scholarly journals Investigating Quantum Coherence by Negative Excursions of the Wigner Quasi-Distribution

2019 ◽  
Vol 9 (7) ◽  
pp. 1344 ◽  
Author(s):  
Mauro Ballicchia ◽  
David Ferry ◽  
Mihail Nedjalkov ◽  
Josef Weinbub
Keyword(s):  
Wave Function ◽  
Quantum Interference ◽  
Quantum Coherence ◽  
Quantum Communication ◽  
Electron System ◽  
Physical Problem ◽  
Quantum Superposition ◽  
Negative Part ◽  
Initial Momentum ◽  
The Past

Quantum information and quantum communication are both strongly based on concepts of quantum superposition and entanglement. Entanglement allows distinct bodies, that share a common origin or that have interacted in the past, to continue to be described by the same wave function until evolution is coherent. So, there is an equivalence between coherence and entanglement. In this paper, we show the relation between quantum coherence and quantum interference and the negative parts of the Wigner quasi-distribution, using the Wigner signed-particle formulation. A simple physical problem consisting of electrons in a nanowire interacting with the potential of a repulsive dopant placed in the center of it creates a quasi two-slit electron system that separates the wave function into two entangled branches. The analysis of the Wigner quasi-distribution of this problem establishes that its negative part is principally concentrated in the region after the dopant between the two entangled branches, maintaining the coherence between them. Moreover, quantum interference is shown in this region both in the positive and in the negative part of the Wigner function and is produced by the superposition of Wigner functions evaluated at points of the momentum space that are symmetric with respect to the initial momentum of the injected electrons.

scholarly journals Observing quantum coherence from photons scattered in free-space

2021 ◽  
Vol 10 (1) ◽  
Author(s):  
Shihan Sajeed ◽  
Thomas Jennewein
Keyword(s):  
Free Space ◽  
Quantum Coherence ◽  
Quantum Communication ◽  
Single Photon ◽  
Detector Array ◽  
Line Of Sight ◽  
Background Suppression ◽  
Single Photon Detector ◽  
High Background ◽  
Non Line Of Sight

AbstractQuantum channels in free-space, an essential prerequisite for fundamental tests of quantum mechanics and quantum technologies in open space, have so far been based on direct line-of-sight because the predominant approaches for photon-encoding, including polarization and spatial modes, are not compatible with randomly scattered photons. Here we demonstrate a novel approach to transfer and recover quantum coherence from scattered, non-line-of-sight photons analyzed in a multimode and imaging interferometer for time-bins, combined with photon detection based on a 8 × 8 single-photon-detector-array. The observed time-bin visibility for scattered photons remained at a high 95% over a wide scattering angle range of −450 to +450, while the individual pixels in the detector array resolve or track an image in its field of view of ca. 0.5°. Using our method, we demonstrate the viability of two novel applications. Firstly, using scattered photons as an indirect channel for quantum communication thereby enabling non-line-of-sight quantum communication with background suppression, and secondly, using the combined arrival time and quantum coherence to enhance the contrast of low-light imaging and laser ranging under high background light. We believe our method will instigate new lines for research and development on applying photon coherence from scattered signals to quantum sensing, imaging, and communication in free-space environments.

scholarly journals Quantum Computers - An Emerging Cybersecurity Threat

2021 ◽  
Vol 3 (2) ◽  
Author(s):  
Dajana Jelčić Dubček
Keyword(s):  
Information Processing ◽  
Quantum Coherence ◽  
Quantum Physics ◽  
Quantum Computers ◽  
Practical Implementation ◽  
Quantum Superposition ◽  
Processing And Storage ◽  
And Storage ◽  
Superposition Of States

Quantum computational supremacy may potentially endanger the current cryptographic protection methods. Although quantum computers are still far from a practical implementation in information processing and storage, they should not be overlooked in the context of cybersecurity. Quantum computers operate with qubits - units of information that are governed by the fundamental principles of quantum physics, such as quantum superposition of states and quantum coherence. In order to address the new challenge that quantum computers pose to cybersecurity, the very principles of their operation have to be understood and are overviewed in this contribution.

Quantum Interference

2019 ◽  
pp. 66-88
Author(s):  
Jeffrey A. Barrett
Keyword(s):  
Hilbert Space ◽  
Wave Function ◽  
Quantum Mechanics ◽  
Quantum Interference ◽  
Standard Theory ◽  
Quantum Decoherence ◽  
Entangled States ◽  
Quantum Mechanical ◽  
Standard Formulation ◽  
Complex Valued

Moving to more subtle experiments, we consider how the standard formulation of quantum mechanics predicts and explains interference phenomena. Tracking the conditions under which one observes interference phenomena leads to the notion of quantum decoherence. We see why one must sharply distinguish between collapse phenomena and decoherence phenomena on the standard formulation of quantum mechanics. While collapses explain determinate measurement records, environmental decoherence just produces more complex, entangled states where the physical systems involved lack ordinary physical properties. We characterize the quantum-mechanical wave function as both an element of a Hilbert space and a complex-valued function over a configuration space. We also discuss how the wave function is interpreted in the standard theory.

Density Functional Approaches to Relativistic Quantum Mechanics

2007 ◽  
Author(s):  
Kenneth G. Dyall ◽  
Knut Faegri
Keyword(s):  
Wave Function ◽  
Quantum Mechanics ◽  
Electron Density ◽  
Density Functional ◽  
Relativistic Quantum ◽  
Electron System ◽  
State Density ◽  
Dft Methods ◽  
Particle Wave Function ◽  
Spatial Coordinates

The wave function is an elusive and somewhat mysterious object. Nobody has ever observed the wave function directly: rather, its existence is inferred from the various experiments whose outcome is most rationally explained using a wave function interpretation of quantum mechanics. Further, the N-particle wave function is a rather complicated construction, depending on 3N spatial coordinates as well as N spin coordinates, correlated in a manner that almost defies description. By contrast, the electron density of an N-electron system is a much simpler quantity, described by three spatial coordinates and even accessible to experiment. In terms of the wave function, the electron density is expressed as . . . ρ(r) = N ∫ Ψ* (r1,r2,...,rN)Ψ (r1,r2,...,rN)dr2dr3 ...drN (14.1) . . . where the sum over spin coordinates is implicit. It might be much more convenient to have a theory based on the electron density rather than the wave function. The description would be much simpler, and with a greatly reduced (and constant) number of variables, the calculation of the electron density would hopefully be faster and less demanding. We also note that given the correct ground state density, we should be able to calculate any observable quantity of a stationary system. The answer to these hopes is density functional theory, or DFT. Over the past decade, DFT has become one of the most widely used tools of the computational chemist, and in particular for systems of some size. This success has come despite complaints about arbitrary parametrization of potentials, and laments about the absence of a universal principle (other than comparison with experiment) that can guide improvements in the way the variational principle has led the development of wave-function-based methods. We do not intend to pursue that particular discussion, but we note as a historical fact that many important early contributions to relativistic quantum chemistry were made using DFT-like methods. Furthermore, there is every reason to try to extend the success of nonrelativistic DFT methods to the relativistic domain. We suspect that their potential for conquering a sizable part of this field is at least as large as it has been in the nonrelativistic domain.

TWO-ELECTRON SYSTEM IN THE FIELD OF GENERALIZED SCREENED POTENTIAL

2011 ◽  
Vol 25 (19) ◽  
pp. 1619-1629 ◽  
Author(s):  
ARIJIT GHOSHAL ◽  
Y. K. HO
Keyword(s):  
Wave Function ◽  
Ground State ◽  
Electron System ◽  
Wave Functions ◽  
Expectation Values ◽  
Screening Parameter ◽  
System Ground State ◽  
Highly Correlated ◽  
First Time ◽  
Ground State Energies

Ground states of a two-electron system in generalized screened potential (GSP) with screening parameter λ: [Formula: see text] where ∊ is a constant, have been investigated. Employing highly correlated and extensive wave functions in Ritz's variational principle, we have been able to determine accurate ground state energies and wave functions of a two-electron system for different values of the screening parameter λ and the constant ∊. Convergence of the ground state energies with the increase of the number of terms in the wave function are shown. We also report various geometrical expectation values associated with the system, ground state energies of the corresponding one-electron system and the ionization potentials of the system. Such a calculation for the ground state of a two-electron system in GSP is carried out for first time in the literature.

scholarly journals Space-time from Collapse of the Wave-function

2019 ◽  
Vol 74 (2) ◽  
pp. 147-152 ◽  
Author(s):  
Tejinder P. Singh
Keyword(s):  
Hilbert Space ◽  
Wave Function ◽  
Quantum Theory ◽  
Minkowski Space ◽  
Time Scales ◽  
Quantum Interference ◽  
Quantum Dynamics ◽  
Space Time ◽  
Minkowski Space Time ◽  
Macroscopic Objects

AbstractWe propose that space-time results from collapse of the wave function of macroscopic objects, in quantum dynamics. We first argue that there ought to exist a formulation of quantum theory which does not refer to classical time. We then propose such a formulation by invoking an operator Minkowski space-time on the Hilbert space. We suggest relativistic spontaneous localisation as the mechanism for recovering classical space-time from the underlying theory. Quantum interference in time could be one possible signature for operator time, and in fact may have been already observed in the laboratory, on attosecond time scales. A possible prediction of our work seems to be that interference in time will not be seen for ‘time slit’ separations significantly larger than 100 attosecond, if the ideas of operator time and relativistic spontaneous localisation are correct.

ATOMIC VELOCITY SELECTION BY INTERFERENCE IN TWO-PHOTON IONIZATION

1996 ◽  
Vol 05 (04) ◽  
pp. 911-919
Author(s):  
J.C. GARREAU ◽  
D. WILKOWSKI ◽  
D. HENNEQUIN ◽  
V. ZEHNLÉ
Keyword(s):  
Electromagnetic Field ◽  
Quantum Interference ◽  
Quantum Coherence ◽  
Degrees Of Freedom ◽  
Internal State ◽  
Center Of Mass ◽  
Mass Motion ◽  
Two Photon ◽  
Selective Ionization ◽  
Velocity Selection

This paper discusses a new scheme for generating quantum coherence between different degrees of freedom of an atom interacting with two modes of the electromagnetic field. The presence of quantum interference in a two-photon coupling between the ground state of the atom and the continuum through two quasi-resonant intermediate states induces selective ionization of the atoms for particular combinations of the different parameters characterizing the degrees of freedom of the system, leading to quantum coherence between the internal state, the center-of-mass motion of the atom, and the electromagnetic field. The application of this method to the selection of an atomic velocity class is discussed.

scholarly journals Experimental Test of Tracking the King Problem

Research ◽  
2019 ◽  
Vol 2019 ◽  
pp. 1-6 ◽  
Author(s):  
Cheng-Qiu Hu ◽  
Jun Gao ◽  
Lu-Feng Qiao ◽  
Ruo-Jing Ren ◽  
Zhu Cao ◽  
...  
Keyword(s):  
Experimental Test ◽  
Quantum Communication ◽  
Quantum Mechanical ◽  
Probabilistic Theory ◽  
Classical Physics ◽  
Classical Counterpart ◽  
The Past ◽  
Long Time ◽  
Future Events ◽  
The Mean

In quantum theory, the retrodiction problem is not as clear as its classical counterpart because of the uncertainty principle of quantum mechanics. In classical physics, the measurement outcomes of the present state can be used directly for predicting the future events and inferring the past events which is known as retrodiction. However, as a probabilistic theory, quantum-mechanical retrodiction is a nontrivial problem that has been investigated for a long time, of which the Mean King Problem is one of the most extensively studied issues. Here, we present the first experimental test of a variant of the Mean King Problem, which has a more stringent regulation and is termed “Tracking the King.” We demonstrate that Alice, by harnessing the shared entanglement and controlled-not gate, can successfully retrodict the choice of King’s measurement without knowing any measurement outcome. Our results also provide a counterintuitive quantum communication to deliver information hidden in the choice of measurement.

scholarly journals Quantum Interference in Real-Time Electron-Dynamics: Gaining Insights from Time-Dependent Configuration Interaction Simulations

2019 ◽  
Author(s):  
Raghunathan Ramakrishnan
Keyword(s):  
Configuration Interaction ◽  
Quantum Interference ◽  
Quantum Coherence ◽  
Gold Atom ◽  
Electron Injection ◽  
Time Dependent ◽  
Destructive Interference ◽  
Electron Dynamics ◽  
Initial State ◽  
Donor States

<p>Femtosecond electron dynamics based on time-dependent configuration interaction (TDCI) is a numerically rigorous approach for quantitative modeling of electron-injection across molecular junctions. Our simulations of cyanobenzene thiolates---para- and meta-linked to an acceptor gold atom---corroborate aromatic resonance stabilization effects and show donor states \emph{conjugating} with the benzene $\pi$-network to exhibit superior electron-injection dynamics across the para-linked isomer compared to the meta counterpart. For a \emph{non-conjugating} initial state, we find electron-injection through the meta-channel to stem from non-resonant quantum mechanical tunneling. Furthermore, we demonstrate quantum interference to drive para- vs. meta- selectivity in the coherent evolution of superposed $\pi$(CN)- and $\sigma$(NC-C)-type wavepackets. Analyses reveal that in the para-linked molecule, $\sigma$, and $\pi$ MOs localized at the donor terminal are \emph{in-phase} leading to constructive interference of electron density distribution while phase-flip of one of the MOs in the meta-linked molecule results in destructive interference. The findings reported here suggest that \emph{a priori} detection of orbital phase-flip and quantum coherence conditions can aid in molecular device design strategies.</p><p></p>

Sign in / Sign up

Export Citation Format

Share Document