Cyclical Quantum Memory for Photonic Qubits

We have performed a proof-of-principle experiment in which qubits encoded in the polarization states of single-photons from a parametric down-conversion source were coherently stored and read-out from a quantum memory device. The memory device utilized a simple free-space storage loop, providing a cyclical read-out that could be synchronized with the cycle time of a quantum computer. The coherence of the photonic qubits was maintained during switching operations by using a high-speed polarizing Sagnac interferometer switch.Comment: 4 pages, 5 figure

High-speed noise-free optical quantum memory

Quantum networks promise to revolutionise computing, simulation, and communication. Light is the ideal information carrier for quantum networks, as its properties are not degraded by noise in ambient conditions, and it can support large bandwidths enabling fast operations and a large information capacity. Quantum memories, devices that store, manipulate, and release on demand quantum light, have been identified as critical components of photonic quantum networks, because they facilitate scalability. However, any noise introduced by the memory can render the device classical by destroying the quantum character of the light. Here we introduce an intrinsically noise-free memory protocol based on two-photon off-resonant cascaded absorption (ORCA). We consequently demonstrate for the first time successful storage of GHz-bandwidth heralded single photons in a warm atomic vapour with no added noise; confirmed by the unaltered photon statistics upon recall. Our ORCA memory platform meets the stringent noise-requirements for quantum memories whilst offering technical simplicity and high-speed operation, and therefore is immediately applicable to low-latency quantum networks

Quantum Optical Systems for the Implementation of Quantum Information Processing

We review the field of Quantum Optical Information from elementary considerations through to quantum computation schemes. We illustrate our discussion with descriptions of experimental demonstrations of key communication and processing tasks from the last decade and also look forward to the key results likely in the next decade. We examine both discrete (single photon) type processing as well as those which employ continuous variable manipulations. The mathematical formalism is kept to the minimum needed to understand the key theoretical and experimental results

Quantum repeaters and quantum key distribution: analysis of secret key rates

We analyze various prominent quantum repeater protocols in the context of long-distance quantum key distribution. These protocols are the original quantum repeater proposal by Briegel, D\"ur, Cirac and Zoller, the so-called hybrid quantum repeater using optical coherent states dispersively interacting with atomic spin qubits, and the Duan-Lukin-Cirac-Zoller-type repeater using atomic ensembles together with linear optics and, in its most recent extension, heralded qubit amplifiers. For our analysis, we investigate the most important experimental parameters of every repeater component and find their minimally required values for obtaining a nonzero secret key. Additionally, we examine in detail the impact of device imperfections on the final secret key rate and on the optimal number of rounds of distillation when the entangled states are purified right after their initial distribution.Comment: Published versio

On-demand semiconductor single-photon source with near-unity indistinguishability

Single photon sources based on semiconductor quantum dots offer distinct advantages for quantum information, including a scalable solid-state platform, ultrabrightness, and interconnectivity with matter qubits. A key prerequisite for their use in optical quantum computing and solid-state networks is a high level of efficiency and indistinguishability. Pulsed resonance fluorescence (RF) has been anticipated as the optimum condition for the deterministic generation of high-quality photons with vanishing effects of dephasing. Here, we generate pulsed RF single photons on demand from a single, microcavity-embedded quantum dot under s-shell excitation with 3-ps laser pulses. The pi-pulse excited RF photons have less than 0.3% background contributions and a vanishing two-photon emission probability. Non-postselective Hong-Ou-Mandel interference between two successively emitted photons is observed with a visibility of 0.97(2), comparable to trapped atoms and ions. Two single photons are further used to implement a high-fidelity quantum controlled-NOT gate.Comment: 11 pages, 11 figure

Room temperature single-photon sources and atomic quantum memories for broadband quantum networks

Quantum networks are envisioned to overcome current limitations in quantum communication and computation. The building blocks that enable the realization of such networks are quantum memories and single-photon sources. A promising path to realize these networks is to make use of heterogeneous interconnects. By interfacing quantum memories based on atomic ensembles with solid-state sources, the best of two worlds could be exploited. However, achieving compatibility between different systems, in order to realize a hybrid quantum network node, has remained an open challenge. In this thesis I report on broadband quantum memories implemented in warm rubidium vapor and on a compatible single-photon source based on non-degenerate cavity-enhanced spontaneous parametric downconversion. The goal is to implement an elementary interconnect where the experimental complexity is kept low by operating all components at or above room temperature. Choosing a monolithic cavity design, the source is inherently robust and reaches high efficiencies. It generates heralded single photons with hundreds of \si{\mega\hertz} bandwidth and reaches high heralding efficiencies of $\geq 40\%$, measured after coupling into a single-mode optical fiber. The memories presented here are based on electromagnetically induced transparency (EIT) in a lambda-level scheme in warm rubidium vapor. To suppress the read-out noise, which is a limiting factor in common ground-state memory implementations, two separate approaches are followed. In the first one, the atomic structure is modified by applying a tesla-order magnetic field and working in the hyperfine Paschen-Back regime. This results in large splittings between the energy levels, which allows us to optically address individual sublevels in the warm vapor. A spectroscopic study of EIT and optical pumping in this regime is presented. Our proof-of-principle implementation of a quantum memory in a miniaturized vapor cell has delivered promising results. This setup was capable of storing and retrieving weak coherent pulses attenuated to the single-photon level, yielding an end-to-end efficiency of $\eta_{e2e} \approx 3\%$ and a SNR of up to $7.9(8)$. The second approach to suppressing read-out noise relies on exploiting polarization selection rules in a Zeeman-pumped vapor. This memory implementation was successfully interfaced with \SI{370}{\mega\hertz}-broad single photons from the heralded downconversion source. The stored photons maintained their non-classical signature after retrieval, yielding a $g^{(2)}(0) = 0.177(23)$. This constitutes the first demonstration of single photon storage and retrieval from the ground state of a warm atomic vapor. The developed platform operates in a technologically relevant regime for future experiments, paving the way for the exploration of promising quantum-network protocols at high bandwidth

Quantum cryptography: key distribution and beyond

Uniquely among the sciences, quantum cryptography has driven both foundational research as well as practical real-life applications. We review the progress of quantum cryptography in the last decade, covering quantum key distribution and other applications.Comment: It's a review on quantum cryptography and it is not restricted to QK