Government Girls General Degree College, Ekbalpur

Violation of Bell's inequality has been the mainspring for secure key generation in an entanglement assisted Quantum Key Distribution (QKD) protocol. Various contributions have relied on the violation of Bell inequalities to build an appropriate QKD protocol. Residing between Bell nonlocality and entanglement, there exists a hybrid trait of correlations, namely correlations exhibited through the violation of steering inequalities. However, such correlations have not been put to use in QKD protocols as much as their stronger counterpart, the Bell violations. In the present work, we show that the violations of the Cavalcanti–Jones–Wiseman–Reid (CJWR) steering inequalities can act as key ingredients in an entanglement assisted QKD protocol. We work with arbitrary two-qubit entangled states, and characterize them by their utility in such protocols. The characterization is based on the quantum bit error rate and violation of the CJWR inequality. Furthermore, we show that subsequent applications of local filtering operations on initially entangled states exhibiting no violation, lead to violations necessary for the successful implementation of the protocol. An additional vindication of our protocol is provided by the use of absolutely Bell–Clauser–Horne–Shimony–Holt (Bell–CHSH) local states, states which remain Bell–CHSH local even under global unitary operations. Quanta 2023; 12: 1–21.

Understanding the resource consumption in distributed scenario is one of the main goals in resource theory. Entanglement swapping is an important example in such scenario. It involves at least three parties. Initially two pairs are entangled and other is separable. Performing Bell measurement on common party, resource is swapped in the separable pair. In resource theory of coherence, the initial separable pair may be in free state or in resource state. This feature leads to development of the idea of coherence swapping and coherence concentration. The latter one is the generalization of the first one. Both the protocols have important role in enhancement of coherence as well as coherence fraction and in reveling resource from a noisy state.

Following an exact relativistic formalism (Ghosh et al. in Ann Phys 312, 398 (2004) [1]), we study the transport properties of dense stellar electron-proton plasma in a strong quantizing magnetic field. The transport coefficients, namely the coefficients of shear and bulk viscosities as well as thermal and electrical conductivities are obtained from the relativistic version of Boltzmann kinetic equation by linearizing the distribution function and using relaxation time approximation. The dependence of the kinetic coefficients on the strength of the magnetic field is discussed. The variation of these coefficients with magnetic fields are found to be insensitive for the field strengths \(\le \) \(10^{17}\)G beyond which decreases with magnetic field. As a consequence, in presence of ultra-strong magnetic field, the electron-proton plasma behaves like a superfluid insulator. Since the electrical conductivity of the medium becomes extremely low (almost zero) in presence of ultra-strong magnetic field, the magnetic field at the core region must, therefore, decay very quickly. Hence, strong magnetic field can not exist at the core of magnetars.

The neutron star low-mass X-ray binary \(4U 1608{-}52\) is known to show kHz QPOs as well as low-frequency noise. The energy dependence of the fractional r.m.s. amplitude of the kHz QPO reflects the underlying radiative mechanism responsible for the QPOs irrespective of their dynamical origin. In this work, we compute the energy dependence for 26 instances of kHz QPO observed by \( {RXTE}\). We typically find as reported before that the r.m.s increases with energy in the slope of \(\sim \) 0.4. This indicates that the variation is in the hot thermal compotonization component and, in particular, the QPO is likely to be driven by variation in the thermal heating rate of the hot plasma. For the same data, we compute the energy-dependent r.m.s. variability of the low-frequency noise component by considering the light curves. In contrast to the behavior seen for the kHz QPO, the energy dependence is nearly flat, i.e., the r.m.s. is energy independent. This indicates that the driver here may be the soft photon source. Thus, the radiative mechanism driving the low-frequency noise and the high-frequency QPO is different in nature.

Experimental demonstration of entanglement needs to have a precise control of experimentalist over the system on which the measurements are performed as prescribed by an appropriate entanglement witness. To avoid such trust problem, recently, device-independent entanglement witnesses (DIEWs) for genuine tripartite entanglement have been proposed where witnesses are capable of testing genuine entanglement without precise description of Hilbert space dimension and measured operators i.e. apparatus are treated as black boxes. Here, we design a protocol for enhancing the possibility of identifying genuine tripartite entanglement in a device independent manner. We consider three mixed tripartite quantum states none of whose genuine entanglement can be detected by applying certain DIEWs, but their genuine tripartite entanglement can be detected by applying the same when distributed in some suitable entanglement swapping network.