arXiv subjects

Arvind

Publications and source records attributed to Arvind.

At least 91 records · Page 5Linked to original sources

arXiv2018

Revisiting the admissibility of non-contextual hidden variable models in quantum mechanics

We construct a non-contextual hidden variable model consistent with all the kinematic predictions of quantum mechanics (QM). The famous Bell-KS theorem shows that non-contextual models which satisfy a further reasonable restriction are inconsistent with QM. In our construction, we define a weaker variant of this restriction which captures its essence while still allowing a non-contextual description of QM. This is in contrast to the contextual hidden variable toy models, such as the one by Bell, and brings out an interesting alternate way of looking at QM. The results also relate to the Bohmian model, where it is harder to pin down such features.

quant-ph↗

arXiv2018

On `orbital' and `spin' angular momentum of light in classical and quantum theories -- a general framework

We develop a general framework to analyze the two important and much discussed questions concerning (a) `orbital' and `spin' angular momentum carried by light and (b) the paraxial approximation of the free Maxwell system both in the classical as well as quantum domains. After formulating the classical free Maxwell system in the transverse gauge in terms of complex analytical signals we derive expressions for the constants of motion associated with its Poincaré symmetry. In particular, we show that the constant of motion corresponding to the total angular momentum ${\bf J}$ naturally splits into an `orbital' part ${\bf L}$ and a `spin' part ${\bf S}$ each of which is a constant of motion in its own right. We then proceed to discuss quantization of the free Maxwell system and construct the operators generating the Poincaré group in the quantum context and analyze their algebraic properties and find that while the quantum counterparts $\hat{\bf L}$ and $\hat{\bf S}$ of ${\bf L}$ and ${\bf S}$ go over into bona fide observables, they fail to satisfy the angular momentum algebra precluding the possibility of their interpretation as `orbital' and `spin' operators at the classical level. On the other hand $\hat{\bf J}=\hat{\bf L}+ \hat{\bf S}$ does satisfy the angular momentum algebra and together with $\hat{\bf S}$ generates the group $E(3)$. We then present an analysis of single photon states, paraxial quantization both in the scalar as well as vector cases, single photon states in the paraxial regime. All along a close connection is maintained with the Hilbert space $\mathcal{M}$ that arises in the classical context thereby providing a bridge between classical and quantum descriptions of radiation fields.

quant-ph↗

arXiv2018

Experimental demonstration of selective quantum process tomography on an NMR quantum information processor

We present the first NMR implementation of a scheme for selective and efficient quantum process tomography without ancilla. We generalize this scheme such that it can be implemented efficiently using only a set of measurements involving product operators. The method allows us to estimate any element of the quantum process matrix to a desired precision, provided a set of quantum states can be prepared efficiently. Our modified technique requires fewer experimental resources as compared to the standard implementation of selective and efficient quantum process tomography, as it exploits the special nature of NMR measurements to allow us to compute specific elements of the process matrix by a restrictive set of sub-system measurements.To demonstrate the efficacy of our scheme, we experimentally tomograph the processes corresponding to `no operation', a controlled-NOT (CNOT), and a controlled-Hadamard gate on a two-qubit NMR quantum information processor, with high fidelities.

quant-ph↗

arXiv2018

Majorana representation, qutrit Hilbert space and NMR implementation of qutrit gates

We report a study of the Majorana geometrical representation of a qutrit, where a pair of points on a unit sphere represents its quantum states. A canonical form for qutrit states is presented, where every state can be obtained from a one-parameter family of states via $SO(3)$ action. The notion of spin-1 magnetization which is invariant under $SO(3)$ is geometrically interpreted on the Majorana sphere. Furthermore, we describe the action of several quantum gates in the Majorana picture and experimentally implement these gates on a spin-1 system (an NMR qutrit) oriented in a liquid crystalline environment. We study the dynamics of the pair of points representing a qutrit state under various useful quantum operations and connect them to different NMR operations. Finally, using the Gell Mann matrix picture we experimentally implement a scheme for complete qutrit state tomography.

quant-ph↗

arXiv2018

Evolution of tripartite entangled states in a decohering environment and their experimental protection using dynamical decoupling

We embarked upon the task of experimental protection of different classes of tripartite entangled states, namely the maximally entangled GHZ and W states and the ${\rm W \bar{W}}$ state, using dynamical decoupling. The states were created on a three-qubit NMR quantum information processor and allowed to evolve in the naturally noisy NMR environment. Tripartite entanglement was monitored at each time instant during state evolution, using negativity as an entanglement measure. It was found that the W state is most robust while the GHZ-type states are most fragile against the natural decoherence present in the NMR system. The ${\rm W \bar{W}}$ state which is in the GHZ-class, yet stores entanglement in a manner akin to the W state, surprisingly turned out to be more robust than the GHZ state. The experimental data were best modeled by considering the main noise channel to be an uncorrelated phase damping channel acting independently on each qubit, alongwith a generalized amplitude damping channel. Using dynamical decoupling, we were able to achieve a significant protection of entanglement for GHZ states. There was a marginal improvement in the state fidelity for the W state (which is already robust against natural system decoherence), while the ${\rm W \bar{W}}$ state showed a significant improvement in fidelity and protection against decoherence.

quant-ph↗

arXiv2017

BigSparse: High-performance external graph analytics

We present BigSparse, a fully external graph analytics system that picks up where semi-external systems like FlashGraph and X-Stream, which only store vertex data in memory, left off. BigSparse stores both edge and vertex data in an array of SSDs and avoids random updates to the vertex data, by first logging the vertex updates and then sorting the log to sequentialize accesses to the SSDs. This newly introduced sorting overhead is reduced significantly by interleaving sorting with vertex reduction operations. In our experiments on a server with 32GB to 64GB of DRAM, BigSparse outperforms other in-memory and semi-external graph analytics systems for algorithms such as PageRank, BreadthFirst Search, and Betweenness-Centrality for terabyte-size graphs with billions of vertices. BigSparse is capable of highspeed analytics of much larger graphs, on the same machine configuration.

cs.DB↗

arXiv2017

Entanglement and Complete Positivity: Relevance and Manifestations in Classical Scalar Wave Optics

Entanglement of states and Complete Positivity of maps are concepts that have achieved physical importance with the recent growth of quantum information science. They are however mathematically relevant whenever tensor products of complex linear (Hilbert) spaces are involved. We present such situations in classical scalar paraxial wave optics where these concepts play a role: propagation characteristics of coherent and partially coherent Gaussian beams; and the definition and separability of the family of Twisted Gaussian Schell Model (TGSM) beams. In the former, the evolution of the width of a projected one-dimensional beam is shown to be a signature of entanglement in a two-dimensional amplitude. In the latter, the partial transpose operation is seen to explain key properties of TGSM beams.

quant-ph↗

arXiv2017

Efficient experimental design of high-fidelity three-qubit quantum gates via genetic programming

We have designed efficient quantum circuits for the three-qubit Toffoli (controlled-controlled NOT) and the Fredkin (controlled-SWAP) gate, optimized via genetic programming methods. The gates thus obtained were experimentally implemented on a three-qubit NMR quantum information processor, with a high fidelity. Toffoli and Fredkin gates in conjunction with the single-qubit Hadamard gates form a universal gate set for quantum computing, and are an essential component of several quantum algorithms. Genetic algorithms are stochastic search algorithms based on the logic of natural selection and biological genetics and have been widely used for quantum information processing applications. The numerically optimized rf pulse profiles of the three-qubit quantum gates achieve $> 99\%$ fidelity. The optimization was performed under the constraint that the experimentally implemented pulses are of short duration and can be implemented with high fidelity. Therefore the gate implementations do not suffer from the drawbacks of rf offset errors or debilitating effects of decoherence during gate action. We demonstrate the advantage of our pulse sequences by comparing our results with existing experimental schemes.

quant-ph↗

arXiv2017

Quantum key distribution protocol based on contextuality monogamy

The security of quantum key distribution protocols hinges upon features of physical systems that are uniquely quantum in nature. We explore the role of quantumness as qualified by quantum contextuality, in quantum key distribution schemes. We present a new quantum key distribution protocol whose security depends uponthe KCBS inequality and contextuality monogamy. We explicitly calculate the key rate, error rate introduced between Alice-Bob communication due to the presence of Eve, and the information-gain by Eve. This protocol provides a new framework for quantum key distribution which has conceptual and practical advantages over other quantum protocols.

quant-ph↗

arXiv2017

An Operational Framework for Specifying Memory Models using Instantaneous Instruction Execution

There has been great progress recently in formally specifying the memory model of microprocessors like ARM and POWER. These specifications are, however, too complicated for reasoning about program behaviors, verifying compilers etc., because they involve microarchitectural details like the reorder buffer (ROB), partial and speculative execution, instruction replay on speculation failure, etc. In this paper we present a new Instantaneous Instruction Execution (I2E) framework which allows us to specify weak memory models in the same style as SC and TSO. Each instruction in I2E is executed instantaneously and in-order such that the state of the processor is always correct. The effect of instruction reordering is captured by the way data is moved between the processors and the memory non-deterministically, using three conceptual devices: invalidation buffers, timestamps and dynamic store buffers. We prove that I2E models capture the behaviors of modern microarchitectures and cache-coherent memory systems accurately, thus eliminating the need to think about microarchitectural details.

cs.PL↗

arXiv2017

A classical optical approach to the `non-local Pancharatnam-like phases' in Hanbury-Brown-Twiss correlations

We examine a recent proposal to show the presence of nonlocal Pancharatnam type geometric phases in a quantum mechanical treatment of intensity interferometry measurements upon inclusion of polarizing elements in the setup. It is shown that a completely classical statistical treatment of such effects is adequate for practical purposes. Further we show that the phase angles that appear in the correlations, while at first sight appearing to resemble Pancharatnam phases in their mathematical structure, cannot actually be interpreted in that manner. We also describe a simpler Mach-Zehnder type setup where similar effects can be observed without use of the paraxial approximation.

quant-ph↗

arXiv2017

Experimental protection of arbitrary states in a two-qubit subspace by nested Uhrig dynamical decoupling

We experimentally demonstrate the efficacy of a three-layer nested Uhrig dynamical decoupling (NUDD) sequence to preserve arbitrary quantum states in a two-dimensional subspace of the four-dimensional two-qubit Hilbert space, on an NMR quantum information processor. The effect of the state preservation is studied first on four known states, including two product states and two maximally entangled Bell states. Next, to evaluate the preservation capacity of the NUDD scheme, we apply it to eight randomly generated states in the subspace. Although, the preservation of different states varies, the scheme on the average performs very well. The complete tomographs of the states at different time points are used to compute fidelity. State fidelities using NUDD protection are compared with those obtained without using any protection. The nested pulse schemes are complex in nature and require careful experimental implementation.

quant-ph↗

arXiv2017

Global aspects of polarization optics and coset space geometry

We use group theoretic ideas and coset space methods to deal with problems in polarization optics of a global nature. These include the possibility of a globally smooth phase convention for electric fields for all points on the Poincaré sphere, and a similar possibility of real or complex bases of transverse electric vectors for all possible propagation directions. It is shown that these methods help in understanding some known results in an effective manner, and in answering new questions as well. We find that apart from the groups $SU(2)$ and $SO(3)$ which occur naturally in these problems, the group $SU(3)$ also plays an important role.

physics.class-ph↗

arXiv2017

Witnessing nonclassical correlations via a single-shot experiment on an ensemble of spins using NMR

A bipartite quantum system in a mixed state can exhibit nonclassical correlations, which can go beyond quantum entanglement. While quantum discord is the standard measure of quantifying such general quantum correlations, the nonclassicality can be determined by simpler means via the measurement of witness operators. We experimentally construct a positive map to witness nonclassicality of two qubits in an NMR system. The map can be decomposed so that a single run of an experiment on an ensemble of spins suffices to detect the nonclassicality in the state, if present. We let the state evolve in time and use the map to detect nonclassicality as a function of time. To evaluate the efficacy of the witness operator as a means to detect nonclassicality, we measure quantum discord by performing full quantum state tomography at each time point and obtained a fairly good match between the two methods.

quant-ph↗

arXiv2016

In-Storage Embedded Accelerator for Sparse Pattern Processing

We present a novel architecture for sparse pattern processing, using flash storage with embedded accelerators. Sparse pattern processing on large data sets is the essence of applications such as document search, natural language processing, bioinformatics, subgraph matching, machine learning, and graph processing. One slice of our prototype accelerator is capable of handling up to 1TB of data, and experiments show that it can outperform C/C++ software solutions on a 16-core system at a fraction of the power and cost; an optimized version of the accelerator can match the performance of a 48-core server.

cs.AR↗

arXiv2016

Experimentally freezing quantum discord in a dissipative environment using dynamical decoupling

The discovery of the intriguing phenomenon that certain kinds of quantum correlations remain impervious to noise up to a specific point in time and then suddenly decay, has generated immense recent interest. We exploit dynamical decoupling sequences to prolong the persistence of time-invariant quantum discord in a system of two NMR qubits decohering in independent dephasing environments. We experimentally prepare two-qubit Bell-diagonal quantum states that interact with individual dephasing channels and demonstrate the effect of dynamical decoupling on the preservation of both quantum and classical correlations. We are able to freeze quantum discord over long time scales in the presence of noise, using dynamical decoupling. We use robust state-independent dynamical decoupling schemes for state preservation and demonstrate that these schemes are able to successfully preserve quantum discord.

quant-ph↗

arXiv2016

Entanglement detection on an NMR quantum information processor using random local measurements

Random local measurements have recently been proposed to construct entanglement witnesses and thereby detect the presence of bipartite entanglement. We experimentally demonstrate the efficacy of one such scheme on a two-qubit NMR quantum information processor. We show that a set of three random local measurements suffices to detect the entanglement of a general two-qubit state. We experimentally generate states with different amounts of entanglement, and show that the scheme is able to clearly witness entanglement. We perform complete quantum state tomography for each state and compute state fidelity to validate our results. Further, we extend previous results and perform a simulation using random local measurements to optimally detect bipartite entanglement in a hybrid system of $2\otimes 3$ dimensionality.

quant-ph↗

arXiv2016

Validating Simplified Processor Models in Architectural Studies

Cycle-accurate software simulation of multicores with complex microarchitectures is often excruciatingly slow. People use simplified core models to gain simulation speed. However, a persistent question is to what extent the results derived from a simplified core model can be used to characterize the behavior of a real machine. We propose a new methodology of validating simplified simulation models, which focuses on the trends of metric values across benchmarks and architectures, instead of errors of absolute metric values. To illustrate this methodology, we conduct a case study using an FPGA-accelerated cycle-accurate full system simulator. We evaluated three cache replacement polices on a 10-stage in-order core model, and then re-conducted all the experiments by substituting a 1-IPC core model for the 10-stage core model. We found that the 1-IPC core model generally produces qualitatively the same results as the accurate core model except for a few mismatches. We argue that most observed mismatches were either indistinguishable from experimental noise or corresponded to the cases where the policy differences even in the accurate model showed inconclusive results. We think it is fair to use simplified core models to study a feature once the influence of the simplification is understood. Additional studies on branch predictors and scaling properties of multithread benchmarks reinforce our argument. However, the validation of a simplified model requires a detailed cycle-accurate model!

cs.AR↗