arXiv subjects

Tabish Qureshi

Publications and source records attributed to Tabish Qureshi.

At least 19 recordsLinked to original sources

arXiv2026

Measuring Entanglement in Qubit System

An operational way of measuring entanglement in a balanced two-path interferometers is presented, where path information is carried by some internal degree of freedom which, in turn, gets entangled with an ancilla system. The analysis is based on a tripartite description involving paths, an internal qubit degree of freedom, and some ancillary states entangled with the internal degree of freedom. It is then applied to two physically distinct experimental situations: a modified Stern-Gerlach interferometer with spin-1/2 particles and a Mach-Zehnder interferometer with photons carrying polarization. The ancilla degree of freedom may not be experimentally accessible. Tracing out the ancillary system, and employing a quantum erasing procedure based on the internal degree of freedom, it is demonstrated that a concurrence-based measure of the entanglement, between the internal degree of freedom and the ancilla, can be extracted directly from measurable asymmetry of the two output channels. These results show that loss of coherence, quantum erasure, and entanglement estimation in interferometric experiments arise from the same underlying correlation structure and provide a compact experimentally accessible framework for quantifying entanglement in qubit systems.

quant-ph↗

arXiv2026

Bohr's complementarity

Quantum complementarity is a fundamental feature of quantum systems and has captivated the physics research community for nearly a century, with significant advancements emerging in recent decades. This review traces the historical evolution of the concept of complementarity, beginning with Bohr's original formulation. It then explores its modern quantification through complementarity relations and its profound connection to the foundational postulates of quantum theory. Furthermore, it delves into key related developments, such as the operational definition of complementarity in the context of incompatible observables, its potential links with quantum uncertainty relations and contextuality, its various applications, and other pertinent topics. This review aims to serve physicists interested in quantum resources, quantum correlations, and the foundational principles of quantum mechanics.

quant-ph↗

arXiv2026

Complementarity Beyond Definite Causal Order

Wave--particle duality is a cornerstone of quantum mechanics, traditionally formulated under definite causal order. We investigate how complementarity is modified when the temporal order of operations is coherently superposed, as in the quantum switch. We show that no universal linear additive complementarity relation exists that simultaneously captures path distinguishability, spatial coherence, and coherence between causal orders. This reveals a fundamental separation between spatial and causal resources, which reside on different subsystems and are therefore not jointly constrained by a single quantum state. While tracing out the order qubit recovers the standard duality relation at the level of the reduced quanton--detector state, coherence between causal orders is not accessible at the level of the reduced description. To capture this contribution, we introduce \emph{causal coherence}, defined as the coherence of the order qubit, which quantifies interference between alternative causal orders and is operationally measurable; we construct explicit processes in which spatial duality is saturated while causal coherence is maximal. We further show that complementarity admits a state-dependent entropic formulation based on incompatible measurements on the causal degree of freedom; unlike generic state-dependent relations, this formulation arises from a universal uncertainty principle and provides a canonical operationally meaningful description. These results establish that complementarity is fundamentally shaped by causal structure and cannot, in general, be fully captured at the level of reduced quantum states alone.

quant-ph↗

arXiv2026

Unifying Quantum and Classical Dynamics

Classical and quantum physics represent two distinct theories; however, quantum physics is regarded as the more fundamental of the two. It is posited that classical mechanics should arise from quantum mechanics under certain limiting conditions. Nevertheless, this remains a challenging objective. In this work, we explore the potential for unifying the dynamics of classical and quantum physics. This discussion does not suggest that classical behavior emerges from quantum mechanics; rather, it demonstrates the exact equivalence between the dynamics of quantum observables and their classical counterparts. It is shown that the Heisenberg equations of motion can be cast in a form that is structurally equivalent to Newton's equations of motion, with $\hbar$ dropping out from the equations. In a generalized analysis, the Heisenberg equations are cast in a form that is identical to the classical Hamilton's equations of motion. This implies that both quantum and classical dynamics are governed by the same form of equations, with the Heisenberg operators substituting the classical observables.

quant-ph↗

arXiv2025

Quantifying Decoherence

Quantum decoherence refers to the phenomenon when the interaction of a quantum system with its environment results in the degradation of quantum coherence. Decoherence is considered to be the most popular mechanism responsible for the emergence of classicality from quantum mechanics. The issue of formulating a measure of decoherence is addressed here. The approach taken here is that decoherence results from the entanglement of a quantum system with certain environment degrees of freedom, and quantifying this entanglement should yield the most natural measure of decoherence. A simple measure of decoherence is presented based on this notion, and it is examined for various example systems. The measure proves to be effective and is relatively straightforward to compute. In addition, a method has been proposed to measure decoherence in a Mach-Zehnder interferometer which may be useful in neutron interferometry.

quant-ph↗

arXiv2025

The Enigma of Delayed Choice Quantum Eraser

The delayed-choice quantum eraser represents an interesting experiment that exemplifies Bohr's principle of complementarity in a beautiful way. According to the complementarity principle, in a two-path interference experiment, the knowledge of which path was taken by the particle and the appearance of interference are mutually exclusive. Even when the which-path information is merely retained in specific quantum path-markers, without being actually read, it suffices to eliminate interference. Nevertheless, if this path information is ``erased'' in some manner, the interference re-emerges, a phenomenon referred to as the quantum eraser. An intriguing aspect of this experiment is that if the path information is erased \emph{after} the particle has been detected on the screen, the interference still reappears, a phenomenon known as the delayed-choice quantum eraser. This observation has led to the interpretation that the particle can be influenced to exhibit characteristics of either a particle or a wave based on a decision made long after it has been registered on the screen. This idea has sparked considerable debate and discussions surrounding retrocausality. This controversy is reviewed here, and a detailed resolution provided.

quant-ph↗

arXiv2025

Characterizing the Generalized Einstein-Podolsky-Rosen State and Extensions

In their seminal paper, Einstein Podolsky and Rosen (EPR) had introduced a momentum entangled state for two particles. That state, referred to as the EPR state, has been widely used in studies on entangled particles with continuous degrees of freedom. Later that state was generalized to a form that allows varying degree of entanglement, known as the generalized EPR state. In a suitable limit it reduces to the EPR state. The generalized EPR state is theoretically analyzed here and its entanglement quantified in terms of a recently introduced generalized entanglement measure. This state can also be applied to entangled photons produced from spontaneous parametric down conversion (SPDC). The present analysis is then used in quantifying the entanglement of photons produced from the SPDC process, in terms of certain experimental parameters. A comparison is also made with the Schmidt number, which is normally used as an entanglement measure in such situations. A procedure for experimentally determining the entanglement of SPDC photons has also been described. Furthermore, an additional state exhibiting non-Gaussian entanglement has been examined, and its entanglement has been quantified.

quant-ph↗

arXiv2025

A Generalized Formulation of Two-Particle Interference

Two-photon interference is an interesting quantum phenomenon that is usually captured in two distinct types of experiments, namely the Hanbury-Brown-Twiss (HBT) experiment and the Hong- Ou-Mandel (HOM) experiment. While the HBT experiment was carried out much earlier in 1956, with classical light, the demonstration of the HOM effect came much later in 1987. Unlike the former, the latter has frequently been argued to be a purely quantum effect. A generalized formulation of two-particle interference is presented here. The HOM and the quantum HBT effects emerge as special cases in the general analysis. A realizable two-particle interference experiment, which is intermediate between the two effects, is proposed and analyzed. Thus two-particle interference is shown to be a single phenomenon with various possible implementations, including the HBT and HOM setups.

quant-ph↗

arXiv2025

Diffusion Limited Aggregation in Ants Biting off Wall Paint

Diffusion limited aggregation (DLA) is a well studied phenomenon in which diffusing particles cumulatively aggregate on a starting fixed seed point, forming a pattern which is fractal in structure. Here we report an interesting DLA process arising from ants biting off wall paint. The structures arising from this long process are analyzed and found to have a fractional dimension between 1 and 2. The DLA formations observed in nature are mostly a result of various kinds of inanimate particles diffusing and aggregating. What makes the present observation interesting is that it involves live beings which may be guided by sight, smell etc. A simple theoretical model of the process is simulated, and the results agree well with the experimental observations.

nlin.PS↗

arXiv2023

Understanding Modified Two-Slit Experiments using Path Markers

Some modified two-slit interference experiments were carried out showing an apparent paradox in wave-particle duality. In a typical such experiment, the screen, where the interference pattern is supposed to be formed, is replaced by a converging lens. The converging lens forms the images of the two slits at two spatially separated detectors. It was claimed that each of these two detectors give information about which slit a photon came from, even though they come from the region of interference. These experiments generated a lot of debate. The various refutations pointed out that the controversial claims involved some questionable assumptions. However the refutations were largely philosophical in nature, and one may like to substantiate those with arguments which are testable, at least in principle. Here such an experiment is theoretically analyzed by introducing path markers which are two orthogonal polarization states of the photon. Analyzing the polarization at the two detectors shows that the photons which give rise to interference, and reach a particular detector, always come from both the slits. This provides clarity in understanding such experiments by making use of testable quantum correlations.

quant-ph↗

arXiv2023

Emergence of Classicality in Stern-Gerlach Experiment via Self-Gravity

Emergence of classicality from quantum mechanics, a hotly debated topic, has had no satisfactory resolution so far. Various approaches including decoherence and gravitational interactions have been suggested. In the present work, the Schrödinger-Newton model is used to study the role of semi-classical self-gravity in the evolution of massive spin-1/2 particles in a Stern-Gerlach experiment. For small mass, evolution of the initial wavepacket in a spin superposition shows a splitting in the magnetic field gradient into two trajectories as in the standard Stern-Gerlach experiment. For larger mass, the deviations from the central path are less than in the standard Stern-Gerlach case, while for high enough mass, the wavepacket does not split, and instead follows the classical trajectory for a magnetic moment in inhomogeneous magnetic field. This indicates the emergence of classicality due to self-gravitational interaction when the mass is increased. In contrast, decoherence which is a strong contender for emergence of classicality, leads to a \emph{mixed state} of two trajectories corresponding to the spin-up and spin-down states, and not the classically expected path. The classically expected path of the particle probably cannot be explained even in the many-worlds interpretation of quantum mechanics. Stern-Gerlach experiments in the macroscopic domain are needed to settle this question.

quant-ph↗

arXiv2022

Characterization of Two-Particle Interference by Complementarity

Bohr's Complementarity Principle is quantitatively formulated in terms of the distinguishability of various paths a quanton can take, and the measure of the interference it produces. This phenomenon results from the interference of single-quanton amplitudes for various paths. The distinguishability of paths puts a bound on the sharpness of the interference the quanton can produce. However there exist other kinds of quantum phenomena where interference of two-particle amplitudes results in a two-particle interference, if the particles are indistinguishable. The Hong-Ou-Mandel (HOM) effect and the Hanbury-Brown-Twiss (HBT) effect are two well known examples. However, two-particle interference is not as easy to define as its single particle counterpart, and the realization that it involves interference of two-particle amplitudes, came much later. In this work, a duality relation, between the particle distinguishability and the visibility of two-particle interference, is derived. The distinguishability of the two particles, arising from some internal degree of freedom, puts a bound on the sharpness of the two-particle interference they can produce, in a HOM or HBT kind of experiment. It is argued that the existence of this kind of complementarity can be used to characterize two-particle interference, which in turn leads one to the conclusion that the HOM and the HBT effects are equivalent in essence, and may be treated as a single two-particle interference phenonmenon.

quant-ph↗

arXiv2022

Testing Gravitational Self-interaction via Matter-Wave Interferometry

The Schrodinger-Newton equation has frequently been studied as a nonlinear modification of the Schrodinger equation incorporating gravitational self-interaction. However, there is no evidence yet as to whether nature actually behaves this way. This work investigates a possible way to experimentally test gravitational self-interaction. The effect of self-gravity on interference of massive particles is studied by numerically solving the Schrodinger-Newton equation for a particle passing through a double-slit. The results show that the presence of gravitational self-interaction has an effect on the fringe width of the interference that can be tested in matter-wave interferometry experiments. Notably, this approach can distinguish between gravitational self-interaction and environment induced decoherence, as the latter does not affect the fringe width. This result will also provide a way to test if gravity requires to be quantized on the scale of ordinary quantum mechanics.

quant-ph↗

arXiv2022

Coherence, Path-Predictability and I-Concurrence: A Triality

It is well known that fringe contrast is not a good quantifier of the wave nature of a quanton in multipath interference. A new interference visibility, based on the Hilbert-Schmidt coherence is introduced. It is demonstrated that this visibility is a good quantifier of wave nature, and can be experimentally measured. A generalized path predictability is introduced, which reduces to the predictability of Greenberger and Yasin, for the case of two paths. In a multipath, which-way interference experiment, the new visibility, the predictability and the I-concurrence (quantifying the entanglement between the quanton and the path-detector), are shown to follow a tight triality relation. It quantifies the essential role that entanglement plays in multipath quantum complementarity, for the first time.

quant-ph↗

arXiv2022

Quantifying Entanglement with Coherence

Quantifying entanglement is a work in progress which is important for the active field of quantum information and computation. A measure of bipartite pure state entanglement is proposed here, named entanglement coherence, which is essentially the normalized coherence of the entangled state in its Schmidt basis. Its value is 1 for maximally entangled states, and 0 for separable states, irrespective of the dimensionality of the Hilbert space. So a maximally entangled state is also the one which is maximally coherent in its Schmidt basis. Quantum entanglement and quantum coherence are thus intimately connected. Entanglement coherence turns out to be closely related to the unified entropy of the reduced state of one of the subsystems. Additionally it is shown that the entanglement coherence is closely connected to the Wigner-Yanase skew information of the reduced density operator of one of the subsystems, in an interesting way.

quant-ph↗

arXiv2021

The Delayed-Choice Quantum Eraser Leaves No Choice

A realizable delayed-choice quantum eraser, using a modified Mach-Zehnder (MZ) interferometer and polarization entangled photons, is theoretically analyzed here. The signal photon goes through a modified MZ interferometer, and the polarization of the idler photon provides path information for the signal photon. The setup is very similar to the delayed-choice quantum eraser experimentally studied by the Vienna group. In the class of quantum erasers with discrete output states, it is easy to see that the delayed mode leaves no choice for the experimenter. The which-way information is always erased, and every detected signal photon fixes the polarization state of the idler, and thus gives information on precisely how the signal photon traversed the two paths. The analysis shows that the Vienna delayed-choice quantum eraser is the first experimental demonstration of the fact that the delayed mode leaves no choice for the experimenter, and the which-way information is always erased. Additionally it is shown that this argument holds even in a conventional two-slit quantum eraser. Every photon registered anywhere on the screen, fixes the state of the two-state which-way detector in a unique mutually unbiased basis. In the delayed-choice quantum eraser experiments, the role of mutually unbiased basis sets for the which-way detector, has been overlooked till now.

quant-ph↗

arXiv2021

Momentum Kicks in Imperfect Which-Way Measurement

There has been an intense debate on the question as to whether a quanton, passing through a double-slit, experiences a 'momentum kick' due to the act of which-way detection. There have been conflicting points of view on this issue over many decades. This issue is addressed here in the general setting where the which-way detection may be imperfect. It is shown here that the loss of interference may still be interpreted as arising out of tiny momentum kicks which the quanton appears to receive, irrespective of the nature of the which-way detector. Interestingly, the magnitude of the random momentum kicks is always h/2d, d being the slit separation, irrespective of how perfect or imperfect the which-way detection is. This is contrary to what has been suggested in the earlier literature. The imperfection of which-way detection decides how frequent are the momentum kicks. It has been shown earlier that for perfect which-way detection, the quanton receives a momentum kick fifty percent of the time. Here it is shown that for imperfect which-way detection, the quanton receives momentum kicks of the same magnitude, but less often. A precise relation between the frequency of kicks and the visibility of interference is found here.

quant-ph↗

arXiv2021

Multipath wave-particle duality with a path detector in a quantum superposition

According to Bohr's principle of complementarity, a quanton can behave either as a wave or a particle, depending on the choice of the experimental setup. Some recent two-path interference experiments have devised methods where one can have a quantum superposition of the two choices, thus indicating that a quanton may be in a superposition of wave and particle nature. These experiments have been of interest from the point of view of Wheeler's delayed-choice experiment. However, it has also been claimed that this experiment can violate complementarity. Here we theoretically analyze a multipath interference experiment that has a which-path detector in a quantum superposition of being present and absent. We show that a tight multipath wave-particle duality relation is respected in all such situations, and complementarity holds well. The apparent violation of complementarity may be due to incorrect evaluation of path distinguishability in such scenarios.

quant-ph↗