Fast control of a strong optical beam by a few photons is an outstanding challenge that limits the performance of quantum sensors and optical processing devices. We report that a fast and efficient optical gate can be realized in an optical fiber that has been engineered with molecular-scale accuracy. Highly efficient, distributed phase-matched photon-photon interaction was achieved in the fiber with locally controlled, nanometer-scale core variations. A three-photon input was used to manipulate a Watt-scale beam at a speed exceeding 500 gigahertz. In addition to very fast beam control, the results provide a path to developing a new class of sensitive receivers capable of operating at very high rates.
Fast control of a strong optical beam by a few photons is an outstanding challenge that limits the performance of quantum sensors and optical processing devices. We report that a fast and efficient optical gate can be realized in an optical fiber that has been engineered with molecular-scale accuracy. Highly efficient, distributed phase-matched photon-photon interaction was achieved in the fiber with locally controlled, nanometer-scale core variations. A three-photon input was used to manipulate a Watt-scale beam at a speed exceeding 500 gigahertz. In addition to very fast beam control, the results provide a path to developing a new class of sensitive receivers capable of operating at very high rates.
In this Internet era, rapid transitioning of many aspects of medical, defense and commercial applications to digital platforms dictates their heavy reliance on optical communication systems. This trend places severe challenges on contemporary communication systems, expected to provide wide-bandwidth services and high degree of information security, with minimum energy dissipation. To meet the rising bandwidth demand, current bandwidth limited technology requires implementation of parallel processing; however, energy dissipation of such a system is globally and environmentally significant. A solution to this problem is envisioned in a form of an optical processor, a device recognized for its high speed, low loss and signal quality preservation. In this dissertation we investigate two optical processor roles: direct signal processing role (all-optical switching) and preprocessing role in preamplified few photon detection. \tIn the direct signal processing role, an optical switch controls the state of one wave by a low-energy control signal. It is realized in a highly nonlinear fiber, a medium acclaimed for its inherent femtosecond response time, a low loss and a high figure of merit. The switching functionality is implemented by employing the four wave mixing process in the depleted pump regime. To maximize the energy exchange, we revisit the saturated parametric mixer phase matching condition and propose a novel dispersion engineering method enhancing the performance. This study advances the understanding of the phase-matched interaction in the saturated parametric mixer and enables the first realization of a few-photon controlled silica-fiber-based device. \tIn the preprocessing role, the all-fiber photonic sampler is implemented prior to detection of a preamplified few photon signal with a low duty cycle; the sampler function is limited to amplified spontaneous emission noise alleviation. The proposed few photon detection method shows an order of magnitude improvement in the Bit Error Ratio and detection efficiency with respect to conventional preamplified receiver, and operates at speed beyond the reach of current single photon detector technology.
The ability to control light with light at ultralow powers has been a major avenue of research in photonics with applications to optical communications, computation, and signal processing. Such light-by-light scattering is achieved in a medium with a strong light-matter interaction, and for the development of quantum information networks it is important to demonstrate such effects near the single-photon level. Alkali-metal vapors such as rubidium (Rb) enable strong light-matter interactions due to the large cross section per atom and well-defined energy level structure, while the use of optical fibers offers the advantage of possible integration with modern optical communication systems. Hollow-core photonic band-gap fibers (PBGFs) can combine both these technologies such that both the atoms and the optical fields are transversely confined to a region that is a few wavelengths in size, which offers the prospect of exploring few-photon nonlinear interactions. We generate large optical depths in such a Rb-PBGF system, and the tight light confinement, high vapor density and long interaction length allow us to perform nonlinear optics at ultralow power. We demonstrate large signal amplification (>100) and frequency conversion using a four-wave mixing process with only microwatts of pump power. This is, to our knowledge, the largest gain observed at such low power. We perturb the coherence of this four-wave mixing to demonstrate all-optical modulation at unprecedented bandwidths (~300 MHz) for an atomic-vapor system, with an energy density of only tens of pho- tons per atomic cross-section, comparable to that achieved in more elaborate setups based on cold-atomic clouds. We then demonstrate an enhancement of several orders of magnitude in degenerate two-photon absorption in our RbPBGF system over that achieved in bulk vapor cells in a focused beam geometry. This allows us to directly measure two-photon absorption from a beam by detecting its intensity on a photodiode. Further, employing a near-resonant, non-degenerate two-photon transition in Rb, we demonstrate all-optical intensity modulation with just a few photons (<20), or only a few attojoules of energy, at relatively large bandwidths (~50 MHz) for such a sensitive scheme. This result takes us to within an order of magnitude of single-photon switching, and improves upon previous experiments for freely propagating optical fields, including those in cold-atoms. Finally, we produce relatively large cross-phase shifts of a few milliradians on a meter beam with <20 signal photons by tuning slightly away from resonance on the same non-degenerate two-photon transition. This corresponds to a phase shift of 0.3 milliradian per photon, with a fast response time of <5 ns. This represents, to our knowledge, the largest such nonlinear phase shift induced in a single-pass through a room temperature medium. Our Rb-PBGF system can thus potentially be employed to realize weak-nonlinearity based quantum computation and quantum non-demolition measurement of photon number. Through these experiments, we show the potential of a Rb-PBGF system for exploring quantum nonlinear optics at ultralow powers. Moreover, our system is simpler and easier to control and manipulate than setups based on cold atomic clouds and/or high-finesse cavities, and holds promise for integration with fiber-optic communication networks.
The ultrafast detection of single photons is currently restricted by the limited time resolution (a few picoseconds) of the available single-photon detectors. Optical gates offer a faster time resolution, but so far they have been applied mostly to ensembles of emitters. Here, we demonstrate through a semi-analytical model that the ultrafast time-resolved detection of single quantum emitters can be possible using an optical Kerr shutter at gigahertz rates under focused illumination. This technique provides sub-picosecond time resolution, while keeping a gate efficiency at around 85%. These findings lay the ground for future experimental investigations on the ultrafast dynamics of single quantum emitters, with implications for quantum nanophotonics and molecular physics.
A crucial component of photonic quantum information processing platforms is the ability to modulate, route, convert, and switch quantum states of light noiselessly with low insertion loss. For instance, a high-speed, low-loss optical switch is crucial for scaling quantum photonic systems that rely on measurement-based feed-forward approaches. Such a device will also ideally be capable of operating on photon-number states, which can act as non-Gaussian resources. Here, we demonstrate ultrafast all-optical switching of heralded photon-number states, of up to 6 photons, using the optical Kerr effect in a single-mode fiber. A local birefringence is created by a high-intensity pump pulse at a center wavelength of 1030 nm which overlaps temporally with the 1550 nm photons in the fiber. A switching efficiency of $>$99 % is reached with a resolution of 2.3 ps, an insertion loss of 2.27±0.08 dB, and a signal-to-noise ratio of 32,000.
Rapid and programmable shaping of light fields is central to modern microscopy, display technologies, optical communications and sensing, quantum engineering, and quantum information processing. Current wavefront shaping technologies face a fundamental dichotomy: liquid-crystal-on-silicon spatial light modulators (LCoS-SLMs) offer high pixel count but suffer from low refresh rates, while acousto-optic deflectors (AODs) provide moderate speed with restricted optical beam geometries. Though recent advances in photonic integrated circuits achieve fast switching, there is currently no tool that provides MHz-rate, continuous motion, and arbitrarily reconfigurable control over a set of diffraction-limited spots. Here we introduce a new class of spatial light modulator that provides both 2D pixel geometry and high speed. The device operates by encoding spatial information in frequency bins via a broadband optical phase modulator, and decoding them via a first-of-its-kind, high-resolution 2D spectrometer. The spectrometer, based on the architecture which we call the Re-Imaging Phased Array (RIPA), achieves its sensitivity through long path-lengths, enabled by intra-spectrometer re-imaging lens-guides. We demonstrate site-resolved optical pulsing with a 44(1)~ns rise time, corresponding to frame rates exceeding 10 million frames per second, as well as arbitrary, reconfigurable 2D addressing and multi-site operations, including asynchronous, independent beam motion, splitting, and recombination. Leveraging these tools opens new horizons in rapid optical manipulation of matter across science, from fast, scalable control that approaches the inertial and radiation limits of atoms in quantum processors, to dynamically programmable, microsecond-resolved illumination in microscopy and neuro-biological imaging.
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.