We demonstrate the first implementation of polarization encoding measurement-deviceindependent quantum key distribution (MDI-QKD), which is immune to all detector side-channel attacks. Active phase randomization of each individual pulse is implemented to protect against attacks on imperfect sources. By optimizing the parameters in the decoy state protocol, we show that it is feasible to implement polarization encoding MDI-QKD over large optical fiber distances. A 1600-bit secure key is generated between two parties separated by 10 km of telecom fibers. Our work suggests the possibility of building a MDI-QKD network, in which complicated and expensive detection system is placed in a central node and users connected to it can perform confidential communication by preparing polarization qubits with compact and low-cost equipment. Since MDI-QKD is highly compatible with the quantum network, our work brings the realization of quantum internet one step closer. Quantum key distribution (QKD) allows two parties, normally referred to as Alice and Bob, to generate a private key even with the presence of an eavesdropper, Eve [1,2]. With perfect single photon sources and single photon detectors, the security of QKD is guaranteed by quantum mechanics [3]. However, the aforementioned perfect devices are not available today and the security of QKD cannot be guaranteed in real life implementation. For example, attenuated coherent laser pulses are commonly used in practical QKD setups, which makes the QKD system vulnerable to the photon number splitting (PNS) attack [4]. Fortunately, it has been shown that the unconditional security of QKD can still be assured with phase randomized weak coherent pulses [5]. Furthermore, by applying decoy state techniques [6], secure key rate can be dramatically increased in practical implementations [7]. Nonetheless, other imperfections in practical QKD systems still present loopholes that can be exploited by Eve to steal the secret key [8,9]. We remark that most of the identified security loopholes are due to imperfections in the detection systems [8].Much effort has been put to build loophole-free QKD systems with practical devices. On one hand, people have been trying to build a better model to understand all the imperfections in a QKD detection system [10], but it is almost impossible to guarantee that all the loopholes have been fixed. On the other hand, full device-independent QKD (DI-QKD) has been proposed to close all the loopholes due to devices' imperfections [11]. The security of DI-QKD relies on the violation of Bell's inequality and does not require any knowledge of how practical QKD devices work. However, the demand for single photon detectors with near unity detection efficiency and the low key rate make this protocol highly impractical [12].Fortunately, measurement-device-independent QKD (MDI-QKD), which removes all loopholes in detec- arXiv:1306.6134v2 [quant-ph]

We present a feasible method that can make quantum key distribution (QKD) both ultra-long-distance and immune to all attacks in the detection system. This method is called measurement-device-independent QKD (MDI-QKD) with entangled photon sources in the middle. By proposing a model and simulating a QKD experiment, we find that MDI-QKD with one entangled photon source can tolerate 77dB loss (367km standard fiber) in the asymptotic limit and 60dB loss (286km standard fiber) in the finite-key case with state-of-the-art detectors. Our general model can also be applied to other non-QKD experiments involving entanglement and Bell state measurements.The global quantum internet 1 is believed to be the next-generation information processing platform promising an exponentially speed-up computation 2 and a secure means of communication. The long-distance distribution of quantum states is a key ingredient for such a global platform, and recently, it has attracted significant scientific attention 3,4 . Among the applications of global quantum internet, quantum key distribution (QKD) 5,6 has been identified as the first technology in quantum information science to reach practical applications. Tremendous effort has been dedicated to creating a global QKD network during the past decade 7-9 . Nonetheless, a reallife QKD network is still limited by two important factors -performance and security.For performance, long-distance QKD remains challenging. In experiment, the maximal transmission distances are 200km through standard telecom fiber for the decoystate BB84 protocol 10,11 and 144km through free space for the entanglement based QKD 12 . In theory, the decoystate BB84 protocol with excellent detectors can tolerate a maximal loss of around 50 dB in the asymptotic limit of an infinitely long key 13 ; in practice, however, the finitekey effect 14 of the data transmission will substantially lower the tolerable loss, e.g., to less than 35 dB 15 . On the other hand, the entanglement based QKD with sophisticated post-processing can in principle tolerate higher losses of 70 dB in the asymptotic limit and around 50 dB in the case of a finite key 16 (also, a more rigorous finite-key analysis will further decrease the tolerable loss to less than 40 dB 15 ). Nevertheless, we remark that all the above schemes are vulnerable to various detector sidechannel attacks (see discussion below).For security, the unconditional security of QKD has been rigorously proved based on the laws of quantum mechanics 17-20 . However, real-life implementations of QKD may contain overlooked imperfections, which are missing in the theoretical model of security proofs. By exa) Electronic mail: feihu.xu@utoronto.ca ploiting these imperfections, especially those in detectors, researchers have demonstrated various quantum attacks including time-shift attack 21 , phase-remapping attack 22 , detector-control attack 23-27 , detector dead-time attack 28 and others 29,30 . These attacks suggest that quantum hacking has become a major problem for the real-life security o...

There is a need to establish in vitro lung alveolar epithelial culture models to better understand the fundamental biological mechanisms that drive lung diseases. While primary alveolar epithelial cells (AEC) are a useful option to study mature lung biology, they have limited utility in vitro. Cells that survive demonstrate limited proliferative capacity and loss of phenotype over the first 3-5 days in traditional culture conditions. To address this limitation, we generated a novel physiologically relevant cell culture system for enhanced viability and maintenance of phenotype. Here we describe a method utilizing e-beam lithography, reactive ion etching, and replica molding to generate poly-dimethylsiloxane (PDMS) substrates containing hemispherical cavities that mimic the architecture and size of mouse and human alveoli. Primary AECs grown on these cavity-containing substrates form a monolayer that conforms to the substrate enabling precise control over cell sheet architecture. AECs grown in cavity culture conditions remain viable and maintain their phenotype over one week. Specifically, cells grown on substrates consisting of 50 μm diameter cavities remained 96 ± 4% viable and maintained expression of surfactant protein C (SPC), a marker of type 2 AEC over 7 days. While this report focuses on primary lung alveolar epithelial cells, our culture platform is potentially relevant and useful for growing primary cells from other tissues with similar cavity-like architecture and could be further adapted to other biomimetic shapes or contours.

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Topographical cues are known to influence cell organization both in native tissues and in vitro. In the trachea, the matrix beneath the epithelial lining is composed of collagen fibres that...

We report on the demonstration of a spot size converter (SSC) for monolithic photonic integration at a wavelength of 850 nm on a GaAs substrate. We designed and fabricated a dual-waveguide AlGaAs chip. The design consists of a lower waveguide layer for efficient end-fire coupling to a single-mode fiber, an upper waveguide layer for high refractive index contrast waveguides, and a vertical SSC to connect the two waveguide layers. We measured a SSC conversion efficiency of 91% (or −0.4 dB) between the upper and lower waveguide layers for the TE mode at a wavelength of 850 nm.

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