren moderne Lasersysteme die sog. „Chirped-Pulse Amplification“. Dies reduziert zwar die Spitzenleistung während des Verstärkungsprozesses, erfordert jedoch Einheiten für Streckung und Kompression vor und nach dem Verstärker. Das führt zu erheblichen Nachteilen des Systems: Zum einen verringern die erhöhte Komplexität und die langen Freistrahl-Wege die Stabilität des Systems. Zum anderen müssen in Laserverstärkern mit hohen mittleren Leistungen Reflexionsgitter eingesetzt werden, die besonders teuer, groß und, trotz der technologischen Fortschritte, eine Verlustquelle in der Kompressionseinheit darstellen. Die resultierende hohe Komplexität solcher Verstärker stellt eine wesentliche Einschränkung für hochempfindliche Messungen dar und verhindert die Aufnahme von Messdaten über lange Zeiträume. Die Konstruktion von Laserquellen, die den hohen Ansprüchen an Langzeitstabilität und Reproduzierbarkeit der Leistung genügen, verlangt eine konzeptionelle Vereinfachung der Laserarchitektur. Vor diesem Hintergrund behandelt diese Arbeit die ersten Schritte für die Implementierung von Verstärkerkonzepten, welche auf „Chirped-Pulse Amplification“ verzichten und ebnet den Weg für kompaktere, kosteneffiziente und benutzerfreundliche multi-Millijoule Lichtquellen mit hohen mittleren Leistungen. Der entwickelte Yb:YAG thin-disk regenerative Verstärker erreicht eine Pulsenergie von 2 mJ bei einer Wiederholrate von 100 kHz mit 200Wmittlerer Leistung. Die Integration nicht-linear optischer Effekte führt zu einer außergewöhnlichen Pulsdauer von 210 fs. Dies ist im Vergleich zu anderen Yb:YAG Systemen, die auf „Chirped-Pulse“ Verstärkung beruhen, um das 5-fache kürzer. Das Konzept der direkten Verstärkung von Femtosekundenpulsen wird in der Pulsenergie auf 6.6 mJ bei 5 kHz oder 10 kHz mittels eines neuartigen Hybridverstärkers skaliert. Zudem werden die Pulse mit einer nicht-linearen Pulskompressionsstufe, die auf einer Argon gefüllten Multipasszelle beruht, verkürzt. Die Pulskompressionsstufe formt einen Wellenleiter, der jedoch nicht mittels einer optischen Faser implementiert wird. Vielmehr wird der Wellenleiter durch ein optisches Abbildungssystem realisiert, welches vergleichbare Bedingungen schafft. Durch die technische Analyse der nicht-linearen Prozesse wird es ermöglicht Pulse mit einer Pulsdauer von 37 fs bei 100 kHz Repetitionsrate, 1.9 mJ Pulsenergie, 35GW Spitzenleistung und mehr als 190W Durchschnittsleistung zu erzeugen. Die entwickelte Technologie stellt den Grundstein für Ultrakurzzeitmesstechnik mit hohen Repetitionsraten im 100 kHz Bereich dar. Erstmals können sich Yb:YAG Laserverstärker als direkte Lichtquelle für Experimente in der Ultrakurzzeitphysik behaupten