A model independent approach towards resource count and precision limits in a general measurement

A formulation towards quantifying resource count used in a measurement, that is independent of the model of the measurement dynamics(Quantum/Classical), is considered. For any general measurement with $(M+1)$ discrete outcomes, it is found that there is a unique probability distribution that minimizes the measurement error, with the error scaling as $1/M$. For a measurement with a finite resource$(R)$, this absolute bound implies the resource count to be equal to the possible outcomes i.e. $R=M$. This formulation therefore provides a model independent route towards estimating resource count used in any general measurement scheme.Comment: 4+1 pages and supplementary informatio

Nonlinear, anisotropic and giant photoconductivity in intrinsic and doped graphene

We present a framework to calculate the anisotropic and non-linear photoconductivity for two band systems with application to graphene. In contrast to the usual perturbative (second order in the optical field strength) techniques, we calculate photoconductivity to all orders in the optical field strength. In particular, for graphene, we find the photoresponse to be giant (at large optical field strengths) and anisotropic. The anisotropic photoresponse in graphene is correlated with polarization of the incident field, with the response being similar to that of a half-wave plate. We predict that the anisotropy in the simultaneous measurement of longitudinal ($\sigma_{xx}$) and transverse $(\sigma_{yx})$ photoconductivity, with four probes, offers a unique experimental signature of the photo-voltaic response, distinguishing it from the thermal-Seebeck and bolometric effects in photoresponse.Comment: 5 pages, 4 figures. Comments and feedback are invite

Terahertz modulated optical sideband generation in graphene

Exploration of optical non-linear response of graphene predominantly relies on ultra-short time domain measurements. Here we propose an alternate technique that uses frequency modulated continuous wavefront optical fields, thereby probing graphene's steady state non-linear response. We predict frequency sideband generation in the reflected field that originates from coherent electron dynamics of the photo-excited carriers. The corresponding threshold in input intensity for optimal sideband generation provides a direct measure of the third order optical non-linearity in graphene. Our formulation yields analytic forms for the generated sideband intensity, is applicable to generic two-band systems and suggests a range of applications that include switching of frequency sidebands using non-linear phase shifts and generation of frequency combs.Comment: To appear in Phys. Rev.

Decay dynamics in a strongly driven atom-molecule coupled system

Within the framework of master equation, we study decay dynamics of an atom-molecule system strongly coupled by two photoassociation lasers. Summing over the infinite number of electromagnetic vacuum modes that are coupled to the laser-dressed atom-molecule system, we obtain an integro-differential master equation for the the system's reduced density matrix. The equation is numerically solved to describe system dynamics in the presence of decay. In particular, we discuss correlated spontaneous emission from a pair of electronically excited diatomic ro-vibrational states due to their laser induced coupling to the ground continuum of atomic scattering states. This allows us to calculate time-dependence of emitted radiation intensity. It exhibits quantum beats due to coherent dynamics. The phase difference between the two driving fields is found to significantly affect the decay dynamics and the beats. Our results demonstrate the possibility to control decay from the molecular excited states and the decoherence between them by changing the relative intensity and the phase between the lasers. We further show that, if the ground-state continuum has a shape resonance at a low energy, then the quantum beats show two distinctive time scales of oscillations in the strong coupling regime. One of the time scales originates from the energy gap between the two excited states while the other time scale corresponds to the collision energy at which free-bound Franck-Condon overlap is resonantly peaked due to the shape resonance.Comment: 23 pages, 10 figures, accepted in J. Phys. B: At. Mol. Opt. Phy

A comparative study of performance of fpga based mel filter bank & bark filter bank

The sensitivity of human ear is dependent on frequency which is nonlinearly resolved across the audio spectrum .Now to improve the recognition performance in a similar non linear approach requires a front -end design, suggested by empirical evidences. A popular alternative to linear prediction based analysis is therefore filter bank analysis since this provides a much more straightforward route to obtain the desired non-linear frequency resolution. MEL filter bank and BARK filter bank are two popular filter bank analysis techniques. This paper presents FPGA based implementation of MEL filter bank and BARK filter bank with different bandwidths and different signal spectrum ranges. The designs have been implemented using VHDL, simulated and verified using Xilinx 11.1.For each filter bank, the basic building block is implemented in Spartan 3E. A comparative study among these two mentioned filter banks is also done in this paper.Comment: 16 pages, 20 figures, 6 tables; International Journal of Artificial Intelligence & Applications (IJAIA), Vol.3, No.3, May 201

Simplified measurement of the Bell parameter within quantum mechanics

We point out that, if one accepts the validity of quantum mechanics, the Bell parameter for the polarization state of two photons can be measured in a simpler way than by the standard procedure [Clauser, Horne, Shimony, and Holt, Phys. Rev. Lett. 23, 880 (1969)]. The proposed method requires only two measurements with parallel linear-polarizer settings for Alice and Bob at 0 and 45 degrees, and yields a significantly smaller statistical error for a large Bell parameter.Comment: 4 pages, 1 figure. Submitted to Physical Review

A controllable single photon beam-splitter as a node of a quantum network

A model for a controlled single-photon beam-splitter is proposed and analysed. It consists of two crossed optical-cavities with overlapping waists, dynamically coupled to a single flying atom. The system is shown to route a single photon with near-unity efficiency in an effective "weak-coupling" regime. Furthermore, two such nodes, forming a segment of a quantum network, are shown to perform several controlled quantum operations. All one-qubit operations involve a transfer of a photon from one cavity to another in a single node, while two-qubit operations involve transfer from one node to a next one, coupled via an optical fiber. Novel timing protocols for classical optical fields are found to simplify possible experimental realizations along with achievable effective parameter regime. Though our analysis here is restricted to a cavity-QED scenario, basic features of the model can be extended to various other physical systems including gated quantum dots, circuit-QED or opto-mechanical elements.Comment: 24 pages, 7 figure

Quantum counterpart of Measure synchronization: A study on a pair of Harper systems

Measure synchronization is a well-known phenomenon in coupled classical Hamiltonian systems over last two decades. In this paper, synchronization for coupled Harper system is investigated in both classical and quantum contexts. The concept of measure synchronization involves with the phase space and it seems that the measure synchronization is restricted in classical limit. But, on the contrary, here, we have extended the aforesaid synchronization in quantum domain. In quantum context, the coupling occurs between two many body systems via a time and site dependent potential. The coupling leads to the generation of entanglement between the quantum systems. We have used a technique, which is already accepted in the classical domain, in both the contexts to establish a connection between classical and quantum scenarios. Interestingly, results corresponding to both the cases lead to some common features.Comment: 13 pages, 9 figure

Quantum enhanced precision in a collective measurement

We explore the role of $\textit{collective measurements}$ on precision in estimation of a single parameter. Collective measurements are represented by observables which commute with all permutations of the probe particles. We show that with this constraint, quantum bits(qubits) outperform classical bits(non-superposable bits) in optimizing precision. Specifically, we prove that while precision in a collective measurement is loosely bounded by $O\left(\frac{1}{N}\right)$ for $N$ classical bits, using qubits it is tightly bounded by $O\left(\frac{1}{N^2}\right)$. This bound is consistent with quantum metrology protocols with the collective measurement requiring an entangled probe state to saturate. Finally, we construct a canonical measurement protocol that saturates this bound.Comment: 16 Pages, 1 Figure, 2 Table

Nonlinear optical conductivity of a generic two band systems, with application to doped and gapped graphene

We present a general formulation to calculate the dynamic optical conductivity, beyond the linear response regime, of any electronic system whose quasiparticle dispersion is described by a two band model. Our phenomenological model is based on the optical Bloch equations. In the steady state regime it yields an analytic solution for the population inversion and the interband coherence, which are nonlinear in the optical field intensity, including finite doping and temperature effects. We explicitly show that the optical nonlinearities are controlled by a single dimensionless parameter which is directly proportional to the incident field strength and inversely proportional to the optical frequency. This identification leads to a unified way to study the dynamical conductivity and the differential transmission spectrum across a wide range of optical frequencies, and optical field strength. We use our formalism to analytically calculate the nonlinear optical conductivity of doped and gapped graphene, deriving the well known universal ac conductivity of $\sigma_0={e^2}/4\hbar$ in the linear response regime of low optical intensities (or equivalently high frequencies) and non-linear deviations from it which appear at high laser intensities (or low frequencies) including the impact of finite doping and band-gap opening.Comment: 12 pages, 6 figures; Criticism/Comments are welcom