IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

We propose an implementation of quantum key distribution (QKD) protocol using magnon-photon interactions in dielectric waveguides. Starting from the Hamiltonian of spin wave-optical interactions in a symmetric dielectric waveguide, we derive the single-photon transformations that allow us to define two non-orthogonal basis sets, which are isomorphic to polarization basis of single-photon. Implementation of the BB84 and B92 protocols is then straightforward. The principal advantage of our scheme is the higher quantum bit error rate (QBER) (37.5%) compared to polarization-coded scheme (25%) for simple intercept/resend attack. This considerably relaxes the specifications of optical components. Finally, we consider the effect of low conversion efficiency on QBER.

Department of Electrical Engineering

Fulltext

Journal of Physics: Conference Series

Magnon-photon interactions for quantum key distribution

We propose the use of spin wave-optical interactions to implement the BB84 and B92 quantum key distribution (QKD) protocols. Spin waves couple the transverse magnetic (TM) and transverse electric (TE) optical modes of a waveguide. We derive the interaction Hamiltonian to describe the coupling, and solve the resulting equations to determine the time evolution of quantized TM and TE modes. With a judicious choice of coupling coefficient, a set of four nonorthogonal states-formally equivalent to the set of polarization states of a single photon-can be generated. These states form a conjugate basis suitable as a QKD basis set. The proposed scheme can mitigate any medium-induced polarization fluctuations. The spin wave-optical interactions can be used to implement the BB84 protocol using frequency-coded coherent optical states. © 2010 IEEE.

IEEE Journal of Quantum Electronics

Quantum key distribution using spin wave-optical interactions

We describe a system that can temporally separate photons in a multi-photon resolving detector. A time delay, using 239m of optical fibre in a feedback loop sends a part of the incident photon stream back to the system, through a directional coupler. The residual optical power is detected using a gated avalanche photo-detector. The two primary aspects considered while studying this system were the separation of photon pulse in space by the coupler, and the timing aspects of the setup required to achieve synchronization between detector and the source. A timing analysis permits us to verify some properties of the input coherent optical state. ©2010 IEEE.

GENERAL

Proceedings of 16th National Conference on Communications, NCC 2010

Characterizing multi-photon states using gated photo-detection

The quantum bit error rate (QBER) is a measure of the performance of a frequency coded quantum key distribution system. We present a detailed analysis of the system, taking into account the statistics of light at different points in the link. We show that the statistics depends on the choice of phase of both the transmitter and receiver. We also evaluate the effects of crosstalk, out of band noise, and dark count of the gated avalanche photodetector on the QBER. Finally we use QBER to evaluate our implementation of the B92 protocol. © 2009 Elsevier B.V. All rights reserved.

Optics Communications

Bit error rates in a frequency coded quantum key distribution system

We present a quantum mechanical model to evaluate the time evolution of quantum states of light in an electro-optic phase modulator (EOPM). The phase modulator is analogous to a multilevel atomic system with equally spaced energy levels, each of which corresponds to the sidebands of the phase modulated optical field. Using perturbation theory, we solve the equations governing the interaction of the optical field with a modulating field both with and without phase velocity mismatch. A unitary operator that evolves any given quantum state allows us to evaluate the response of EOPM for single photon, coherent and squeezed states. We apply these results to the analysis of frequency coded quantum key distribution scheme. © 2008 IEEE.

Evolution of quantum states in an electro-optic phase modulator

We modify the macroscopic equations proposed by Valet and Fert for current perpendicular to the plane of a magnetic bilayer. Considering different spin bands for the spin-up and spin-down electrons, we calculate the charge accumulation near the interface along with the spin accumulation. This gives rise to a voltage drop at the interface, and causes the resistance to depend on the relative degree of magnetization of the two layers. We obtain expressions for the spatial distribution of charge and the accompanying electric field across the interface between the magnetic layers. An additional resistance caused by the potential step at the interface is added to get a better estimate of the magnetoresistance. ©2009 IEEE.

2009 2nd International Workshop on Electron Devices and Semiconductor Technology, IEDST '09

Charge estimation due to band mismatch in CPP GMR

Spin Waves: Theory and Applications covers topics foundational to understanding spin waves such as the physics of magnetism and electromagnetic waves in anisotropic media, as well as both classical and quantum mechanical treatments of spin wave excitations. The authors discuss many applications including microwave delay lines, spin wave-optical devices, and microwave oscillations induced by spin transfer torques in multilayer structures. The material provides explanations and developments of concepts that assist students and researchers interested in beginning work on topics related to magnetic excitations. The book can be divided into three major parts. The first is comprised of Chapters 1-4 and is concerned with the physics of magnetism and electromagnetic waves in magnetic media. The second part, Chapters 5-8, focuses on magnetostatic modes and dipolar spin waves, their properties, how to excite them, and how they interact with light. Finally, Chapters 9 and 10 treat nonlinear phenomena and advanced applications of spin wave excitations. Spin Waves: Theory and Applications provides an introduction to an active area of research and a handy reference for workers in the field. The material is useful for graduate students in physics, materials science, or electrical engineering as well as active researchers in fundamental and applied magnetism. © Springer Science+Business Media, LLC 2009. All rights reserved.

Spin Waves: Theory and Applications

Spin waves: Theory and applications

We demonstrate a multiple wavelength laser, employing wavelength dependent cavity losses, with up to four lasing wavelengths on the ITU-grid in the C-band and report on its stability characteristics. The linewidth of each wavelength was estimated to be less than 4 pm and we observed antiphase oscillations around 3 kHz and relaxation oscillations around 25 kHz. The antiphase oscillations in the multiwavelength laser lie on a limit cycle in a relative phase diagram. We present an analytic lumped model to determine the output power and relaxation oscillation frequency of the multiple lasing wavelengths in the Erbium doped fiber. The model is validated against our implementation of a multiwavelength ring laser. © 2009 Elsevier B.V. All rights reserved.

Multiwavelength Erbium doped fiber ring lasers

We report an all-fibre interferometric autocorrelation measurement of femtosecond optical pulses in the C-band. An all-fibre Mach-Zehnder interferometer with piezo-electric fiber stretcher in one arm provides the interference signal. A highly nonlinear and dispersion shifted fibre with Kerr coefficient 10.5 /W-km generates an idler by the process of degenerate four wave mixing of the pump and target pulse signals. The idler intensity is proportional to the interferometric autocorrelation of target pulse. We have measured approximately 100 fs optical pulses at 1552.52nm from an actively mode locked erbium doped fibre laser operating at 280 MHz repetition rate.

World University root node

All-fibre autocorrelator for optical pulse width measurement

Fibre interferometry has many advantages, especially in the 1550nm region. In this wavelength region, not only are components easily available, but they are also typically inexpensive. This means that it is easy to have a completely optical system right up to a fibre pigtailed photodetector for detecting the output interference signal. Having such a system means minimum or no alignment of components is necessary. However, a fibre interferometer is far more sensitive to environmental disturbances than a free space one. Even a small movement of the fibre, will result in fluctuation of the interference output, as will thermal fluctuations. It, therefore, becomes important to have some form of stabilisation for the interferometer. Without stabilisation, the phase fluctuates through 2π causing the output intensity to vary through its entire gamut of values. In this paper, initial results on the design and fabrication of a completely automated dual wavelength fibre interferometer will be presented. The ultimate goal is to develop an interferometer, which can be used for surface height measurements in the tens of nm scale. The various parts of the interferometer such as the multi-wavelength erbium doped fibre source developed specifically for this interferometer, the optical calibration that was carried out to arrive at the best configuration, and the electrical control required for the stabilisation, will all be discussed in detail.

17th Symposium on Photonic Measurements 2008, 18th IMEKO TC 2 Symposium on Photonics in Measurements 2008

Development of a fully automated fibre interferometer

Spin torque due to spin polarized tunneling current can be used to switch the free layer in a magnetic tunnel junction (MTJ). This current also gives rise to an Ampere torque, which influences the switching threshold of the MTJ. We modified the Landau-Lifschitz-Gilbert equation (LLGE) to include an Ampere torque term and solved for the magnetization dynamics under the single domain approximation using a linear solver in SPICE. We also extend the model to a square array of MTJs to study the effect of nearest neighbour interactions in addition to effects like demagnetization and magnetostatic interactions with the pinned layer. The interlayer exchange field between the free and pinned layers of a MTJ and the spin torque are competing factors that decide the threshold current density for switching the MTJ. We used a two current model to study the effects of barrier height and barrier thickness on spin torque and exchange energy. We observe that both the spin torque and exchange energy decrease with an increase in barrier height (for ferromagnetic coupling) and barrier thickness. We find that the inclusion of Ampere torque causes a reduction in the switching current. Varying the thickness of MgO and Al 2O 3 barriers allows us to minimize the switching threshold voltage. © 2008 WILEY-VCH Verlag GmbH &amp; Co. KGaA.

Physica Status Solidi (A) Applications and Materials Science

Switching thresholds in MTJ using SPICE model - Effects of spin and Ampere torques

A magnetic tunnel junction, used as a memory element in a magnetic random access memory, can be switched by a spin polarized current flowing through the MTJ. We developed a single domain model for the MTJ and magnetostatically coupled interaction model for an array of MTJs. The simulations are performed using SPICE and we solve the Landau Lifschitz Gilbert equation under the influence of exchange fields, spin torque effects and magnetostatic fields from the pinned layer and neighbouring MTJ elements. The exchange interactions and the spin torque effects are estimated via a two current model. It is observed that the neighbouring elements in a MTJ array increase the maximum current for switching an MTJ by 17.6%. © 2007 IEEE.

Proceedings of the 14th International Workshop on the Physics of Semiconductor Devices, IWPSD

SPICE models for single domain magnetic nano-pillar tunnel junction arrays

Vasomotion refers to spontaneous variations in the lumen size of small vessels, with a plausible role in regulation of various aspects of microcirculation. We propose a computational model of vasomotion in skeletal muscle in which the pattern of vasomotion is shown to critically determine the efficiency of oxygenation of a muscle fiber. In this model, precapillary sphincters are modeled as nonlinear oscillators. We hypothesize that these sphincters interact via exchange of vasoactive substances. As a consequence, vasomotion is described as a phenomenon associated with a network of nonlinear oscillators. As a specific instance, we model the vasomotion of precapillary sphincters surrounding an active fiber. The sphincters coordinate their rhythms so as to minimize oxygen deficit in the fiber. Our modeling studies indicate that efficient oxygenation of the fiber depends crucially on the mode of interaction among the vasomotions of individual sphincters. While chaotic forms of vasomotion enhanced oxygenation, regular patterns of vasomotion failed to meet the oxygenation demand accurately. © 2007 Elsevier Inc. All rights reserved.

Department Of Biotechnology

Microvascular Research

Effect of chaotic vasomotion in skeletal muscle on tissue oxygenation

In this paper, we estimate the stiction force from electrical (current-voltage) measurements on surface micromachined polysilicon cantilever beams. A bias voltage was applied between the beam and the substrate. At the pull-in voltage, the beam collapsed to the substrate and the current rose rapidly from zero. Similarly, at the pull-out voltage during bias sweep back, the current dropped rapidly to zero when the contact between the beam and the substrate was broken. An analytic model for the stiction force was developed in terms of the pull-in and pull-out voltages and was used to estimate a stiction force of about 70 nN from the measured electrical characteristics. This method of characterization is suitable for use in packaged devices. An analytic model was developed to estimate stiction force from optical surface-profile measurements of the curvature of long collapsed cantilever beams in a cantilever-beam array, in the absence of any electrostatic actuation. The force per unit length of about 14 nN/m thus obtained was used to compare the effects of surface roughness on stiction. © 2007 IEEE.

Journal of Microelectromechanical Systems

Estimation of stiction force from electrical and optical measurements on cantilever beams

The magnetization dynamics of a rectangular magnetic thin film are studied using micromagnetic simulations. The film is excited by an inplane external magnetic field that is perpendicular to the easy axis. The dynamics that govern the rotation of the magnetization were extracted assuming quasistatic equilibrium before the external field was changed. The power spectral density (PSD) of the time series, spatially averaged over the sensor geometry, reveal the existence of a fundamental precession frequency and an upper sideband. At a critical angle of rotation, the collision of edge domains causes the excitation of spin waves which appear as multiple peaks in the PSD. The process of collision and formation of domain walls causes the central portion of the sensor to switch abruptly between horizontal and vertical directions. The distance betwen the domain walls before collision becomes a measure of the quiescent state stability of the sensor. We also observe wall displacement waves along the edges of the domains using temporal snapshots of the precession frequency within a micromagnetic cell. © 2007 IEEE.

IEEE Transactions on Magnetics

Incoherent rotation and domain wall collision in rectangular thin magnetic films

The low frequency noise observed in giant magnetoresistive (GMR) sensors exhibited a bistability at certain critical values of bias current. As the sensor oscillated between the two states of a bistable system, the power spectral density (PSD) of the noise showed two peaks; a narrow low frequency peak corresponding to twice the average residence time in a bistable system and a broad high frequency peak corresponding to intrawell vibration. The peaks were visible in the PSD even when the bistability was not pronounced in a time trace. The PSD showed deviations from a Lorentzian behavior indicating that the energy minima and the barrier between them depended on the bias current. A simple analytic model was developed to describe the change in the energy of the system with bias current. The model was based on the physical structure of the sensor and the estimated path of current flow. At particular current induced bias field values, the system was described by multiple energy minima permitting thermal noise excitation of the free layer magnetization. © 2006 American Institute of Physics.

Journal of Applied Physics

Current induced bistability in giant magnetoresistive multilayer thin films

Automatic identification of bird calls without manual intervention has been a challenging task for meaningful research on the taxonomy and monitoring of bird migrations in ornithology. In this paper we apply several techniques used in speech recognition to the automatic identification of bird calls. A new technique which computes the ensemble average on the FFT spectrum is proposed for identification of bird calls. This ensemble average is computed on the FFT spectrum of each bird and is called the Spectral Ensemble Average Voice Print (SEAV) of that particular bird. The SEAV of various birds are computed and are found to be different when compared to each other. A database of bird calls is created from the available recordings of fifteen bird species. The SEAV is then used for the identification of bird calls from this database. The results of identification using SEAV are then compared against the results derived from common classifiers used in speech recognition like dynamic time warping (DTW), Gaussian mixture modeling (GMM). A one level and two level classifier combination is also tried by combining SEAV classifier with the DTW classifier. The SEAV is computationally less expensive when compared to DTW or the GMM based classifiers while performing better than the DTW technique. Several new possibilities in automatic bird call identification using SEAV are also listed.

Department of Computer Science and Engineering

14th European Signal Processing Conference

Automatic identification of bird calls using spectral ensemble average voice prints

An estimate of stiction force, rather than the more commonly reported surface energy, helps design reliable structures. Stiction is a major cause of failure in surface micromachined structures. We report on the modeling and estimation of the stiction force from simple I-V curves on cantilever beams which can be measured even on packaged devices. We have fabricated oxide anchored cantilever beams of polysilicon by surface micromachining. Current is measured for an applied bias between the beam and the substrate. Pull-in and pull-out voltages are determined as the points of maximum slope calculated by differentiating a cubic spline fit to the measured I-V data. The commercial package CoventorWare was used to develop an empirical model for estimating the pull-out voltage for the cases when there is no stiction and in the presence of stiction. A model is developed for finding the stiction force from the simulated and the experimental pull-out voltages. The method uses only measured values of pull-in and pull-out voltages and the beam length and does not require the value of Young's modulus. We also discuss an independent visual method to estimate the process stiction force from the cantilever beam array that is normally used to estimate the surface adhesive energy. An analytical model is developed to calculate the stiction force from the attachment length of long stuck cantilever beams that are released in the same process.

Proceedings of SPIE - The International Society for Optical Engineering

Stiction force estimation from attachment length and electrostatic measurements on cantilever beams

Low signal-to-noise ratios at submicron read-widths is a limiting factor in measurements of the magnetic read-width from a single micro-track. Instead, a number of adjacent microtracks, of decreasing widths were used to generate a series of microtrack profiles. There was a quadratic dependence of the fullwidth at half-maximum of the cross-track profile on the microtrack width. This result was also proved analytically for profiles with a Gaussian and Lorentzian shapes. The asymptotic limit of the quadratic fit yielded the magnetic width of the reader. A further improvement in the characterization method was possible by assuming that the microtracks had sharp edges. With a rectangular profile for the magnetization from the track, the area under the microtrack profile became proportional to the width of the track on the media. A least squares fit of profile width versus area was used to estimate read-width. The method was relatively insensitive to position errors on the spin-stand.

Estimation of submicron read-width in longitudinal magnetic recording

Self-similarity has been observed in various traffic measurements and can be attributed in some instances to the effect of network protocols. We have developed a discrete map for packet transmission using the stop and wait protocol under the limitations of a finite size buffer at the receiver. We have shown that the resultant traffic is self-similar under Poisson distributed input. We then generalized the discrete map for buffer overflow by using it to simulate the self-similarity in TCP traffic. A map of total packets transmitted in a given time interval versus that transmitted in the previous interval provides an easy visualization of self-similarity. The map from a simulation with 10 transmitters captured the salient features of a map from experimental data.