Hybrid photonic circuit for multiplexed heralded single photons

A key resource for quantum optics experiments is an on‐demand source of single and multiple photon states at telecommunication wavelengths. This letter presents a heralded single photon source based on a hybrid technology approach, combining high efficiency periodically poled lithium niobate waveguides, low‐loss laser inscribed circuits, and fast (>1 MHz) fibre coupled electro‐optic switches. Hybrid interfacing different platforms is a promising route to exploiting the advantages of existing technology and has permitted the demonstration of the multiplexing of four identical sources of single photons to one output. Since this is an integrated technology, it provides scalability and can immediately leverage any improvements in transmission, detection and photon production efficiencies.     

一种基于标记单光子源的态制备误差容忍量子密钥分发协议

在实际量子密钥分发系统中,由于设备、器件存在缺陷,在量子态制备过程中往往存在误差,而这些态制备误差会导致一定的系统安全性漏洞.本文在Tamaki等(Phys. Rev.A 90 052314)的工作基础之上,提出了一种基于标记单光子源的态制备误差容忍量子密钥分发协议.本文将发送端制备态误差进行参数刻画并带入量子密钥协议安全性分析之中,避免了实际应用中由于态制备装置的不理想可能引入的安全性漏洞,提高了系统的安全性.同时,为了方便起见,本文采用三强度诱骗态方案开展建模分析与数值仿真计算.仿真结果显示,本文提出的协议对态制备误差具有很好的鲁棒性.同时,由于标记单光子源具有真空脉冲概率低的优点,与此前基于弱相干态脉冲的同类协议相比,我们的协议在传输距离较远时能够显示出更优的性能.因而,该工作有望为未来发展长距离量子保密通信应用与研究提供重要的参考价值.     

Modulator‐Free Coherent‐One‐Way Quantum Key Distribution

Time‐bin encoding is an attractive method for transmitting photonic qubits over long distances with minimal decoherence. It allows a simple receiver for quantum key distribution (QKD) that extracts a key by measuring time of arrival of photons and detects eavesdropping by measuring interference of pulses in different time bins. In the past, coherent pulses have been generated using a CW laser and an intensity modulator. A greatly simplified transmitter is proposed and demonstrated here that works by directly modulating the laser diode. Coherence between pulses is maintained by a weak seed laser. The modulator‐free source creates time‐bin encoded pulses with a high extinction ratio (29.4 dB) and an interference visibility above 97 %. The resulting QKD transmitter gives estimated secure key rates up to 4.57 Mbit/s, the highest yet reported for coherent‐one‐way QKD, and can be programmed for all protocols using weak coherent pulses.     

Heralded linear optical quantum Fredkin gate based on one auxiliary qubit and one single photon detector

Linear optical quantum Fredkin gate can be applied to quantum computing and quantum multi-user communication networks. In the existing linear optical scheme, two single photon detectors （SPDs） are used to herald the success of the quantum Fredkin gate while they have no photon count. But analysis results show that for non-perfect SPD, the lower the detector efficiency, the higher the heralded success rate by this scheme is. We propose an improved linear optical quantum Fredkin gate by designing a new heralding scheme with an auxiliary qubit and only one SPD, in which the higher the detection efficiency of the heralding detector, the higher the success rate of the gate is. The new heralding scheme can also work efficiently under a non-ideal single photon source. Based on this quantum Fredkin gate, large-scale quantum switching networks can be built. As an example, a quantum Bene~ network is shown in which only one SPD is used.     

Hamiltonian Ratchets with Ultra‐Cold Atoms

Quantum‐resonance ratchets have been realized over the last ten years for the production of directed currents of atoms. These non‐dissipative systems are based on the interaction of a Bose‐Einstein condensate with an optical standing wave potential to produce a current of atoms in momentum space. In this paper we provide a review of the important features of these ratchets with a particular emphasis on their optimization using more complex initial states. We also examine their stability close to resonance conditions of the kicking. Finally we discuss the way in which these ratchets may pave the way for applications in quantum (random) walks and matter‐wave interferometry.     

Entanglement distribution over 150 km in wavelength division multiplexed channels for quantum cryptography

Granting information privacy is of crucial importance in our society, notably in fiber communication networks. Quantum cryptography provides a unique means to establish, at remote locations, identical strings of genuine random bits, with a level of secrecy unattainable using classical resources. However, several constraints, such as non‐optimized photon number statistics and resources, detectors' noise, and optical losses, currently limit the performances in terms of both achievable secret key rates and distances. Here, these issues are addressed using an approach that combines both fundamental and off‐the‐shelves technological resources. High‐quality bipartite photonic entanglement is distributed over a 150 km fiber link, exploiting a wavelength demultiplexing strategy implemented at the end‐user locations. It is shown how coincidence rates scale linearly with the number of employed telecommunication channels, with values outperforming previous realizations by almost one order of magnitude. Thanks to its potential of scalability and compliance with device‐independent strategies, this system is ready for real quantum applications, notably entanglement‐based quantum cryptography.

     

Ultra‐fast nano‐optics

Ultra‐fast nano‐optics is a comparatively young and rapidly growing field of research aiming at probing, manipulating and controlling ultrafast optical excitations on nanometer length scales. This ability to control light on nanometric length and femtosecond time scales opens up exciting possibilities for probing dynamic processes in nanostructures in real time and space. This article gives a brief introduction into the emerging research field of ultrafast nano‐optics and discusses recent progress made in it. A particular emphasis is laid on the recent experimental work performed in the authors' laboratories. We specifically discuss how ultrafast nano‐optical techniques can be used to probe and manipulate coherent optical excitations in individual and dipole‐coupled pairs of quantum dots, probe the dynamics of surface plasmon polariton excitations in metallic nanostructures, generate novel nanometer‐sized ultrafast light and electron sources and reveal the dipole interaction between excitons and surface plasmon polaritons in hybrid metal‐semiconductor nanostructures. Our results indicate that such hybrid nanostructures carry significant potential for realizing novel nano‐optical devices such as ultrafast nano‐optical switches as well as surface plasmon polariton amplifiers and lasers.     

Active demultiplexing of single photons from a solid‐state source

A scheme for active temporal‐to‐spatial demultiplexing of single photons generated by a solid‐state source is introduced. The scheme scales quasi‐polynomially with photon number, providing a viable technological path for routing n photons in the one temporal stream from a single emitter to n different spatial modes. Active demultiplexing is demonstrated using a state‐of‐the‐art photon source—a quantum‐dot deterministically coupled to a micropillar cavity—and a custom‐built demultiplexer—a network of electro‐optically reconfigurable waveguides monolithically integrated in a lithium niobate chip. The measured demultiplexer performance can enable a six‐photon rate three orders of magnitude higher than the equivalent heralded SPDC source, providing a platform for intermediate quantum computation protocols.

     

Ultra‐short and ultra‐intense X‐ray free‐electron laser single pulse in one‐dimensional photonic crystals

The propagation within a one‐dimensional photonic crystal of a single ultra‐short and ultra‐intense pulse delivered by an X‐ray free‐electron laser is analysed with the framework of the time‐dependent coupled‐wave theory in non‐linear media. It is shown that the reflection and the transmission of an ultra‐short pulse present a transient period conditioned by the extinction length and also the thickness of the structure for transmission. For ultra‐intense pulses, non‐linear effects are expected: they could give rise to numerous phenomena, bi‐stability, self‐induced transparency, gap solitons, switching, etc., which have been previously shown in the optical domain.     

On‐chip single photon sources using planar photonic crystals and single quantum dots

We review the basic light‐matter interactions and optical properties of chip‐based single photon sources, that are enabled by integrating single quantum dots with planar photonic crystals. A theoretical framework is presented that allows one to connect to a wide range of quantum light propagation effects in a physically intuitive and straightforward way. We focus on the important mechanisms of enhanced spontaneous emission, and efficient photon extraction, using all‐integrated photonic crystal components including waveguides, cavities, quantum dots and output couplers. The limitations, challenges, and exciting prospects of developing on‐chip quantum light sources using integrated photonic crystal structures are discussed.     

Generation of quantum‐entangled twin photons by waveguide nonlinear‐optic devices

This paper reviews the quasi‐phase‐matched (QPM) waveguide nonlinear‐optic device technologies for generation of quantum‐entangled twin photons indispensable for quantum‐information techniques. After a brief introduction to the concept of entanglement, quantum theory analysis of twin‐photon generation (TPG) is outlined to clarify the properties of twin photons. Then, methods for entangled‐photon generation are discussed. Practical design and theoretical performances of LiNbO₃ waveguide QPM TPG devices, as well as the fabrication techniques, are described. Finally, experimental demonstrations of polarization‐entangled twin‐photon generation by waveguide Type‐I and Type‐II QPM TPG devices are presented.     

Frequency‐sensitive switching control effect induced by two‐photon resonance in an EIT‐based layered medium

The optical response of an atomic vapor can be coherently manipulated by tunable quantum interference occurring in atomic transition processes. A periodic layered medium whose unit cells consist of a dielectric and an EIT (electromagnetically induced transparency) atomic vapor is designed for light propagation manipulation. Such an EIT‐based periodic layered medium exhibits a flexible frequency‐sensitive optical response, where a very small change in probe frequency can lead to a drastic variation of reflectance and transmittance. As the destructive quantum interference relevant to two‐photon resonance arises in EIT atoms interacting with both control and probe fields, the controllable optical processes that depend sensitively on the external control field will take place in this EIT‐based periodic layered medium. Such a frequency‐sensitive and field‐controlled optical behavior of reflection and transmission in the EIT photonic crystal can be applicable to designs of new devices such as photonic switches, photonic logic gates and photonic transistors, where one laser field can be controlled by the other one, and would have potential applications in the areas of integrated optical circuits and other related techniques (e.g., all‐optical instrumentations).     

超导转变边沿单光子探测器原理与研究进展

量子信息技术近十多年来的快速发展对单光子探测器的性能提出了更高的要求,高性能单光子探测器也因此受到了更多的关注.与传统的单光子探测器相比,超导转变边沿(TES)单光子探测器在探测效率、能量分辨、光子数分辨和暗计数等方面具有突出优势.目前,超导TES单光子探测器已经被成功地应用在量子光学实验和量子密钥分配系统中,未来在量子信息技术等研究领域具有更广泛的应用.本文从超导TES单光子探测器的工作原理、制备流程、测试系统、主要性能指标以及研究现状和进展等方面对该探测器技术进行简要综述.     

Two‐photon absorption induced anti‐Stokes emission in single InGaN/GAN quantum‐dot‐like objects

We observed crossed transitions and anti‐Stokes emissions in single quantum‐dot‐like objects embedded in the active layer of InGaN/GaN quantum disks by two‐photon absorption techniques. We proposed a phenomenological model based on the interplay between Auger effect and crossed transitions to explain the origin of anti‐Stokes emissions and the preferential excitation of 0D objects at the expense of their surroundings. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Particle injection into a chain: decoherence versus relaxation for Hermitian and non‐Hermitian dynamics

A model system for the injection of fermionic particles from filled source sites into an empty chain is investigated. The ensuing dynamics for Hermitian as well as for non‐Hermitian time evolution, where the particles cannot return to the bath sites (quantum ratchet), is studied. A non‐homogeneous hybridization between bath and chain sites permits transient currents in the chain. Non‐interacting particles show decoherence in the thermodynamic limit: the average particle number and the average current density in the chain become stationary for long times, whereas the single‐particle density matrix displays large fluctuations around its mean value. Using the numerical time‐dependent density‐matrix renormalization group (t‐DMRG) method it is demonstrated, on the other hand, that sizable density‐density interactions between the particles introduce relaxation which is by orders of magnitudes faster than the decoherence processes.     

Fiber‐based light sources for biomedical applications of coherent anti‐Stokes Raman scattering microscopy

During the past decade coherent anti‐Stokes Raman scattering (CARS) microscopy has evolved to one of the most powerful imaging techniques in the biomedical sciences, enabling the label‐free visualization of the chemical composition of tissue in vivo in real time. While the acquisition of high‐contrast images of single cells up to large tissue sections enables a wide range of medical applications from routine diagnostics to surgical guidance, to date CARS imaging is employed in fundamental research only, essentially because the synchronized multiple wavelength pulsed laser sources required for CARS microscopy are large, expensive and require regular maintenance. Laser sources based on optical fibers can overcome these limitations combining highest efficiency and peak powers with an excellent spatial beam profile and thermal stability. In this review we summarize the different fiber‐based approaches for laser sources dedicated to coherent Raman imaging, in particular active fiber technology and passive fiber‐based frequency conversion processes, i.e. supercontinuum generation, soliton self‐frequency shift and four‐wave mixing. We re‐evaluate the ideal laser parameters for CARS imaging and discuss the suitability of different laser concepts for turn‐key operation required for routine application in clinics.

     

Sustainability of Environment‐Assisted Energy Transfer in Quantum Photobiological Complexes

It is shown that quantum sustainability is a universal phenomenon which emerges during environment‐assisted electronic excitation energy transfer (EET) in photobiological complexes (PBCs), such as photosynthetic reaction centers and centers of melanogenesis. We demonstrate that quantum photobiological systems must be sustainable for them to simultaneously endure continuous energy transfer and keep their internal structure from destruction or critical instability. These quantum effects occur due to the interaction of PBCs with their environment which can be described by means of the reduced density operator and effective non‐Hermitian Hamiltonian (NH). Sustainable NH models of EET predict the coherence beats, followed by the decrease of coherence down to a small, yet non‐zero value. This indicates that in sustainable PBCs, quantum effects survive on a much larger time scale than the energy relaxation of an exciton. We show that sustainable evolution significantly lowers the entropy of PBCs and improves the speed and capacity of EET.     

Diffraction of X‐ray free‐electron laser femtosecond pulses on single crystals in the Bragg and Laue geometry

A solution of the problem of dynamical diffraction for X‐ray pulses with arbitrary dimensions in the Bragg and Laue cases in a crystal of any thickness and asymmetry coefficient of reflection is presented. Analysis of pulse form and duration transformation in the process of diffraction and propagation in a vacuum is conducted. It is shown that only the symmetrical Bragg case can be used to avoid smearing of reflected pulses.     

Nonlinear optical fiber based high resolution all‐optical waveform sampling

When there is a need to accurately characterize optical waveforms and, it is not surprising that some of the best, albeit only recently established, techniques to do this rely on all‐optical phenomena. Some basic reasons why all‐optical sampling holds great promise as a very useful tool well into the foreseeable future are that there are no ringing phenomena with associated waveform distortion as in electronic sampling due to impedance mismatch, and that the time resolution can be made extremely high (⩽ 1 ps) while yet also offering high sensitivity for e.g. eye diagram (a superposition of all ‘1’ and ‘0’ in a data sequence that is widely used in telecommunications testing) and statistical analysis. In this paper, we review recent developments in optical fiber‐based sampling of optical waveforms. In particular, we describe the state‐of‐the‐art in terms of the various performance measures as well as their trade‐offs.     

Quantum‐optical analogies using photonic structures

Engineered photonic waveguides have provided in the past decade an extremely rich laboratory tool to visualize with optical waves the classic analogues of a wide variety of coherent quantum phenomena encountered in atomic, molecular or condensed‐matter physics. As compared to quantum systems, optics offers the rather unique advantage of a direct mapping of the wave function evolution in coordinate space by simple fluorescence imaging or scanning tunneling optical microscopy techniques. In this contribution recent theoretical and experimental advances in the field of quantum‐optical analogies are reviewed. Special attention is devoted to some relevant optical analogies based on the use of curved photonic structures, including: coherent destruction of tunneling in driven bistable potentials; coherent population transfer and adiabatic passage in laser‐driven multilevel atomic systems; quantum decay control and Zeno dynamics; electronic Bloch oscillations and Zener tunneling, Anderson localization and dynamic localization in crystalline potentials.