arXiv subjects

Pankaj Agrawal

Publications and source records attributed to Pankaj Agrawal.

At least 19 recordsLinked to original sources

arXiv2026

Quantifying Entangling Power of Controlled Unitary Gates

Applying controlled unitary gates to generate entanglement between qubits is a routine task in both quantum communication and computation. The existing tools for predicting how much entanglement a given gate can generate require either simulation of the entangling circuit or averaging over a distribution of inputs for a given controlled unitary gate. Here, we introduce a computable quantity $\zeta$ that not only determines whether a controlled unitary gate generates entanglement, but also quantifies the entanglement for any specific input without requiring the construction of the output state. For two-qubit controlled unitary gates, we establish the physical conditions corresponding to the extremum values of the proposed quantity. Extending the dimension of control and target registers to arbitrary size through a generalized controlled unitary architecture, we derive a universal upper bound on $\zeta$ and identify the conditions for its saturation. Later we establish functional relation between the quantity and other known quantities, such as purity, normalized linear entropy, and von Neumann entropy. Finally, we compare our results with previous research work and show that the quantity proposed in this work achieves the previously known optimal values of entangling power of controlled unitary gates for some specific dimensions of target and control registers.

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arXiv2026

Associated Higgs production in lepton-photon collisions at FCC-ee

Following the HL-LHC era, proposed lepton colliders highlight the need to study various important Higgs boson production mechanisms to precisely probe the Standard Model Higgs sector. We propose a novel mechanism $e^\pm \gamma \rightarrow {\bar \nu}_e (\nu_e) H W^\pm$, which can be useful to study Higgs boson properties. This channel is relatively free from the background and can be used to measure the Higgs boson properties, in particular $WWH$ coupling. We examine the viability of this production mechanism. We show that the process can be observed at the planned FCC-ee with the center-of-mass energy of 365 GeV. At the center-of-mass energy of 500 GeV, the process can be observed within a few months of the operation. We use an in-house Monte Carlo event generator that simultaneously incorporates the photon distribution and electron/positron distribution. Our work is also a step towards realistic simulations of lepton- and photon-initiated processes at lepton colliders.

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arXiv2026

Violation of Bell Monogamy Relations

The entangled multipartite systems, specially in pure states, exhibit the phenomenon of entanglement monogamy. Such systems also display the phenomenon of Bell nonlocality. Like entanglement monogamy relations, there are Bell monogamy relations. These relations suggest a sharing of nonlocality across the subsystems. The nonlocality, as characterized by Bell inequalities, of one subsystem limits the nonlocality exhibited by another subsystem. We show that the Bell monogamy relations can be violated by using local filtering operations. We consider permutation-symmetric multipartite pure states, in particular $W$ states, to demonstrate the violation.

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arXiv2025

Deterministic distribution of W-class states in quantum networks

Multipartite entangled states possess a number of non-intuitive properties, making them a useful resource for various quantum information-processing tasks. The three-qubit W-state is one such example where every state is robust to single-qubit loss. However, this state is not suitable for deterministic distribution, and deterministic communication protocols. Here, we focus on the distribution of a non-symmetric version of such states, namely $W_{\mathrm{mod}}$ states. These states belong to the W-class, and have one ebit of entanglement across a specific bipartition, enabling deterministic teleportation and superdense coding. In particular, we describe a few protocols through which these multipartite entangled states can be distributed {\it deterministically} in a quantum network by first preparing them locally in a central node and then transmitting individual qubits to the end nodes. We analyse the performance of these protocols based on the fidelity of the final distributed state, considering all types of noises that can act during the distribution. Finally, we compare the performance of the protocols to the case where the distribution is performed without any central node.

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arXiv2025

Modified security analysis of device-independent quantum key distribution with random key basis

Security analysis is a critical part in any cryptographic protocol, may it be classical or quantum. Without security analysis, one cannot ensure the secrecy of the distributed keys. To perform a conclusive security analysis, it is very often necessary to frame the problem as an optimization problem. However, solving such optimization problems is quite challenging. In this article, we focus on the security analysis of device-independent quantum key distribution (DIQKD) with random key basis protocol. We show that the optimization cost of the existing security analysis can be reduced without compromising the key rate. In particular, we reframe the entire security analysis of this protocol as a strongly convex optimization problem and demonstrate that unlike the original security proof, optimization of Bob's measurement angles for finding a lower bound on Eve's uncertainty about Alice's key generation basis can be done with lesser cost. We derive an explicit form of the pessimistic error that arises while optimizing the measurement angles of both the parties. We also clarify a few parts of the original security proof, making the analysis more rigorous and complete.

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arXiv2025

Effect of anomalous $HHH$ coupling on the decay $H\rightarrow Z\,Z^*\rightarrow$ 4 charged leptons

We have computed the electroweak corrections to $H\rightarrow Z\,Z^*\rightarrow$ 4 charged leptons, including the effect of anomalous $HHH$ coupling in the $\kappa$-framework. The results of this scaling are gauge invariant. We have computed the results for $ H \to e^+ e^- \mu^+ \mu^-$ and $ H \to e^+ e^- e^+ e^-$ processes. The corrections for the both processes depend on the input parameter scheme. In the $G_F$ scheme, the electroweak corrections are about $1.26\%$ for the $ H \to e^+ e^- \mu^+ \mu^-$ and about $0.25\%$ for the $ H \to e^+ e^- e^+ e^-$ process. However changing the $\kappa$ from $4$ to $-4$, the corrections vary from less than $1\%$ to about $-6\%$. We have plotted a number of kinematic distributions. The corrections over most of the phase space regions are similar. These large corrections can be used to put a bound on the $HHH$ coupling. This can help in determining the structure of the Higgs potential.

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arXiv2025

Breaking Quantum Key Distributions under Quantum Switch-Based Attack

Quantum key distribution (QKD) enables secure key sharing between distant parties, with several protocols proven resilient against conventional eavesdropping strategies. Here, we introduce a new attack scenario where an eavesdropper, Eve, exploits a quantum switch using the indefinite causal order to intercept and manipulate quantum communication channel. Using multiple metrics such as the information gain, mutual information, and Bell violation, we demonstrate that the presence of a quantum switch significantly compromises QKD security. Our results highlight a previously overlooked vulnerability, emphasizing the need for countermeasures against quantum-controlled adversarial strategies.

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arXiv2024

Next-to-leading order QCD corrections to $Z\to q\bar{q}\gamma$, $q\bar{q}\gamma\gamma$

We consider the rare decay channels of the $Z$ boson: $Z \to \text{two}\ \textrm{jets} + \gamma$ and $Z \to \text{two}\ \textrm{jets} +2\, \gamma$. To obtain the widths and distributions for these processes, we compute the effect of NLO QCD corrections to the processes $Z \to q {\bar q}+ \gamma$ and $Z \to q {\bar q} +2\, \gamma$. We find that these corrections reduce the widths of these processes by about $6.03\%$ and $12.39\%$, respectively. The reduction in the partial widths is larger at the jet level. These NLO-improved decay observables may be tested in future runs of the LHC or at future $e^{+}e^{-}$ colliders.

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arXiv2024

Effect of anomalous $HHH$ and $ZZHH$ couplings on the decay width of $H\rightarrow ν_e\barν_eν_μ\barν_μ$

Despite the discovery of the Higgs boson, the Higgs sector of the standard model is still not fully established. In particular, the self couplings of the Higgs boson, and its couplings with gauge bosons, are still to be fully determined. We consider electroweak corrections to the process $H\rightarrow ν_e\barν_eν_μ\barν_μ$. The corrections depend on the $HHH$ and $ZZHH$ couplings. We investigate this dependence in $κ$-framework. We find that the width depends on $HHH$ coupling significantly. The dependence on $ZZHH$ coupling is only marginal. We also discuss the dependence on $ZZWW$ coupling.

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arXiv2023

Minimal scenario facet Bell inequalities for multi-qubit states

Facet inequalities play an important role in detecting the nonlocality of a quantum state. The number of such inequalities depends on the Bell test scenario. With the increase in the number of parties, measurement outcomes, or/and the number of measurement settings, there are more nontrivial facet inequalities. For several Bell scenarios, by involving two dichotomic measurement settings for two parties and one dichotomic measurement by other parties, we show that the local polytope has only one nontrivial facet. For three parties, we have three variants of this inequality, depending upon which party is doing one dichotomic measurement. This measurement scenario for a multipartite state may be considered as the minimal scenario involving multipartite correlations that can detect nonlocality. We show that this inequality is violated by all generalized GHZ states. Being the only facet Bell inequality, this inequality is also violated by any entangled three-qubit pure state. We also show that for noisy W states, our inequality is more effective than the well-known Mermin inequality.

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arXiv2022

Inferred-variance uncertainty relations in the presence of quantum entanglement

Uncertainty relations play a significant role in drawing a line between classical physics and quantum physics. Since the introduction by Heisenberg, these relations have been considerably explored. However, the effect of quantum entanglement on uncertainty relations was not probed. Berta et al. [Nature Physics 6, 659-662 (2010)] removed this gap by deriving a conditional-entropic uncertainty relation in the presence of quantum entanglement. In the same spirit, using inferred-variance, we formulate uncertainty relations in the presence of entanglement for general two-qubit systems and arbitrary observables. We derive lower bounds for the sum and product inferred-variance uncertainty relations. Strikingly, we can write the lower bounds of these inferred-variance uncertainty relations in terms of measures of entanglement of two-qubit states, as characterized by concurrence, or $G$ function. Presumably, the presence of entanglement in the lower bound of inferred-variance uncertainty relation is new and unique. We also explore the violation of local uncertainty relations in this context and an interference experiment. Furthermore, we discuss possible applications of these uncertainty relations.

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arXiv2021

Di-vector boson production in association with a Higgs boson at hadron colliders

We consider the production of a Higgs boson in association with two electroweak vector bosons at hadron colliders. In particular, we examine $γγH$, $γZH$, $ZZH$, and $W^{+}W^{-}H$ production at the LHC (14 TeV), HE-LHC (27 TeV), and FCC-hh (100 TeV) colliders. Our main focus is to estimate the gluon-gluon ($gg$) channel ($gg \to VV^\prime H$) contributions to $pp \to VV^\prime H~(V,V^\prime=γ,Z,W)$ and compare them with corresponding contributions arising from the quark-quark ($qq$) channel. Technically, the leading order $gg$ channel contribution to $pp \to VV^\prime H$ cross section is an NNLO correction in $α_s$. In the processes under consideration, we find that in the $gg$ channel, $W^{+}W^{-}H$ has the largest cross section. However, relative contribution of the $gg$ channel is more important for the $pp \to ZZH$ production. At the FCC-hh, $gg \to ZZH$ contribution is comparable with the NLO QCD correction to $qq \to ZZH$. We also compute the cross sections when $W$ and $Z$-bosons are polarized. In the production of $W^{+}W^{-}H$ and $ZZH$, we find that the $gg$ channel contributes more significantly when the vector bosons are longitudinally polarized. By examining such events, one can increase the fraction of the $gg$ channel contribution to these processes. Further, we have studied beyond-the-standard-model effects in the $κ$-framework. We find that the $gg$ channel processes $ZZH$ and $WWH$ have very mild dependence on $κ_λ$, but strong dependence on $κ_t$ and $κ_V$. The $qq$ channel processes mainly depend on $κ_V$. Dependence of the $gg$ channel contribution on $κ_V$ is stronger than that of the $qq$ channel contribution. Therefore focusing on events with longitudinally polarized $W$ and $Z$-bosons, one can find stronger dependence on $κ_V$ that can help us measure this parameter.

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arXiv2021

$W^+W^-H$ production through bottom quarks fusion at hadron colliders

With the standard model working well in describing the collider data, the focus is now on determining the standard model parameters as well as for any hint of deviation. In particular, the determination of the couplings of the Higgs boson with itself and with other particles of the model is important to better understand the electroweak symmetry breaking sector of the model. In this letter, we look at the process $pp \to WWH$, in particular through the fusion of bottom quarks. Due to the non-negligible coupling of the Higgs boson with the bottom quarks, there is a dependence on the $WWHH$ coupling in this process. This sub-process receives largest contribution when the $W$ bosons are longitudinally polarized. We compute one-loop QCD corrections to various final states with polarized $W$ bosons. We find that the corrections to the final state with the longitudinally polarized $W$ bosons are large. It is shown that the measurement of the polarization of the $W$ bosons can be used as a tool to probe the $WWHH$ coupling in this process. We also examine the effect of varying $WWHH$ coupling in the $κ$-framework.

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arXiv2020

Resource state structure for controlled quantum key distribution

Quantum entanglement plays a pivotal role in many communication protocols, like secret sharing and quantum cryptography. We consider a scenario where more than two parties are involved in a protocol and share a multipartite entangled state. In particular, we considered the protocol of Controlled Quantum Key Distribution (CoQKD), introduced in the Ref. Chin. Phys. Lett. 20, 183-185 (2003), where, two parties, Alice and Bob establish a key with the cooperation of other parties. Other parties control/supervise whether Alice and Bob can establish the key, its security and key rate. We discuss the case of three parties in detail and find suitable resource states. We discuss the controlling power of the third party, Charlie. We also examine the usefulness of the new resource states for generating conference key and for cooperative teleportation. We find that recently introduced Bell inequalities can be useful to establish the security of the conference key. We also generalize the scenario to more than three parties.

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arXiv2020

Determining the Shape of Higgs Potential at Future Colliders

Although the Higgs boson has been discovered, its self-couplings are poorly constrained. This leaves the nature of the Higgs boson undetermined. Motivated by different Higgs potential scenarios other than the Landau-Ginzburg type in the standard model, we systematically organize various new physics scenarios -- elementary Higgs, Nambu-Goldstone Higgs, Coleman-Weinberg Higgs, and Tadpole-induced Higgs, etc. We find that double-Higgs production at the 27 TeV high energy LHC can be used to discriminate different Higgs potential scenarios, while it is necessary to use triple-Higgs production at a future 100 TeV proton-proton collider to fully determine the shape of the Higgs potential.

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arXiv2019

Quantum mutual information and quantumness vectors for multi-qubit systems

We introduce a new information theoretic measure of quantum correlations for multiparticle systems. We use a form of multivariate mutual information -- the interaction information and generalize it to multiparticle quantum systems. There are a number of different possible generalizations. We consider two of them. One of them is related to the notion of quantum discord and the other to the concept of quantum dissension. This new measure, called dissension vector, is a set of numbers -- quantumness vector. This can be thought of as a fine-grained measure, as opposed to measures that quantify some average quantum properties of a system. These quantities quantify/characterize the correlations present in multiparticle states. We consider some multiqubit states and find that these quantities are responsive to different aspects of quantumness, and correlations present in a state. We find that different dissension vectors can track the correlations (both classical and quantum), or quantumness only. As physical applications, we find that these vectors might be useful in several information processing tasks. We consider the role of dissension vectors -- (a) in deciding the security of BB84 protocol against an eavesdropper and (b) in determining the possible role of correlations in the performance of Grover search algorithm. Especially, in the Grover search algorithm, we find that dissension vectors can detect the correlations and show the maximum correlations when one expects.

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arXiv2018

Two-qubit mixed states and teleportation fidelity: Purity, concurrence, and beyond

To explore the properties of a two-qubit mixed state, we consider quantum teleportation. The fidelity of a teleported state depends on the resource state purity and entanglement, as characterized by concurrence. Concurrence and purity are functions of state parameters. However, it turns out that a state with larger purity and concurrence, may have comparatively smaller fidelity. By computing teleportation fidelity, concurrence and purity for two-qubit X-states, we show it explicitly. We further show that fidelity changes monotonically with respect to functions of parameters - other than concurrence and purity. A state with smaller concurrence and purity, but larger value of one of these functions has larger fidelity. These functions, thus characterize nonlocal classical and/or quantum properties of the state that are not captured by purity and concurrence alone. In particular, concurrence is not enough to characterize the entanglement properties of a two-qubit mixed state.

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arXiv2018

Distinguishing different classes of entanglement of three-qubit pure states

Employing the Pauli matrices, we have constructed a set of operators, which can be used to distinguish six inequivalent classes of entanglement under SLOCC (stochastic local operation and classical communication) for three-qubit pure states. These operators have very simple structure and can be obtained from the Mermin's operator with suitable choice of directions. Moreover these operators may be implemented in an experiment to distinguish the types of entanglement present in a state. We show that the measurement of only one operator is sufficient to distinguish GHZ class from rest of the classes. It is also shown that it is possible to detect and classify other classes by performing a small number of measurements. We also show how to construct such observables in any basis. We also consider a few mixed states to investigate the usefulness of our operators. Furthermore, we consider the teleportation scheme of Lee et al. (Phys. Rev. A 72, 024302 (2005)) and show that the partial tangles and hence teleportation fidelity can be measured. We have also shown that these partial tangles can also be used to classify genuinely entangled state, biseparable state and separable state.

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