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The nature of atomic vapors, their natural alignment with interatomic transitions, and their ease of use make them highly suited for spectrally narrow-banded optical filters. Atomic filters come in two flavors: a filter based on the absorption of light by the Doppler broadened atomic vapor, i.e. a notch filter, and a bandpass filter based on the transmission of resonant light caused by the Faraday effect. The notch filter uses the absorption of resonant photons to filter out a small spectral band around the atomic transition. The off-resonant part of the spectrum is fully transmitted. Atomic vapors based on the Faraday effect allow for suppression of the detuned spectral fraction. Transmission of light originates from the magnetically induced rotation of linear polarized light close to an atomic resonance. This filter constellation allows selective acceptance of specific light frequencies. In this manuscript, we discuss these two types of filters and elucidate the specialties of atomic line filters. We also present a practical guide on building such filter setups from scratch and discuss an approach to achieve an almost perfect atomic spectrum backed by theoretical calculations.

Quantum mechanics implies that a single photon can be in the superposition of two distant spatial modes and enable nonlocal interferences. The most vivid example is the two-photon coalescence on a50∶50beam splitter, known as Hong-Ou-Mandel interference. In the past decade, this experiment has been used to characterize the suitability of different single-photon sources for linear optical quantum gates. This characterization alone cannot guarantee the suitability of the photons in a scalable quantum network. As for a deeper insight, we perform a number of nonclassical interference measurements of single photons emitted by a single organic molecule that are optimized by an atomic Faraday filter. Our measurements reveal near unity visibility of the quantum interference, and a one-port correlation measurement proves the ideal Fourier limited nature of our single-photon source. A delayed choice quantum eraser allows us to observe a constructive interference between the photons, and a Hong-Ou-Mandel peak is formed additionally to the commonly observed dip. These experiments comprehensively characterize the involved photons for their use in a future quantum Internet, and they attest to the fully efficient interaction of the molecular photons with a next subsequent quantum node. They can be adapted to other emitters and will allow us to gain insights to their applicability for quantum information processing. We introduce a quality number that describes the photon’s properties for their use in a quantum network; this states that effectively 97% of the utilized molecular photons can be used in a scalable quantum optical system and interact with other quantum nodes. The experiments are based on a hybridization of solid state quantum optics, atomic systems, and all-optical quantum information processing.

Quantum key distribution (QKD) allows two remote parties to grow a shared secret key. Its security is founded on the principles of quantum mechanics, but in reality it significantly relies on the physical implementation. Technological imperfections of QKD systems have been previously explored, but no attack on an established QKD connection has been realized so far. Here we show the first full-field implementation of a complete attack on a running QKD connection. An installed eavesdropper obtains the entire 'secret' key, while none of the parameters monitored by the legitimate parties indicate a security breach. This confirms that non-idealities in physical implementations of QKD can be fully practically exploitable, and must be given increased scrutiny if quantum cryptography is to become highly secure. A quantum key distribution system allows two remote parties to communicate in secret by a shared key code. This work demonstrates a complete and undetected eavesdropping attack on a quantum key distribution connection, highlighting the need for further security updates on secure communication systems.

Narrow-band optical filtering is required in many spectroscopy applications to suppress unwanted background light. One example is quantum communication where the fidelity is often limited by the performance of the optical filters. This limitation can be circumvented by utilizing the GHz-wide features of a Doppler broadened atomic gas. The anomalous dispersion of atomic vapours enables spectral filtering. These, so-called, Faraday anomalous dispersion optical filters (FADOFs) can be by far better than any commercial filter in terms of bandwidth, transition edge and peak transmission. We present a theoretical and experimental study on the transmission properties of a sodium vapour based FADOF with the aim to find the best combination of optical rotation and intrinsic loss. The relevant parameters, such as magnetic field, temperature, the related optical depth and polarization state are discussed. The non-trivial interplay of these quantities defines the net performance of the filter. We determine analytically the optimal working conditions, such as transmission and the signal to background ratio and validate the results experimentally. We find a single global optimum for one specific optical path length of the filter. This can now be applied to spectroscopy, guide star applications, or sensing.

A single organic dye molecule at cryogenic conditions is resonantly excited in a confocal microscope. Under strong laser illumination it undergoes Rabi oscillations. Mathematically, this was well described and had been experimentally implemented. These oscillations can be measured as side-bands on their resonance fluorescence, e.g. in the Mollow-Triplet. An alternative method is to research this effect by an analysis of the single molecule anti-bunched photon statistics. This has been performed in this work. Here we research on the detuning dependence of this signal-it is experimentally demanding since the utilized laser might drift or single emitters are not necessarily spectrally stable enough, such that the spectrum can be measured indefinitely. We therefore apply a measurement technique in which the photon correlation signal is acquired in detuning dependent steps. This is performed by continuous laser sweeps over the single molecule excitation spectrum. A single recording of the anti-bunched photons takes 20-50 ms. After approx. 1 h of repetitive laser detunings a full anti-bunching curve is reconstructed for each spectral position. An alternative technique with 100 ns laser pulses allows us to acquire a set of comparable data. Our study is derived from a single dibenzanthanthrene molecule with a natural linewidth of 2π×16 MHz. It emits under resonant excitation more than 380.000 photons per second. Under spectral detuning, Rabi-oscillations are observed up to Rabi = 2π×160 MHz.

Hybrid quantum systems integrating semiconductor quantum dots (QDs) and atomic vapours become important building blocks for scalable quantum networks due to the complementary strengths of individual parts. QDs provide on-demand single-photon emission with near-unity indistinguishability comprising unprecedented brightness—while atomic vapour systems provide ultra-precise frequency standards and promise long coherence times for the storage of qubits. Spectral filtering is one of the key components for the successful link between QD photons and atoms. Here we present a tailored Faraday anomalous dispersion optical filter based on the caesium-D 1 transition for interfacing it with a resonantly pumped QD. The presented Faraday filter enables a narrow-bandwidth (Δ ω =2 π × 1 GHz) simultaneous filtering of both Mollow triplet sidebands. This result opens the way to use QDs as sources of single as well as cascaded photons in photonic quantum networks aligned to the primary frequency standard of the caesium clock transition. Hybrid quantum systems combine efficient high-quality quantum dot sources with atomic vapours that can serve as precise frequency standards or quantum memories. Here, Portalupi et al . demonstrate an optimized atomic Cs-Faraday filter working with single photons emitted from a semiconductor quantum dot.

The generation of random bits is of enormous importance in modern information science. Cryptographic security is based on random numbers which require a physical process for their generation. This is commonly performed by hardware random-number generators. These often exhibit a number of problems, namely experimental bias, memory in the system, and other technical subtleties, which reduce the reliability in the entropy estimation. Further, the generated outcome has to be postprocessed to “iron out” such spurious effects. Here, we present a purely optical randomness generator, based on the bistable output of an optical parametric oscillator. Detector noise plays no role and postprocessing is reduced to a minimum. Upon entering the bistable regime, initially the resulting output phase depends on vacuum fluctuations. Later, the phase is rigidly locked and can be well determined versus a pulse train, which is derived from the pump laser. This delivers an ambiguity-free output, which is reliably detected and associated with a binary outcome. The resulting random bit stream resembles a perfect coin toss and passes all relevant randomness measures. The random nature of the generated binary outcome is furthermore confirmed by an analysis of resulting conditional entropies.

Optical detection and spectroscopy of single molecules has become an indispensable tool in biological imaging and sensing. Its success is based on fluorescence of organic dye molecules under carefully engineered laser illumination. In this paper we demonstrate optical detection of single molecules on a wide-field microscope with an illumination based on a commercially available, green light-emitting diode. The results are directly compared with laser illumination in the same experimental configuration. The setup and the limiting factors, such as light transfer to the sample, spectral filtering and the resulting signal-to-noise ratio are discussed. A theoretical and an experimental approach to estimate these parameters are presented. The results can be adapted to other single emitter and illumination schemes.

Defects in silicon carbide have recently been proposed as bright single-photon sources. It is now shown that they can be used as sources of single electron spins having long coherence times at room temperature. Spins in solids are cornerstone elements of quantum spintronics 1 . Leading contenders such as defects in diamond 2 , 3 , 4 , 5 or individual phosphorus dopants in silicon 6 have shown spectacular progress, but either lack established nanotechnology or an efficient spin/photon interface. Silicon carbide (SiC) combines the strength of both systems 5 : it has a large bandgap with deep defects 7 , 8 , 9 and benefits from mature fabrication techniques 10 , 11 , 12 . Here, we report the characterization of photoluminescence and optical spin polarization from single silicon vacancies in SiC, and demonstrate that single spins can be addressed at room temperature. We show coherent control of a single defect spin and find long spin coherence times under ambient conditions. Our study provides evidence that SiC is a promising system for atomic-scale spintronics and quantum technology.

Single organic dye molecules have high-flux, narrow-bandwidth single-photon emission and can be spectrally matched to the transitions of atoms acting as a quantum memory, making them promising for use in quantum information and communication schemes. Single-molecule photons meet atom optics Quantum communication strategies depend on single-photon sources to provide a high photon flux over a very narrow range of wavelengths that in turn is spectrally matched with other system components. Single organic dye molecules show potential in this regard, as they combine high-flux, narrow-bandwidth emissions with a wealth of possible spectral lines that can in principle be selected to match transitions in the systems with which they are to be integrated. Petr Siyushev and colleagues now provide an example of such integration capabilities by interfacing the photons emitted by a molecular system with an atomic vapour quantum memory. Future quantum communication will rely on the integration of single-photon sources, quantum memories and systems with strong single-photon nonlinearities 1 . Two key parameters are crucial for the single-photon source: a high photon flux with a very small bandwidth, and a spectral match to other components of the system. Atoms or ions may act as single-photon sources—owing to their narrowband emission and their intrinsic spectral match to other atomic systems—and can serve as quantum nonlinear elements. Unfortunately, their emission rates are still limited, even for highly efficient cavity designs 2 . Single solid-state emitters such as single organic dye molecules are significantly brighter 3 and allow for narrowband photons 4 ; they have shown potential in a variety of quantum optical experiments 5 , 6 but have yet to be interfaced with other components such as stationary memory qubits. Here we describe the optical interaction between Fourier-limited photons from a single organic molecule and atomic alkali vapours, which can constitute an efficient quantum memory. Single-photon emission rates reach up to several hundred thousand counts per second and show a high spectral brightness of 30,000 detectable photons per second per megahertz of bandwidth. The molecular emission is robust and we demonstrate perfect tuning to the spectral transitions of the sodium D line and efficient filtering, even for emitters at ambient conditions. In addition, we achieve storage of molecular photons originating from a single dibenzanthanthrene molecule in atomic sodium vapour. Given the large set of molecular emission lines matching to atomic transitions, our results enable the combination of almost ideal single-photon sources with various atomic vapours, such that experiments with giant single-photon nonlinearities, mediated, for example, by Rydberg atoms 7 , 8 , become feasible.