Quantum key distribution (QKD) allows two distant parties to share encryption keys with security based on physical laws. Experimentally, QKD has been implemented via optical means, achieving key rates of 1.26 megabits per second over 50 kilometres of standard optical fibre and of 1.16 bits per hour over 404 kilometres of ultralow-loss fibre in a measurement-device-independent configuration . Increasing the bit rate and range of QKD is a formidable, but important, challenge. A related target, which is currently considered to be unfeasible without quantum repeaters, is overcoming the fundamental rate-distance limit of QKD . This limit defines the maximum possible secret key rate that two parties can distil at a given distance using QKD and is quantified by the secret-key capacity of the quantum channel that connects the parties. Here we introduce an alternative scheme for QKD whereby pairs of phase-randomized optical fields are first generated at two distant locations and then combined at a central measuring station. Fields imparted with the same random phase are 'twins' and can be used to distil a quantum key. The key rate of this twin-field QKD exhibits the same dependence on distance as does a quantum repeater, scaling with the square-root of the channel transmittance, irrespective of who (malicious or otherwise) is in control of the measuring station. However, unlike schemes that involve quantum repeaters, ours is feasible with current technology and presents manageable levels of noise even on 550 kilometres of standard optical fibre. This scheme is a promising step towards overcoming the rate-distance limit of QKD and greatly extending the range of secure quantum communications.

Abstract:We demonstrate quantum key distribution (QKD) with bidirectional 10 Gb/s classical data channels in a single fiber using dense wavelength division multiplexing. Record secure key rates of 2.38 Mbps and fiber distances up to 70 km are achieved. Data channels are simultaneously monitored for error-free operation. The robustness of QKD is further demonstrated with a secure key rate of 445 kbps over 25 km, obtained in the presence of data lasers launching conventional 0 dBm power. We discuss the fundamental limit for the QKD performance in the multiplexing environment.

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Quantum key distribution (QKD) is a technology that allows two users to exchange keys securely. The decoy state technique enhances the technology, ensuring keys can be shared at high bit rates over long distances with information theoretic security. However, imperfections in the implementation, known as side-channels, threaten the perfect security of practical QKD protocols. Intensity modulators are required for high-rate decoy state QKD systems, although these are unstable and can display a side channel where the intensity of a pulse is dependent on the previous pulse. Here we demonstrate the superior practicality of a tunable extinction ratio Sagnac-based intensity modulator (IM) for practical QKD systems. The ability to select low extinction ratios, alongside the immunity of Sagnac interferometers to DC drifts, means that random decoy state QKD patterns can be faithfully reproduced with no patterning effects. The inherent stability of Sagnac interferometers also ensures that the modulator output does not wander over time.

The need for an empirically-validated, dimensional system of personality disorders is becoming increasingly apparent. While a number of systems have been investigated in this regard, the fivefactor model of personality has demonstrated the ability to adequately capture personality pathology. In particular, the personality disorder prototypes developed by Lynam and Widiger (2001) have been tested in a number of samples. The goal of the present study is to extend this literature by validating the prototypes in a large, representative community sample of later middleaged adults using both self and informant reports. We found that the prototypes largely work well in this age group. Schizoid, Borderline, Histrionic, Narcissistic, and Avoidant personality disorders demonstrate good convergent validity, with a particularly strong pattern of discriminant validity for the latter four. Informant-reported prototypes show similar patterns to self reports for all analyses. This demonstrates that informants are not succumbing to halo representations of the participants, but are rather describing participants in nuanced ways. Importantly, informant reports add significant predictive validity for Schizoid, Antisocial, Borderline, Histrionic, and Narcissistic personality disorders. Implications of our results and directions for future research are discussed. KeywordsPersonality disorder; prototypes; five-factor model; informant report; validity Over the last few decades, dimensional models of personality disorders (PDs) have garnered a great deal of support and enthusiasm. Of the diagnoses in the Diagnostic and Statistical Manual (DSM-IV; American Psychological Association, 1994), PDs are often considered the most problematic (e.g., in terms of reliability and co-occurrence). However, there are a number of reasons to be hopeful that a dimensional model may yield a dramatic improvement in dealing with these issues. First, evidence has demonstrated that personality pathology is continuously distributed (Livesley, 1998;Widiger, 1992). It appears that, rather than asking whether a PD is or is not present, researchers and clinicians should be asking about the degree to which features are present. Dimensional models of PDs have also gained support through the consistent finding that abnormal personality is highly related to normal personality functioning, which has long been conceptualized as continuous O'Connor, 2002). Finally, Skodol et al. (2005) found that dimensional models of four PDs -Schizotypal, Borderline, Avoidant, and Obsessive-Compulsive PDsaccounted for more variance in functional impairment (e.g., work or interpersonal functioning) than did the DSM-IV categories or simply measures of normal personality. Each Address correspondence to Thomas Oltmanns, Department of Psychology, Washington University, 1 Brookings Drive, Campus Box 1125, St Louis, MO 63130, toltmann@artisci.wustl.edu. NIH Public Access Author ManuscriptPersonal Disord. Author manuscript; available in PMC 2011 December 21. NIH-PA Author ManuscriptN...

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We measured the electro-luminescence of light emitting diodes (LEDs) on substrates with low dislocation densities (LDD) at 10 6 cm −2 and low 10 8 cm −2 , and compared them to LEDs on substrates with high dislocation densities (HDD) closer to 10 10 cm −2. The external quantum efficiencies (EQEs) were fitted using the ABC-model with and without localisation. The non-radiative recombination (NR) coefficient A was constant for HDD LEDs, indicating that the NR was dominated by dislocations at all wavelengths. However, A strongly increased for LDD LEDs by a factor of twenty when increasing the emission wavelength from 440 nm to 540 nm. We attribute this to an increased density of point defects due to the lower growth temperatures used for longer wavelengths. The radiative recombination coefficient B followed the squared wave function overlap for all samples. Using the observed coefficients, we calculated the peak efficiency as a function of the wavelength. For HDD LEDs the change of wave function overlap (i.e. B) is sufficient to reduce the EQE as observed, while for LDD LEDs also the NR coefficient A must increase to explain the observed EQEs. Thus reducing NR is important to improve the EQEs of green LEDs, but this cannot be achieved solely by reducing the dislocation density: point defects must also be addressed.

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