Experimental quantum speed-up in reinforcement learning agents

Increasing demand for algorithms that can learn quickly and efficiently has led to a surge of development within the field of artificial intelligence (AI). An important paradigm within AI is reinforcement learning (RL), where agents interact with environments by exchanging signals via a communication channel. Agents can learn by updating their behaviour based on obtained feedback. The crucial question for practical applications is how fast agents can learn to respond correctly. An essential figure of merit is therefore the learning time. While various works have made use of quantum mechanics to speed up the agent's decision-making process, a reduction in learning time has not been demonstrated yet. Here we present a RL experiment where the learning of an agent is boosted by utilizing a quantum communication channel with the environment. We further show that the combination with classical communication enables the evaluation of such an improvement, and additionally allows for optimal control of the learning progress. This novel scenario is therefore demonstrated by considering hybrid agents, that alternate between rounds of quantum and classical communication. We implement this learning protocol on a compact and fully tunable integrated nanophotonic processor. The device interfaces with telecom-wavelength photons and features a fast active feedback mechanism, allowing us to demonstrate the agent's systematic quantum advantage in a setup that could be readily integrated within future large-scale quantum communication networks.Comment: 10 pages, 4 figure

Design and construction of a fibre-coupled grating-based single-photon spectrometer with sub-nanometer resolution at 1550 nm

Diverse bisher üblicherweise auf Prinzipien der klassischen Physik beruhen de Anwendungen unterschiedlicher Gebiete der Physik können aus der Nutzung intrinsischer Eigenschaften von Quantensystemen Profit ziehen. Diese Tatsache führte zur Entwicklung verschiedener neuer Forschungsgebiete wie, unter Anderem, Quanten Kryptographie, Quanten Computation (das auch das Gebiet der Quanten Simulation umfasst), und Quanten Metrologie. Hinsichtlich vieler Gesichtspunkte stellen Einzelphotonen ideale Quantensysteme dar: Sie können eine sehr hohe Ununterscheidbarkeit aufweisen und ihre intrinsische Mobilität ermöglicht den Transport von Quanteninformation mit Lichtgeschwindigkeit. Für letzteres kann auf bewährte Technologien zurückgegriffen werden, wie in etwa auf optische Glasfasern und integrierte Optik. Des Weiteren verlangt die Verarbeitung von Einzelphotonen nicht nach einem Vakuum oder kryogenen Bedingungen, was die Anwendbarkeit optischer Platformen nochmals unterstreicht. Die rasante Entwicklung photonischer Quantentechnologie führte zu skalierbaren Architekturen, basierend auf integrierten Quellen, Verarbeitungseinheiten und Detektoren. Allerdings basieren diemeisten Multiphotonenverarbeitungsmodelle auf speztifischen optischen Eigenschaften, wie Multiphotoneninterferenz, die eine präzise Charakterisierung der generierten Einzelphotonen unabdingbar machen. Frühe Multiphotonenexperimente basierten auf Quellen die Photonen bei in etwa 800 nm Wellenlänge erzeugten. In den letzten Jahren verschob sich der Schwerpunkt allerdings zu Quellen die im Telekummunikationswellenlängenbereich operieren. Der Grund dafür liegt einerseits darin, dass die Nulldispersionswellenlänge von Quarzglasfasern in diesem Bereich liegt und andererseits dass deren Dämpfung dort am geringsten ist, mit einem Minimum bei 1550 nm. Da Verluste in individuellen Einzelphotonenquellen die Effizienz von Multiphotonenquellen exponentiell vermindern ist diese Wellenlänge ideal für Multiphotonenexperimente. Der Hauptzweck dieser Arbeit bestand darin, einen kosteneffizienten, dabei aber genauen, Monochromator mit Einzelphotonenauflösung bei 1550 nm zu konstruieren um die Charakterisierung von Einzelphotonenquellen bei dieser Wellenlänge zu unterstützen. Das fertige Gerät basiert auf einem planen optischen Beugungsgitter, ist fasergekoppelt und erreicht eine Auflösung von in etwa 0.1 nm. Es wurde zur Charakterisierung von Einzelphotonen von Pikosekunden-gepulsten PPKTP Quellen eingesetzt und half dabei einen Multiphotonenaufbau einzurichten. Diese Arbeit umfasst den Entwicklungsprozess des Gerätes sowie die Implementierung einer Steuerungssoftware und Messungen an den Quellen.The utilisation of intrinsic properties of quantum systems can benefit several applications of different fields in physics which previously relied on classical systems, and has led to the development of new research areas, comprising quantum cryptography, quantum computation (including quantum simulation) and quantum metrology. Single photons serve as ideal quantum systems in many ways: They can offer a high degree of indistinguishability and their intrinsic mobility allows for the transportation of quantum information at the speed of light by exploiting established technology such as optical fibres and integrated optics. In addition, processing single photons does not require vacuum or cryogenic conditions, which underlines the applicability of optical platforms. The vast development of photonic quantum technology has lead to scalable architectures based on integrated sources, processors and detectors. However, almost all multi-photon processing schemes are building on particular optical properties, such as multi-photon interference, that require a precise characterisation of the generated single photons. Early multi-photon experiments relied on sources producing photons at around 800 nm wavelength. In recent years, however, focus has shifted towards sources operating at telecom wavelengths. This is due to the fact that the zero-dispersion wavelength of fused-silica fibres lies in this region and their attenuation is lowest there, with a minimum at 1550 nm. Since losses in individual single-photon sources exponentially degrade the efficiency of multi-photon sources, this wavelength is ideal for multi-photon experiments.The primary purpose of this thesis was to build a cost-effective but accurate monochromator with single-photon resolution at 1550 nm to help characterize single-photon sources operating at this wavelength. The device is based on a planar diffraction grating, fibre coupled and offers a resolution of about 0.1 nm. It was used for the characterisation of single photons from pico-second pulsed PPKTP sources and helped to set up a multi-photon setup. This thesis covers the design process as well as the implementation of a control software and measurements of the sources

Demonstration of universal time-reversal for quantum processes

Although the laws of classical physics are deterministic, thermodynamics gives rise to an arrow of time through irreversible processes. In quantum mechanics the unitary nature of the time evolution makes it intrinsically reversible, however the question of how to revert an unknown time evolution nevertheless remains. Remarkably, there have been several recent demonstrations of protocols for reverting unknown unitaries in scenarios where even the interactions with the target system are unknown. The practical use of these universal rewinding protocols is limited by their probabilistic nature, raising the fundamental question of whether time-reversal could be performed deterministically. Here we show that quantum physics indeed allows for deterministic universal time-reversal by exploiting the non-commuting nature of quantum operators, and demonstrate a recursive protocol for two-level quantum systems with an arbitrarily high probability of success. Using a photonic platform we demonstrate our protocol, reverting the discrete time evolution of a polarization state with an average state fidelity of over 95%. Our protocol, requiring no knowledge of the quantum process to be rewound, is optimal in its running time, and brings quantum rewinding into a regime of practical relevance

Demonstration of quantum-digital payments

Abstract Digital payments have replaced physical banknotes in many aspects of our daily lives. Similarly to banknotes, they should be easy to use, unique, tamper-resistant and untraceable, but additionally withstand digital attackers and data breaches. Current technology substitutes customers’ sensitive data by randomized tokens, and secures the payment’s uniqueness with a cryptographic function, called a cryptogram. However, computationally powerful attacks violate the security of these functions. Quantum technology comes with the potential to protect even against infinite computational power. Here, we show how quantum light can secure daily digital payments by generating inherently unforgeable quantum cryptograms. We implement the scheme over an urban optical fiber link, and show its robustness to noise and loss-dependent attacks. Unlike previously proposed protocols, our solution does not depend on long-term quantum storage or trusted agents and authenticated channels. It is practical with near-term technology and may herald an era of quantum-enabled security

Tuning single-photon sources for telecom multi-photon experiments

Multi-photon state generation is of great interest for near-future quantum simulation and quantum computation experiments. To-date spontaneous parametric down-conversion is still the most promising process, even though two major impediments still exist: accidental photon noise (caused by the probabilistic non-linear process) and imperfect single-photon purity (arising from spectral entanglement between the photon pairs). In this work, we overcome both of these difficulties by (1) exploiting a passive temporal multiplexing scheme and (2) carefully optimizing the spectral properties of the down-converted photons using periodically-poled KTP crystals. We construct two down-conversion sources in the telecom wavelength regime, finding spectral purities of > 91%, while maintaining high four-photon count rates. We use single-photon grating spectrometers together with superconducting nanowire single-photon detectors to perform a detailed characterization of our multi-photon source. Our methods provide practical solutions to produce high-quality multi-photon states, which are in demand for many quantum photonics applications