Search

Shannon proved in 1949 that information-theoretic-secure encryption is possible if the encryption key is used only once, is random, and is at least as long as the message itself. Notwithstanding, when information is encoded in a quantum system, the phenomenon of quantum data locking allows one to encrypt a message with a shorter key and still provide information-theoretic security. We present one of the first feasible experimental demonstrations of quantum data locking for direct communication and propose a scheme for a quantum enigma machine that encrypts 6 bits per photon (containing messages, new encryption keys, and forward error correction bits) with less than 6 bits per photon of encryption key while remaining information-theoretically secure.

We present a loophole-free violation of local realism using entangled photon pairs. We ensure that all relevant events in our Bell test are spacelike separated by placing the parties far enough apart and by using fast random number generators and high-speed polarization measurements. A high-quality polarization-entangled source of photons, combined with high-efficiency, low-noise, single-photon detectors, allows us to make measurements without requiring any fair-sampling assumptions. Using a hypothesis test, we compute p values as small as 5.9×10^{-9} for our Bell violation while maintaining the spacelike separation of our events. We estimate the degree to which a local realistic system could predict our measurement choices. Accounting for this predictability, our smallest adjusted p value is 2.3×10^{-7}. We therefore reject the hypothesis that local realism governs our experiment.

Local realism is the worldview in which physical properties of objects exist independently of measurement and where physical influences cannot travel faster than the speed of light. Bell's theorem states that this worldview is incompatible with the predictions of quantum mechanics, as is expressed in Bell's inequalities. Previous experiments convincingly supported the quantum predictions. Yet, every experiment requires assumptions that provide loopholes for a local realist explanation. Here, we report a Bell test that closes the most significant of these loopholes simultaneously. Using a well-optimized source of entangled photons, rapid setting generation, and highly efficient superconducting detectors, we observe a violation of a Bell inequality with high statistical significance. The purely statistical probability of our results to occur under local realism does not exceed 3.74×10^{-31}, corresponding to an 11.5 standard deviation effect.

Hong-Ou-Mandel (HOM) interference between independent photon sources (HOMI-IPS) is the fundamental block for quantum information processing. All the previous HOMI-IPS experiments were carried out in time-domain, however, the spectral information during the interference was omitted. Here, we investigate the HOMI-IPS in spectral domain using the recently developed fast fiber spectrometer, and demonstrate the spectral distribution during the HOM interference between two heralded single-photon sources, and two thermal sources. This experiment not only can deepen our understanding of HOMI-IPS from the viewpoint of spectral domain, but also presents a tool to test the theoretical predictions of HOMI-IPS using spectrally engineered sources.

Background and Objectives: This paper analyses why and how conflicts occur and their influence on doctors and nurse-anaesthetists' motivation in the provision of maternal and neonatal health care in a specialist hospital., Methodology: The study used ethnographic methods including participant observation, conversation and in-depth interviews over eleven months in a specialist referral hospital in Ghana. Qualitative analysis software Nvivo 8 was used for coding and analysis of data. Main themes identified in the analysis form the basis for interpreting and reporting study findings., Ethics Statement: Ethical clearance was obtained from the Ghana Health Service Ethics Review board (approval number GHS-ERC:06/01/12) and from the University of Wageningen. Written consent was obtained from interview participants, while verbal consent was obtained for conversations. To protect the identity of the hospital and research participants pseudonyms are used in the article and the part of Ghana in which the study was conducted is not mentioned., Results: Individual characteristics, interpersonal and organisational factors contributed to conflicts. Unequal power relations and distrust relations among doctors and nurse-anaesthetists affected how they responded to conflicts. Responses to conflicts including forcing, avoiding, accommodating and compromising contributed to persistent conflicts, which frustrated and demotivated doctors and nurse-anaesthetists. Demotivated workers exhibited poor attitudes in collaborating with co-workers in the provision of maternal and neonatal care, which sometimes led to poor health worker response to client care, consequently compromising the hospital's goal of providing quality health care to clients., Conclusion: To improve health care delivery in health facilities in Ghana, health managers and supervisors need to identify conflicts as an important phenomenon that should be addressed whenever they occur. Effective mechanisms including training managers and health workers on conflict management should be put in place. Additionally promoting communication and interaction among health workers can foster team spirit. Also resolving conflicts using the collaborating response may help to create a conducive work environment that will promote healthy work relations, which can facilitate the delivery of quality maternal and neonatal health care. However, such an approach requires that unequal power relations, which is a root cause of the conflicts is addressed.

When in-vitro fertilization (IVF) was introduced in the 1970s, doctors were criticized for not properly informing prospective users about its possible risks and limited success rates as well as for medicalizing fertility problems. Nowadays, many fertility clinics are seeking to improve their accountability to stakeholders through patient-centred practices. Based on an ethnographic study of a Dutch fertility clinic, outspoken in its aims to provide patient-centred medicine and to empower clients, this paper addresses how patient-centred medicine affects couples' decision-making to use IVF and related reproductive technologies. The author contends that while patient-centred practices facilitate informed decision-making and support couples emotionally, they may also have unintended disciplining and normalizing effects. The information and support provided, the trust couples have in clinic staff, the ongoing visualization of conception mediated by medical technology--all can be seen as practices that strengthen lay people's 'medical gaze' in how they come to view their bodies, fertility problems and possible solutions. These unintended effects are labelled 'the ambiguity of patient-centeredness' as they (may) interfere with processes of autonomous decision-making.

This article provides insight into how ethically sensitive requests for the use of assisted reproductive technologies (ARTs) are dealt within the daily practice of a Dutch fertility clinic. The findings presented are part of an ethnographic study conducted in this clinic from September 2003 until April 2005. Information for this article was gathered by attending the multidisciplinary ethics meetings and conversations with clinic staff. By looking at 'bioethics in practice', this article provides insight into the complex and dynamic interplay between particular couples' situations, contextual features, bioethical principles, doctors' subjective feelings and views, and the employment of medical practices. Our study suggests that personal views to a certain extent inform the agenda of the ethics meetings, but in the end neither these views nor bioethical principles fully determine the decisions made. Clinic staff members employ routine medical practices with the intention to carefully resolve ethically sensitive cases. These practices include: collegial consultation, searching for scientific evidence in the literature, obtaining more medical information, offering medical tests, referring couples to other clinics and ensuring informed consent. Rather than examining hypothetical cases, which evoke principles, observations of practices regarding real life cases of which many details are known, allowed us to identify the influence of routine medical practice on ethical decisions. Despite initial concerns from the side of the medical professionals (some of which might be regarded as paternalistic), at the end the reproductive autonomy of most couples seeking ARTs was not jeopardized. The format of the multidisciplinary ethics meetings seems to be promising as it provides a space for clinic staff members to express and reflect on their subjective views and feelings of unease regarding certain requests for ARTs, while at the same time it diminishes the risk that decision making is (too heavily) shaped by these feelings and views., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

The integrated optical circuit is a promising architecture for the realization of complex quantum optical states and information networks. One element that is required for many of these applications is a high-efficiency photon detector capable of photon-number discrimination. We present an integrated photonic system in the telecom band at 1550 nm based on UV-written silica-on-silicon waveguides and modified transition-edge sensors capable of number resolution and over 40 % efficiency. Exploiting the mode transmission failure of these devices, we multiplex three detectors in series to demonstrate a combined 79 % ± 2 % detection efficiency with a single pass, and 88 % ± 3 % at the operating wavelength of an on-chip terminal reflection grating. Furthermore, our optical measurements clearly demonstrate no significant unexplained loss in this system due to scattering or reflections. This waveguide and detector design therefore allows the placement of number-resolving single-photon detectors of predictable efficiency at arbitrary locations within a photonic circuit - a capability that offers great potential for many quantum optical applications.

We examine the photon statistics of photon-subtracted thermal light using photon-number-resolved detection. We demonstrate experimentally that the photon number distribution transforms from a Bose-Einstein distribution to a Poisson distribution as the number of subtracted photons increases. We also show that second- and higher-order photon correlation functions can be directly determined from the photon-number-resolved detection measurements of a single optical beam.

The violation of a Bell inequality is an experimental observation that forces the abandonment of a local realistic viewpoint--namely, one in which physical properties are (probabilistically) defined before and independently of measurement, and in which no physical influence can propagate faster than the speed of light. All such experimental violations require additional assumptions depending on their specific construction, making them vulnerable to so-called loopholes. Here we use entangled photons to violate a Bell inequality while closing the fair-sampling loophole, that is, without assuming that the sample of measured photons accurately represents the entire ensemble. To do this, we use the Eberhard form of Bell's inequality, which is not vulnerable to the fair-sampling assumption and which allows a lower collection efficiency than other forms. Technical improvements of the photon source and high-efficiency transition-edge sensors were crucial for achieving a sufficiently high collection efficiency. Our experiment makes the photon the first physical system for which each of the main loopholes has been closed, albeit in different experiments.

Single photons are an important prerequisite for a broad spectrum of quantum optical applications. We experimentally demonstrate a heralded single-photon source based on spontaneous parametric down-conversion in collinear bulk optics, and fiber-coupled bolometric transition-edge sensors. Without correcting for background, losses, or detection inefficiencies, we measure an overall heralding efficiency of 83%. By violating a Bell inequality, we confirm the single-photon character and high-quality entanglement of our heralded single photons which, in combination with the high heralding efficiency, are a necessary ingredient for advanced quantum communication protocols such as one-sided device-independent quantum key distribution.

Typically, transition edge sensors resolve photon number of up to 10 or 20 photons, depending on the wavelength and TES design. We extend that dynamic range up to 1000 photons, while maintaining sub-shot noise detection process uncertainty of the number of detected photons and beyond that show a monotonic response up to ≈ 6 · 10(6) photons in a single light pulse. This mode of operation, which heats the sensor far beyond its transition edge into the normal conductive regime, offers a technique for connecting single-photon-counting measurements to radiant-power measurements at picowatt levels. Connecting these two usually incompatible operating regimes in a single detector offers significant potential for directly tying photon counting measurements to conventional cryogenic radiometric standards. In addition, our measurements highlight the advantages of a photon-number state source over a coherent pulse source as a tool for characterizing such a detector.

Quantum steering allows two parties to verify shared entanglement even if one measurement device is untrusted. A conclusive demonstration of steering through the violation of a steering inequality is of considerable fundamental interest and opens up applications in quantum communication. To date, all experimental tests with single-photon states have relied on post selection, allowing untrusted devices to cheat by hiding unfavourable events in losses. Here we close this 'detection loophole' by combining a highly efficient source of entangled photon pairs with superconducting transition-edge sensors. We achieve an unprecedented ∼62% conditional detection efficiency of entangled photons and violate a steering inequality with the minimal number of measurement settings by 48 s.d.s. Our results provide a clear path to practical applications of steering and to a photonic loophole-free Bell test.

We characterize a periodically poled KTP crystal that produces an entangled, two-mode, squeezed state with orthogonal polarizations, nearly identical, factorizable frequency modes, and few photons in unwanted frequency modes. We focus the pump beam to create a nearly circular joint spectral probability distribution between the two modes. After disentangling the two modes, we observe Hong-Ou-Mandel interference with a raw (background corrected) visibility of 86% (95%) when an 8.6 nm bandwidth spectral filter is applied. We measure second order photon correlations of the entangled and disentangled squeezed states with both superconducting nanowire single-photon detectors and photon-number-resolving transition-edge sensors. Both methods agree and verify that the detected modes contain the desired photon number distributions., (© 2011 Optical Society of America)

Some sort of infertility treatments, including the use of advanced reproductive technologies (ARTs), is nowadays provided at several places in sub-Saharan Africa. Yet, to date only a few studies have actually looked into the way these treatments are offered, used and experienced. In this review article the authors present and discuss empirical study findings that give insight into the way biomedical infertility care is provided, considered, experienced and/or used in sub-Saharan African countries. They concentrate on four themes that were often referred to in the reviewed studies and underline the importance of taking into account the local sociocultural context and notions when developing and implementing infertility care, namely: counselling, male involvement, acceptability of ARTs and the use of donor material (semen and embryos). In the conclusion the authors emphasize the importance of preventing infertility as part of integrated reproductive health programs and the need to improve the quality of (low tech) infertility care in the public health sector by means of standardized guidelines, training of health staff and improved counselling. In addition, from a reproductive rights perspective, they support initiatives to introduce low cost ARTs to treat tubal factor related infertility. They also point to potential unintended side effects of the introduction of ARTs and the use of donor material in the sub-Saharan African context, affecting gender inequity and inequity between citizens from different social classes, and argue that such effects should be acknowledged and avoided by all possible means. Finally, they present an agenda for future social science research on this topic in sub-Saharan Africa.

The longitudinal magneto-optical Kerr effect is used to obtain a calibrated measure of the dynamic magnetization response over the ferromagnetic resonance (FMR) profile for in-plane magnetized Permalloy films excited with high power in-plane transverse microwave fields at 1.25 to 3.75 GHz and in-plane precession angles up to about 20 degrees. The data provide a profound demonstration of the Suhl threshold effect for parametric spin wave generation for angles above about 14 degrees, the magnetization precession lock-up just above threshold, and the complicated response over the full FMR profile at very high powers.

Since the invention of the first magnetic memory disk in 1954, much effort has been put into enhancing the speed, bit density and reliability of magnetic memory devices. In the case of magnetic random access memory (MRAM) devices, fast coherent magnetization rotation by precession of the entire memory cell is desired, because reversal by domain-wall motion is much too slow. In principle, the fundamental limit of the switching speed via precession is given by half of the precession period. However, under-critically damped systems exhibit severe ringing and simulations show that, as a consequence, undesired back-switching of magnetic elements of an MRAM can easily be initiated by subsequent write pulses, threatening data integrity. We present a method to reverse the magnetization in under-critically damped systems by coherent rotation of the magnetization while avoiding any ringing. This is achieved by applying specifically shaped magnetic field pulses that match the intrinsic properties of the magnetic elements. We demonstrate, by probing all three magnetization components, that reliable precessional reversal in lithographically structured micrometre-sized elliptical permalloy elements is possible at switching times of about 200 ps, which is ten times faster than the natural damping time constant.