Surfactant-aided exfoliation of molydenum disulphide for ultrafast pulse generation through edge-state saturable absorption

We use liquid phase exfoliation to produce dispersions of molybdenum disulphide (MoS2) nanoflakes in aqueous surfactant solutions. The chemical structures of the bile salt surfactants play a crucial role in the exfoliation and stabilization of MoS2. The resultant MoS2 dispersions are heavily enriched in single and few (<6) layer flakes with large edge to surface area ratio. We use the dispersions to fabricate free-standing polymer composite wide-band saturable absorbers to develop mode-locked and Q- switched fibre lasers, tunable from 1535-1565 and 1030-1070 nm, respectively. We attribute this sub-bandgap optical absorption and its nonlinear saturation behaviour to edge-mediated states introduced within the material band-gap of the exfoliated MoS2 nanoflakes.Comment: 6 pages, 5 figure

Quantified Effects of the Laser Seeding Attack in Quantum Key Distribution

Quantum key distribution (QKD) enables private communications with information-theoretic security. To guarantee the practical security of QKD, it is essential that QKD systems are implemented in accordance to theoretical requirements and robust against side-channel attacks. Here we study a prominent attack on QKD transmitters known as the laser seeding attack (LSA). It consists in injecting photons into the laser of the transmitter in an attempt to modify the outgoing light in some way that is beneficial to the eavesdropper. In this work we measure the response of a QKD transmitter to the LSA as a function of the optical power injected, allowing us to quantify the level of optical attenuation required to mitigate the attack. Further, we employ a laser rate equation model to numerically simulate the effects of the LSA on a gain-switched laser. With this model we are able to reproduce previous experimental results, as well as generate new insight into the LSA by examining the effects of the LSA when the QKD transmitter is operated with different laser current driving parameters

A Hybrid Integrated Quantum Key Distribution Transceiver Chip

Quantum photonic technologies, such as quantum key distribution, are already benefiting greatly from the rise of integrated photonics. However, the flexibility in design of these systems is often restricted by the properties of the integration material platforms. Here, we overcome this choice by using hybrid integration of ultra-low-loss silicon nitride waveguides with indium phosphide electro-optic modulators to produce high-performance quantum key distribution transceiver chips. Access to the best properties of both materials allows us to achieve active encoding and decoding of photonic qubits on-chip at GHz speeds and with sub-1% quantum bit error rates over long fibre distances. We demonstrate bidirectional secure bit rates of 1.82 Mbps over 10 dB channel attenuation and positive secure key rates out to 250 km of fibre. The results support the imminent utility of hybrid integration for quantum photonic circuits and the wider field of photonics.Comment: 13 pages, 5 figures, 1 tabl

Five Years of Mid-Infrared Evolution of the Remnant of SN 1987A: The Encounter Between the Blast Wave and the Dusty Equatorial Ring

We have used the Spitzer satellite to monitor the mid-IR evolution of SN 1987A over a 5 year period spanning the epochs between days 6000 and 8000 since the explosion. The supernova (SN) has evolved into a supernova remnant (SNR) and its radiative output is dominated by the interaction of the SN blast wave with the pre-existing equatorial ring (ER). The mid-IR spectrum is dominated by emission from ~180 K silicate dust, collisionally-heated by the hot X-ray emitting gas with a temperature and density of ~5x10^6 K and 3x10^4 cm-3, respectively. The mass of the radiating dust is ~1.2x10^(-6) Msun on day 7554, and scales linearly with IR flux. The infrared to soft-X-ray flux ratio is roughly constant with a value of 2.5. Gas-grain collisions therefore dominate the cooling of the shocked gas. The constancy of of this ratio suggests that very little grain processing or gas cooling have occurred throughout this epoch. The shape of the dust spectrum remained unchanged during the observations while the total flux increased with a time dependence of t^(0.87), t being the time since the first encounter between the blast wave and the ER. These observations are consistent with the transitioning of the blast wave from free expansion to a Sedov phase as it propagates into the main body of the ER.Comment: Accepted for publication in the ApJ, 11 pages, 11 figure

600 km repeater-like quantum communications with dual-band stabilisation

Twin-field (TF) quantum key distribution (QKD) could fundamentally alter the rate-distance relationship of QKD, offering the scaling of a single-node quantum repeater. Although recent experiments have demonstrated the potential of TF-QKD, formidable challenges remain for its real world use. In particular, new methods are needed to extend both the distance beyond 500 km and key rates above current milli-bit per second values. Previous demonstrations have required intense stabilisation signals at the same wavelength as the quantum channel, thereby unavoidably generating noise due to Rayleigh scattering that limits the distance and bit rate. Here, we introduce a novel dual band stabilisation scheme based on wavelength division multiplexing that allows us to circumvent past limitations. An intense stabilisation signal that is spectrally isolated from the quantum channel is used to reduce the phase drift by three orders of magnitude, while a second, much weaker reference at the quantum wavelength locks the channel phase to a predetermined value. With this strategy, we realise a low noise implementation suitable for all the variants of TF-QKD protocols proposed so far and capable of generating real strings of bits for the first time. The setup provides repeater-like key rates over record communication distances of 555 km and 605 km in the finite-size and asymptotic regimes, respectively, and increases the secure key rate at long distance by two orders of magnitude to values of practical significance.Comment: 14 pages, 5 figures. Methods and supplementary materials are include

Spitzer IRAC Observations of Star Formation in N159 in the LMC

We present observations of the giant HII region complex N159 in the LMC using IRAC on the {\it Spitzer Space Telescope}. One of the two objects previously identified as protostars in N159 has an SED consistent with classification as a Class I young stellar object (YSO) and the other is probably a Class I YSO as well, making these two stars the youngest stars known outside the Milky Way. We identify two other sources that may also be Class I YSOs. One component, N159AN, is completely hidden at optical wavelengths, but is very prominent in the infrared. The integrated luminosity of the entire complex is L $\approx 9\times10^6$L$_{\odot}$, consistent with the observed radio emission assuming a normal Galactic initial mass function (IMF). There is no evidence for a red supergiant population indicative of an older burst of star formation. The N159 complex is 50 pc in diameter, larger in physical size than typical HII regions in the Milky Way with comparable luminosity. We argue that all of the individual components are related in their star formation history. The morphology of the region is consistent with a wind blown bubble $\approx 1-2Myr-old that has initiated star formation now taking place at the rim. Other than its large physical size, star formation in N159 appears to be indistinguishable from star formation in the Milky Way.Comment: 14 figure

Degree of explanation

Partial explanations are everywhere. That is, explanations citing causes that explain some but not all of an effect are ubiquitous across science, and these in turn rely on the notion of degree of explanation. I argue that current accounts are seriously deficient. In particular, they do not incorporate adequately the way in which a cause’s explanatory importance varies with choice of explanandum. Using influential recent contrastive theories, I develop quantitative definitions that remedy this lacuna, and relate it to existing measures of degree of causation. Among other things, this reveals the precise role here of chance, as well as bearing on the relation between causal explanation and causation itself

Investigation of the depolarisation transition in Bi-based relaxor ferroelectrics

The loss of macroscopic polarisation in relaxor ferroelectric (Na0.8K0.2)(1/2)Bi1/2TiO3 ceramics doped with BiZn1/2Ti1/2O3 has been studied by electrical and structural methods. These indicate that the phenomena that are coupled in a displacive phase transition are not necessarily coupled in the depolarisation of Na1/2Bi1/2TiO3-based relaxors and a concept of correlated and uncorrelated switching of dipoles within adjacent unit cells is used to explain this. Second harmonic generation performed on poled ceramics during heating yields values of the freezing temperature and shows a broad temperature range of similar to 100 degrees C across which the structure changes from field-induced ferroelectric to an equilibrium-state ergodic relaxor. Electrical poling at room temperature causes poled regions to increase in size by similar to 2 orders of magnitude. A model illustrating the main steps in thermal depolarisation is described that does not require a phase transition to take place on a unit cell level.open1

600-km repeater-like quantum communications with dual-band stabilization

Twin-field (TF) quantum key distribution (QKD) fundamentally alters the rate-distance relationship of QKD, offering the scaling of a single-node quantum repeater. Although recent experiments have demonstrated the new opportunities for secure long-distance communications allowed by TF-QKD, formidable challenges remain to unlock its true potential. Previous demonstrations have required intense stabilization signals at the same wavelength as the quantum signals, thereby unavoidably generating Rayleigh scattering noise that limits the distance and bit rate. Here, we introduce a dual-band stabilization scheme that overcomes past limitations and can be adapted to other phase-sensitive single-photon applications. Using two different optical wavelengths multiplexed together for channel stabilization and protocol encoding, we develop a setup that provides repeater-like key rates over communication distances of 555 km and 605 km in the finite-size and asymptotic regimes respectively and increases the secure key rate at long distance by two orders of magnitude to values of practical relevance