Quantum information authentication using entangled states

Einstein-Podolsky-Rosen (EPR)-entangled state light field at a telecommunication wavelength of 1.5 μm is an important quantum source for realizing the continuous variable quantum information processing and some quantum protocols over optical fiber channel. When the EPR-entangled state light field is distributed over the optical fiber channel, the disentanglement is always present because the the EPR entangled state interacts with the fiber channel. It affects the performance of quantum information processing. In this paper, we theoretically calculate the positive partial transposition (PPT) of the entangled state distributed over the optical fiber channel in the single-channel and dual-channel distribution scheme, respectively. Three types of initial entangled light field are considered and analyzed, they being an initial EPR entangled state, an EPR entangled state with asymmetric quadratures, and an EPR entangled state with asymmetric modes. Furthermore, the influence of the extra noise in the optical fiber on the transmission distance of EPR entangled state over the optical fiber channel is investigated. In the single-channel scheme or dual-channel scheme, the extra noise in the optical fiber channel leads the entangled state light field to be disentangled, and the transmission distance of EPR entangled state over the optical fiber channel to decrease rapidly with the increase of the extra noise. For maintaining the robustness of EPR entangled states in lossy optical fiber channels, the dual-channel scheme has more stringent requirements for the correlation quadrature symmetry and purity of the initial entangled state than the single-channel scheme. In the single fiber noise channel scheme, the maximum transmission distance and the robustness of the EPR entangled states with asymmetric modes are not sensitive to the asymmetry between modes. The change of asymmetry between modes does not lead to being disentangled. The maximum transmission distance does not change either. However, the decrease of asymmetry between modes results in the disentanglement in the double fiber noise channels’ scheme. The maximum transmission distance is reduced and the sudden death occurs to the entanglement. The present results will lay a foundation for continuous variables quantum information processing based on optical fiber, such as realizing continuous variables quantum communication over optical fiber and constructing metropolitan quantum network over optical fiber.

This study investigates the dynamics of two-qubit systems in open quantum environments, with a focus on the influence of exceptional points (EPs) and phase angle on entanglement dynamics. Two-qubit systems are pivotal in quantum information science, as they are the simplest system that reveals quantum entanglement, a crucial resource for quantum computing and communication. The research explores the effects of coupling, spontaneous emission, and environment on two-qubit states, specifically the $X$-states, within the framework of the Gorini–Kossakowski–Sudarshan–Lindblad (GKSL) or Lindblad master equation. EPs are unique to non-Hermitian systems. They affect the system’s dynamics significantly when the system approaches EP. Analytical and numerical analyses reveal that third-order EPs occur at specific parameters for two-qubit system, leading to critical changes in eigenvalues and eigenvectors of the Liouvillian superoperator. The study demonstrates that EPs can enhance or suppress entanglement. The concurrence, a quantitative measure of entanglement, peaks around the EPs under certain initial conditions. Furthermore, the impact of phase angle, $\theta$, that parameterizes the relative phase of maximally entangled states is analyzed. For initial states labeled as $\rho_\Psi(\theta,p)$ in this thesis, the phase angle significantly influences concurrence evolution, enabling tunability of entanglement dynamics. In contrast, for other class of initial states labeled as $\rho_\Phi(\theta,p)$, concurrence exhibits phase invariance, ensuring stability across the variations in $\theta$. Besides this, the analysis shows that the maximum concurrence occurs around EP for some states, the phase angle can also be tuned to increase the concurrence for some initial states. These findings highlight the importance of both EPs and phase angle in optimizing entanglement generation and control. This research offers valuable insights into the interplay between non-Hermitian physics and quantum entanglement, paving the way for advancements in quantum technologies. The outcomes provide possible practical implications in the field of quantum computation.

Quantum key distribution protocol of mesh network structure based on n + 1 EPR pairs

Quantum‐state decoherence is the major obstacle to the implementation of quantum computing and quantum information. Here we present an experimental proof of principle for the quantum engineering of decohering interactions. An optically generated Einstein‐Podolski‐Rosen (EPR) entangled state was subjected to severe loss by placing a 50% beam splitter in the path of each entangled light beam. The resulting severe degradation of the signature quantum correlations between the two beams was subsequently alleviated when another, independent EPR state impinged upon the other input ports of the beam splitter, effectively entangling the losses inflicted to the initial EPR state. The additional EPR beam pair was classically coherent with the primary one but had no quantum correlations with it. This result may also find application as a “quantum tap” for entanglement.

In this paper, we consider two related problems, unambiguous discrimination of quantum mixed states and unambiguously conveying classical messages using Einstein-Podolsky-Rosen (EPR) pairs. The main technical contributions are as follows. First, we present a general upper bound on the success probability for unambiguous discrimination among arbitrary $N$ mixed quantum states with given a priori probability, which can be directly used to the unambiguous communication setting. Second, we propose a sufficient condition to show that a new upper bound can be achievable, and construct examples to clarify the relationship between the upper bound we derive and the existing upper bounds presented by other authors. Third, in the issue of conveying classical messages using EPR pairs, we also derive an upper bound on the success probability for unambiguously decoding, where Alice wants to convey $n$ bit classical information to Bob by sending $m$ quantum bits.

We investigate the time evolution of entanglement of various entangled states of a two-qubit system exposed to either thermal or squeezed reservoirs. We show that, except for the vacuum reservoir, the sudden-death of entanglement always exists in the thermal and squeezed reservoirs. We present explicit expression for the sudden-death time of entanglement for various entangled states. We find that the sudden-death of entanglement results from the portion of the double excitation component in the initial entangled state. In this sense, the maximally entangled states of a two-qubit system that do not have the double excitation component is more robust against the quantum fluctuations of the vacuum reservoir.

We study the closest disentangled state to a given entangled state in any system (multi-party with any dimension). We obtain the set of equations the closest disentangled state must satisfy, and show that its reduction is strongly related to the extremal condition of the local filtering on each party. Although the equations we obtain are not still tractable, we find some sufficient conditions for which the closest disentangled state has the same reduction as the given entangled state. Further, we suggest a prescription to obtain a tight upper bound of the relative entropy of entanglement in two-qubit systems.

Einstein-Podolsky-Rosen (EPR) is the building block of entanglement-based and entanglement-assisted quantum communication protocols. Prior shared EPR pair and an authenticated classical channel allow two distant users to share a secret key. To build a network architecture where a centralized EPR source creates entangled states by the process of spontaneous parametric down-conversion (SPDC) then routes the states to users in different access networks. We present a metropolitan optical network (MON) architecture for entanglement distribution in a typical telecommunication infrastructure. The architecture allows simultaneous transmission of classical and quantum signals in the network and provides a dynamic routing mechanism to serve the entire MON. However, the strong launch power of the classical signals impairs the weak quantum signals when they coexist in the same optical fiber. Raman scattering Stokes-shift is the major physical impairment on the higher wavelength quantum signals which, caused by the lower wavelength classical signals. Therefore, we also study the physical impairments in the network to reduce the nonlinear effects and improve the quality of the signals. In our architecture, quantum and classical signals travel in the same optical fiber, but in different spectral bands. We show Raman Stokes-shift wavelength range and the peak power gain when simultaneous transmission of both signals occur in the same optical fiber. Reducing the physical impairments increases the traveling distance of the signals and the number of the access networks in the MON.

Entanglement and polarization of pure, two-qubit systems are known to be constrained by the relationship C2+P2=1 . Here, C stands for concurrence, a standard measure of entanglement, and P refers to the degree of polarization of either of the two, single-qubit subsystems. The above constraint may be understood as reflecting that a system cannot be in a pure state, if it is entangled with another one. In order to explore the connection between entanglement and polarization beyond two-qubit systems, we addressed states in which at least one of the parties has dimensionality greater than two. The case of pure, tripartite states offered itself as a timely candidate. We focussed on three qubits and studied the connection between one qubit’s polarization and its entanglement with the other two taken together. We derived new constraints, akin to the above one, and submitted them to experimental tests using single photons entangled in polarization. The latter provided two qubits, each one attached to one photon. The third qubit was chosen to be the path (momentum) of one of the photons. Our experimental results confirmed the validity of the new constraints. Our theoretical results hold also for classical light. Possible experimental tests could be done with so-called structured light beams.

We propose an efficient approach to prepare Einstein-Podolsky-Rosen (EPR) pairs in currently existing Josephson nanocircuits with capacitive couplings. In these fixed coupling circuits, two-qubit logic gates could be easily implemented while, strictly speaking, single-qubit gates cannot be easily realized. For a known two-qubit state, conditional single-qubit operation could still be designed to evolve only the selected qubit and keep the other qubit unchanged; the rotation of the selected qubit depends on the state of the other one. These conditional single-qubit operations allow us to deterministically generate the well-known Einstein-Podolsky-Rosen pairs, represented by EPR-Bell (or Bell) states. Quantum-state tomography is further proposed to experimentally confirm the generation of these states. The decays of the prepared EPR pairs are analyzed using numerical simulations. Possible application of the generated EPR pairs to test Bell's Inequality is also discussed.

Improvement of entanglement via catalytic quantum scissors

Recently, we introduced and generated new types of two-mode entangled states named “entangled coherent-squeezed states”. In these states, two common states for quantum information processing, coherent state, and squeezed states have been used. Now, based on the generated entangled states, we introduce “two-mode photon-added entangled coherent-squeezed states”. These states are obtained via the iterated action of two creation operators on the two modes of the “entangled coherent-squeezed states”. Next, we study the amount of entanglement of the introduced states using concurrence criterion. In the continuation, some of the nonclassical features such as photon-statistics, second-order correlation function, and quadrature squeezing are considered. In addition, we study the influence of photon-addition of two modes on the mentioned properties of the introduced states. We will observe that the entanglement of the introduced photon-added entangled states increases more rapidly as photon-addition of two modes increases. Moreover, some of the nonclassical features for the first mode of the introduced states such as sub-Poissonian photon-statistics and squeezing in p appear and disappear by photon-addition of two modes, respectively.

Disentanglement in a two-qubit system subjected to dissipative environment: Exact analysis

How to concentrate non-maximally entangled states for quantum communication is a fundamental problem in quantum information. In this paper, we will apply generalized measurements to entanglement concentration of known non-maximally entangled pure states in arbitrary dimensional system. How to design the generalized measurements for the unambiguous discrimination of linearly independent non-orthogonal states is crucial for the concentration of the known non-maximally entangled states. The result shows that, any known non-maximally entangled pure state (for arbitrary dimensional system) can be transformed to the maximally entangled state only by introducing a qubit as ancilla and a joint unitary transformation operation on one of the entangled particles and the ancilla. In addition, because the less entangled state of each fail round will be re-concentrated too, the entanglement waste during the concentration process will be greatly reduced.

CDMA 1x evolution–data optimized (1x EV-DO) is defined in the TIA/EIA IS-856 and IS-835 standards. The security of 1x EV-DO offers authentication, integrity, and encryption capabilities. 1x EV-DO supports authentication of the access terminal (AT) to authorize access to the wireless network, as well as authentication of a subscription to authorize access to the Internet Protocol (IP) network. The system also provides session security via re-authenticating the AT during a session to prevent 1x EV-DO session hijacking and to protect integrity of the user packets. This paper will outline security threats to the 1x EV-DO system, security algorithms, authentication and authorization procedures, and ciphering procedures. The 1x EV-DO security will be detailed here for key exchange, authentication, and encryption functions by supporting the following protocols: security protocol, key exchange protocol, authentication protocol, and encryption protocol. Finally, a summary of 1x EV-DO security evolution will be presented © 2007 Alcatel-Lucent.