Department of Physics

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IIT Jodhpur

This paper reports the development of a sealed-off double-gap pseudospark switch and its performance analysis. The modular pseudospark geometry has two gaps that are separated by a cavity drift space region. It utilizes a single ferroelectric trigger unit in a hollow cathode region of the first gap to initiate the discharge for rapid breakdown of both the gaps. The switch has been pinched for deuterium gas and a gas reservoir has been used for maintaining optimum gas pressure in the sealed-off device. The switch design has been analyzed for higher hold-off voltage with high charge transfer capability at higher repetition rates. The developed switch holds a 50 kV voltage in vacuum, and 40 kV during gas discharge at optimum gas pressure with a peak anode current of 8 kA and pulse duration of 1.3 μs while it is operated with a resistive load of 2.9 Ω. The switch has been tested using a constant current charging power supply for pulse repetition frequency up to 50 Hz at different voltages and peak currents corresponding to 200 nF energy storage capacitors. Its performance has been analyzed in terms of pulse parameters including hold-off voltage, jitter, fall time, rise time, peak current and delay time to qualify the design. © 2018 IOP Publishing Ltd.

World University root node

Plasma Sources Science and Technology

A sealed-off double-gap pseudospark switch and its performance analysis

In quantum key distribution, one conservatively assumes that the eavesdropper Eve is restricted only by physical laws, whereas the legitimate parties, namely the sender Alice and receiver Bob, are subject to realistic constraints, such as noise due to environment-induced decoherence. In practice, Eve too may be bound by the limits imposed by noise, which can give rise to the possibility that decoherence works to the advantage of the legitimate parties. A particular scenario of this type is one where Eve can’t replace the noisy communication channel with an ideal one, but her eavesdropping channel itself remains noiseless. Here, we point out such a situation, where the security of the ping–pong protocol (modified to a key distribution scheme) against a noise-restricted adversary improves under a non-unital noisy channel, but deteriorates under unital channels. This highlights the surprising fact that, contrary to the conventional expectation, noise can be helpful to quantum information processing. Furthermore, we point out that the measurement outcome data in the context of the non-unital channel can’t be simulated by classical noise locally added by the legitimate users. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.

Fulltext

Quantum Information Processing

Decoherence can help quantum cryptographic security

We simulated photovoltaic characteristics of single heterojunction solar cell with Cu2ZnSnS4 and Cu2ZnSnSe4 absorber layer numerically using one dimensional solar cell capacitance simulator (SCAPS-1D). n-CdS/ZnO double buffer layer is used for hetrostructure interfaces with the absorber layer. The cell performance is investigated against variation of different material layer properties such as thickness, carrier concentration, and defect density. First, the performance is optimized for the single junction solar cell with Mo as back contact material with ‘∼5 eV work function. A double junction CZTS/CZTSe tandem cell structure is realized keeping the same material properties as is used in the single CZTS and CZTSe solar cell simulation and flat band condition is considered at the interface for efficient carrier transport under matching current conditions. Tandem cell performance is determined after matching the current condition for top and bottom sub-cells. The CZTS/CZTSe short circuit current density is ∼20.98 mA/cm2 for current matched 211.33 nm thick CZTS top cell in conjunction with 2000 nm bottom cell. The maximum efficiency obtained under the flat band condition at the contact is ∼21.7% with open circuit voltage ∼1.324 V. © 2018 Elsevier B.V.

Optical Materials

Theoretical studies of single and tandem Cu2ZnSn(S/Se)4 junction solar cells for enhanced efficiency

Entropic Leggett–Garg inequality is studied in systems like neutrinos in the context of two and three flavor neutrino oscillations and in neutral Bd, Bs and K mesons. The neutrino dynamics is described with the matter effect taken into consideration. For the decohering B / K meson systems, the effect of decoherence and CP violation have also been taken into account, using the techniques of open quantum systems. Enhancement in the violation with increase in the number of measurements has been found, in consistency with findings in spin-s systems. The effect of decoherence is found to bring the deficit parameter D[ n ] closer to its classical value zero, as expected. The violation of entropic Leggett–Garg inequality lasts for a much longer time in K meson system than in Bd and Bs systems. © 2018, The Author(s).

European Physical Journal C

Entropic Leggett–Garg inequality in neutrinos and B(K) meson systems

Quantum non-Markovianity of a quantum noisy channel is typically identified with information backflow or, more generally, with departure of the intermediate map from complete positivity. But here, we also indicate certain non-Markovian channels that cannot be witnessed by the CP-divisibility criterion. In complex systems, non-Markovianity becomes more involved on account of subsystem dynamics. Here we study various facets of non-Markovian evolution, in the context of coined quantum walks, with particular stress on disambiguating the internal vs. environmental contributions to non-Markovian backflow. For the above problem of disambiguation, we present a general power-spectral technique based on a distinguishability measure such as trace-distance or correlation measure such as mutual information. We also study various facets of quantum correlations in the transition from quantum to classical random walks, under the considered non-Markovian noise models. The potential for the application of this analysis to the quantum statistical dynamics of complex systems is indicated. © 2018 World Scientific Publishing Co. Pte Ltd. All rights reserved.

Open Systems and Information Dynamics

Non-Markovian Evolution: A Quantum Walk Perspective

We introduce a method to construct non-Markovian variants of completely positive (CP) dynamical maps, particularly, qubit Pauli channels. We identify non-Markovianity with the breakdown in CP divisibility of the map, i.e., appearance of a not-completely positive intermediate map. In particular, we consider the case of non-Markovian dephasing in detail. The eigenvalues of the Choi matrix of the intermediate map crossover at a point which corresponds to a singularity in the canonical decoherence rate of the corresponding master equation and thus to a momentary noninvertibility of the map. Thereafter, the rate becomes negative, indicating non-Markovianity. We quantify the non-Markovianity by two methods, one based on CP divisibility [Hall, Phys. Rev. A 89, 042120 (2014)PLRAAN1050-294710.1103/PhysRevA.89.042120], which does not require optimization but requires normalization to handle the singularity, and another method, based on distinguishability [Breuer Phys. Rev. Lett. 103, 210401 (2009)PRLTAO0031-900710.1103/PhysRevLett.103.210401], which requires optimization but is insensitive to the singularity. © 2018 American Physical Society.

Physical Review A

Non-Markovian dephasing and depolarizing channels

Non-Markovian quantum effects are typically observed in systems interacting with structured reservoirs. Discrete-time quantum walks are prime example of such systems in which, quantum memory arises due to the controlled interaction between the coin and position degrees of freedom. Here we show that the information backflow that quantifies memory effects can be enhanced when the particle is subjected to uncorrelated static or dynamic disorder. The presence of disorder in the system leads to localization effects in 1-dimensional quantum walks. We shown that it is possible to infer about the nature of localization in position space by monitoring the information backflow in the reduced system. Further, we study other useful properties of quantum walk such as entanglement, interference and its connection to quantum non-Markovianity. © 2018 The Author(s).

Scientific Reports

Enhanced non-Markovian behavior in quantum walks with Markovian disorder

Zirconium carbonitride (ZrC-ZrN) absorber layer has been optimized for the maximum absorptivity alpha similar to 0.9 in ZrOx/ZrC-ZrN/Zr reflector-absorber tandem structures on stainless steel (SS) and aluminum (Al) substrates, using reactive DC/RF magnetron sputtering process and varying reactive nitrogen during the deposition. The mechanical properties such as hardness and Young's modulus values increase from similar to 19.63 to similar to 38.53 GPa and similar to 36.39 to similar to 58.67 GPa, respectively, with increasing nitrogen flow rate during ZrC-ZrN absorber layer deposition on SS substrate. These values also increase with increasing load up to a critical limit and saturates thereafter with further increase in load values. The films with moderate hardness and Young's modulus exhibit enhanced solar thermal performance (alpha similar to 0.9) against films with lower and higher values of these mechanical properties, suggesting optimal nitrogen concentration for desired solar thermal performance.

Department of Mathematics

Department of Mechanical Engineering

Effect of Growth Condition on Mechanical Properties of Zirconium Carbonitride Absorber-Based Spectrally Selective Coatings

Van der Waals (vdW) heterostructures is a rapidly emerging field that promises to produce on-demand properties for novel optoelectronic devices. Assembly of dissimilar two-dimensional atomic crystals in the vdW heterostructure enables unique features and properties which are fundamentally different from individual 2-dimensional (2D) crystals. Currently, most growth and fabrication methods prohibit large scale, micron-thick and robust heterostructures. An alternative approach is the one based on liquid phase exfoliation allowing the possibility of scalable thin films and composites. Such thin films have inherent and predicted advantages: they can display new behaviors due to their extremely high surface area and, as free-standing laminates, can be manipulated with mixing of nanosheets and other materials for novel device attributes. We use the aforementioned route to prepare spray-coated and few microns thick WS2 and hBN/WS2 heterostructure laminates. A combination of photoluminescence and transmission electron microscope measurements show that, despite the disordered layer stacking inherent to the fabrication process, the laminates preserve the few layer optical response. In particular, the hBN/WS2 heterostructure laminates exhibit a 3-layer average distribution. Using optical pump-terahertz probe (OPTP) measurement to access the photocarrier dynamics and photoconductivity, we study and compare the photocarrier dynamics and photoconductivity of pure WS2 and hBN/WS2 samples. The hBN/WS2 samples show an unusual response that is different from what has been previously reported for pure transition metal dichalcogenides. After photoexcitation, instead of a monotonic decay as in pure WS2, an initial fast decay is followed by a rise of the negative differential terahertz (THz) transmission dominating the dynamics for the following 50 ps. By analyzing the time resolved THz complex photoconductivity, we attribute this effect to the presence of free carriers as well as dipoles at the hBN/WS2 interfaces. As previously reported in hBN/Graphene heterostructures, interfacial dipoles can form along with free carriers at the instant of photoexcitation. Whereas free carriers cause a decrease in the transient THz transmission due to Coulomb screening, dipoles can provide an increase in the pump-induced change in transmission. In terms of complex photoconductivity, free carriers have both real and imaginary components while dipoles probed off-resonance provide almost a purely imaginary response. Our results provide a deeper understanding of the photoconductive response of large van der Waals heterostructure laminates fabricated by liquid phase exfoliation, and will enable their use in future optoelectronic applications. © 2018 IOP Publishing Ltd.

Semiconductor Science and Technology

Terahertz photoconductivity and photocarrier dynamics in few-layer hBN/WS2 van der Waals heterostructure laminates

We propose a scheme to enhance the operational photonic bandwidth exploiting the bandgap overlapping of the same order or different orders through judiciously chosen aperiodic geometries of spatial dimension. To implement the scheme, we design a specialty optical fiber with hybrid chirped-cladding. Our designed fiber helps merge the fundamental bandgap with the first higher-order bandgap and thus provides an ultrawide photonic bandwidth of ∼3 μm. The designed fiber structure also provides a much flatter dispersion profile in comparison with conventional periodic cladding fiber carrying the signature of zero dispersion at ∼2.7 μm. This opens up possibilities in the mid-infrared wavelength regime for bandgap-tunable fiber-based devices. The proposed two-solid-material fiber geometry consisted of thermally compatible low-loss chalcogenide glasses, GeAsSe and AsSe as low and high index, respectively. The supported photonic bandwidth of the fiber covers the entire material low-loss window of the chosen glasses. The efficient delivery of short moderate-energy pulses with significantly less distortion in the temporal as well as spectral domain and enhanced spectral broadening of specific high-energy pulses under certain operating conditions are demonstrated through these specialty fibers. © 2018 Optical Society of America