Entanglement has evolved from being a spooky interaction towards a key resource of real-life applications such as absolute secure communication. Today, quantum networking devices are commercially available, typically taking advantage of time-bin or polarisation entanglement, the latter one doubtlessly being easier to analyse thanks to interferometer free set-ups. The communication distance of such systems is generally limited to a few hundreds of kilometres due to intrinsic fibre transmission losses and imperfect detectors. It was shown that the communication distance can be greatly increased when combining entangled pairs of photons and quantum memories. But the narrow absorption bandwidth of current quantum memories is in contrary to the widely used broadband entanglement sources based on non linear inter- actions. More precisely, there is a lack of ultra narrowband sources in the telecom C-band, where fibre losses are minimal and high performance guided-wave optics are available. Here we introduce a high quality polarisation entanglement engineering scheme based on a birefringent de- lay line (BDL). This BDL is applied to generate polarization entanglement from a high efficiency ultra narrowband tele- com photon pair source where no polarisation entanglement is available initially.
Entanglement has evolved from being a spooky interaction towards a key resource of real-life applications such as absolute secure communication. Today, quantum networking devices are commercially available, typically taking advantage of time-bin or polarisation entanglement, the latter one doubtlessly being easier to analyse thanks to interferometer free set-ups. The communication distance of such systems is generally limited to a few hundreds of kilometres due to intrinsic fibre transmission losses and imperfect detectors. It was shown that the communication distance can be greatly increased when combining entangled pairs of photons and quantum memories. But the narrow absorption bandwidth of current quantum memories is in contrary to the widely used broadband entanglement sources based on non linear inter- actions. More precisely, there is a lack of ultra narrowband sources in the telecom C-band, where fibre losses are minimal and high performance guided-wave optics are available. Here we introduce a high quality polarisation entanglement engineering scheme based on a birefringent de- lay line (BDL). This BDL is applied to generate polarization entanglement from a high efficiency ultra narrowband tele- com photon pair source where no polarisation entanglement is available initially.
During the last decade, quantum entanglement has paved the way out to of the lab modern applications such as quantum computation and communication. Today, small scale quantum networks exist already, but they are limited to a few 100 km distance, due to intrinsic fiber transmission losses and non perfect detectors. These networks are typically established using photon pair sources based on spontaneous parametric down conversion (SPDC). Widely used entanglement observables are time-bin and polarization, where the latter one is undoubtedly simpler to analyze due to lack of interferometric devices. It was shown recently, that introducing quantum memories (QM) between the communication parties could increase both communication speed and distance. Such memories already exist, but their narrow absorption bandwidth does not match the broadband emission spectrum of SPDC sources. Our report is twofold. First, we introduce a new polarization entanglement scheme based on a birefringent delay line (BDL). A pair of incoming photons is projected on a polarizing beam splitter (PBS), where vertically polarized photons travel along a polarization maintaining fiber loop accumulating a delay δ compared to horizontally polarized photons, which exit the BDL directly upon reflection at the PBS. Via post selection of only simultaneously arriving photon pair events, we create entangled states of the form α|H>_1|H>_2 + e^(iφ)β|V>_1 |V>_2. The probability amplitudes α and β are simply tuned by controlling the input pair's polarization and the phase φ can be varied by piezo fiber stretcher (PZT) in the delay line. Therefore, any superposition of the polarization entangled Bell states can be created. The scheme may also be used for entanglement distillation of non maximally entangled states, e.g. emitted by quantum dots. We will introduce all requirements needed to create entanglement from such a BDL. Second, we show how to apply this scheme to produce polarization entanglement from one of the most efficient photon pair generators, namely a periodically poled Lithium Niobate waveguide structure (PPLN/W), where the quasi phase matching is optimized for the type-0 SPDC interaction. The pair's wavelength is chosen in the standardized ITU-21 channel of the low loss telecom C-band and a fiber Bragg grating is applied to reduce the initial spectral bandwidth from 5 THz to 540 MHz, which is suitable for some solid-state quantum memories. Using the BDL, we create the maximally entangled states, with near perfect quality, which is proven by a violation of the Bell's inequalities by more than 29 standard deviations. We achieve a source spectral brightness of 211 narrowband polarization entangled pairs created per second, mW of pump power, and MHz of spectral bandwidth, coupled into a single mode fiber. This is on the same level as the best time-bin entanglement sources reported to date. The source is very practical due to the use of standard telecom fiber components. Moreover the choice for polarization instead of time-bin entanglement allows for simple state analysis using only rotating PBSs. This is a non negligible advantage, especially for narrowband photons with long coherence times, where time-bin analysis requires interferometer path length differences on the order of a few meters stabilized to a few tens of nanometers. Non unit visibilities are solely explained by the nonlinear character of our photon pair source, giving rise to double pair contributions. Applying this scheme to deterministic photon pair sources should result in perfect quality entanglement. We believe that our scheme should play an important role for future quantum networking applications.
Entanglement is a key resource for quantum infor- mation protocols. Low-loss optical fibers and reliable guided-wave components make it possible to create and distribute photonic entanglement in the telecom C-band of wavelengths (1530-1565 nm). Entanglement is easily produced using spontaneous parametric down-conversion (SPDC) in nonlinear crystals. Usual observables are time-bin and polarisation, the latter being easier to analyze, thanks to simple experimental apparatus, real-time fiber birefringence compensation, and the natural design of heralded quantum memories for this observable. In quantum networks, where telecom photons are used to distribute, and quantum memories to store, entanglement, the photonic spectral and memory acceptance bandwidths have to be matched. The latter ranges from a few MHz (cold atoms) to a few GHz (solid- state). In the following, we discuss a versatile polar- isation entangling scheme based on a birefringent delay line (BDL) that can be applied to photon bandwidths down to 25 MHz. We demonstrate such versatility by violating, by large amounts, the Bell inequalities for 3 different bandwidths: 25 MHz, 500 MHz, and 100 GHz.
Recent advances in guided-wave optics bring quantum key distribution (QKD) to a new level where less complex, yet more robust, schemes have been proposed. Through continuous research in this field, newer QKD systems achieve higher bit rate, distance, and reliability. Entanglement-based QKD receives enormous attention from the scientific community, in order to increase the communication distance and to go to device-independent systems. In this framework, photons are generally the preferred qubit carrier, taking advantage of low-loss fibre optical networks, and high-performance standard telecom components. Future QKD networks will demand sources exhibiting high brightness, reliability, and photons in the standard telecom ITU channels for compatibility and easy integration with current technology. One of the most exploited approaches towards genera- ting photonic entanglement is based on the process of spontaneous parametric down-conversion (SPDC) in non-linear crystals. Usually employed entanglement observables are energy-time and polarization. The latter one is, without doubt, the easiest to analyze thanks to interferometer-free setups. Polarization entanglement is conveniently generated via the so-called type-II SPDC interaction, possibly in a waveguide non-linear structure for increasing the efficiency. Nevertheless, with a type-II source, there are some precautions that must be taken for achieving high entanglement qualities. In particular, the generated paired photons need to be spectrally and temporally indistinguishable. Typically, this requires spectral filtering, and temporal walk-off compensation stages. So far, these issues have been addressed by using bulk optics arrangements. We demonstrate a fully fibred approach based on a type-II periodically poled lithium niobate waveguide (PPLN/W) generating paired photons at 1540 nm (telecom C-band wavelength). We highlight the use of a standard polarization maintaining fibre (PMF) as an essentially loss-free, guided-wave solution to the temporal walk-off problem. By using standard telecom DWDMs, we perform, at the same time, the tasks of spectral filtering and deterministic photon pair separation in two ITU channels. By the violation of the Bell's inequalities with more than 100 standard deviations, we demonstrate excellent entanglement qualities and high coincidence rates. Our fully fibred approach offers very low losses, high reliability, and would be ready for QKD field tests.
An in-line source of polarization entangled photons based on a commercial birefringent fibre is demonstrated. When two similar sections of this fibre are spliced together, incorporating a 90° rotation between them, and pumped using a linearly polarised pump beam at 45° to the principle polarization axes, the total generated state resulting from the two fibre sections can be entangled. Classical transmission measurements confirm that the splice loss was low (-1(limited by the available pump power). The phase-matched idler wavelength from this device is suitable for long distance transmission through fibre and the corresponding signal wavelength lies within the operating range of efficient silicon-based single photon detectors. Furthermore, group velocity matching of the idler and pump is expected to occur close to this pump wavelength, resulting in the generation of signal photons with naturally narrow bandwidth.
We report a versatile and practical approach for generating high-quality polarization entanglement in a fully guided-wave fashion. Our setup relies on a high-brilliance type-0 waveguide generator producing paired photon at a telecom wavelength associated with an advanced energy-time to polarisation transcriber. The latter is capable of creating any pure polarization entangled state, and allows manipulating single photon bandwidths that can be chosen at will over five orders of magnitude, ranging from tens of MHz to several THz. We achieve excellent entanglement fidelities for particular spectral bandwidths, i.e. 25 MHz, 540 MHz and 100 GHz, proving the relevance of our approach. Our scheme stands as an ideal candidate for a wide range of network applications, ranging from dense division multiplexing quantum key distribution to heralded optical quantum memories and repeaters.
Studies on telecom-band entangled photon-pair sources for entanglement distribution have so far focused on their narrowband operations. Fiber-based sources are seriously limited by spontaneous Raman scattering while sources based on quasi-phase-matched crystals or waveguides are usually narrowband because of long device lengths and/or operations far from degeneracy. An entanglement distributor would have to multiplex many such narrowband sources before entanglement distribution to fully utilize the available fiber transmission bandwidth. In this work, we demonstrate a broadband source of polarization-entangled photon-pairs suitable for wavelength-multiplexed entanglement distribution over optical fiber. We show that our source is potentially capable of simultaneously supporting up to forty-four independent wavelength channels.