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International Telecommunications Symposium (ITS)

Quantum noisy channel modeled by canonical unitary operation in two qubits

Wellington Alves de Brito, Rubens Viana Ramos

Published Sep 1, 2006• 0 citations
Publisher

Abstract

In this work, we analyze the entanglement variation of pure bipartite states during quantum noisy channel propagation. The quantum noisy channel is modeled by the interaction between the bipartite state and the environment through a canonical unitary form of a two-qubit gate. We have found an analytical expression for the entanglement of the bipartite state at the channel output as function of the channel parameters. At last, we used numerical simulation to show that a canonical unitary form of a three qubit gate is not a good model for a quantum noisy channel.

DOI
10.1109/its.2006.4433353

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In this work, we analyze the entanglement variation of pure bipartite states during quantum noisy channel propagation. The quantum noisy channel is modeled by the interaction between the bipartite state and the environment through a canonical unitary form of a two-qubit gate. We have found an analytical expression for the entanglement of the bipartite state at the channel output as function of the channel parameters. At last, we used numerical simulation to show that a canonical unitary form of a three qubit gate is not a good model for a quantum noisy channel.
This paper investigates the change of entanglement for transmitting an arbitrarily entangled two-qubit pure state via one of three typical kinds of noisy quantum channels: amplitude damping quantum channel, phase damping quantum channel and depolarizing quantum channel. It finds, in all these three cases, that the output distant entanglement (measured by concurrence) reduces proportionately with respect to its initial amount, and the decaying ratio is determined only by the noisy characteristics of quantum channels and independent of the form of initial input state.
We investigate the influence of quantum noises on the processes of remote preparation of a qubit by using an Einstein-Podolsky-Rosen （EPR） state and a bipartite partially entangled state as the quantum channel respectively. By solving the master equation in the Lindblad form, we obtain the time evolution of the quantum channels. Then we use the trace distance to describe how close the output state and the initial state are in various noisy conditions. Our results show that the influence of the noise acting on z direction is the weakest, and the influence of the noises acting simultaneously on x, y, and z directions is the strongest.
The entanglement evolution of two qubits under local, single-, and two-sided noisy channels is investigated. It is found that for all pure initial states, the entanglement under a one-sided noisy channel is completely determined by the maximal trace distance which is the main element to construct the measure of non-Markovianity. For the two-sided noisy channel case, when the qubits are initially prepared in a general class of states, either pure or mixed, the entanglement can be expressed as the product of the initial entanglement and the channels' action on the maximally entangled state.
In practice, the interaction between quantum system and environment will lead to the decoherence of entanglement. By quantifying the entanglement through the correlation tensor, we systematically investigate the entanglement robustness of two-qubit states under the four types of noisy channels and corresponding correlated channels. We use an analytical method to find all of the most robust and fragile states under the noisy channels. The puzzling phenomenon that the most robust and fragile states have the same entanglement under noisy channels has been investigated. Moreover, we compare the entanglement robustness of a three-qubit Greenberger-Horne-Zeilinger state with a WWW state. Our method shows the behavior of entanglement under the noisy channels and characterizes the entanglement robustness of the multiqubit system analytically.
This work is divided into two parts. In the first one, the use of the Sagnac interferometer in quantum information is analyzed applying it in three problems: interaction-free measurement, quantum key distribution, and secret sharing. For the interaction-free measurement two systems using Sagnac interferometer were proposed. Considering ideal detectors and loss less devices, one of them has a success probability of 25% for each photon used while the other presents the probability of getting success to detect the presence of the object close to 100% for each photon used. For quantum key distribution, it was proposed a different setup, where the main difference is that the pulse sent by the transmitter does not come back to him/her as happen with the systems based on Sagnac proposed before. This avoids the Trojan horse attack. Finally, it was proposed an optical configuration where it is possible to share a secret among five users, locally distant, that could be used only when all five persons agreed. The second part of this dissertation presents an analytical and numerical study of the entanglement variation of bipartite states of qubits during propagation in a quantum noisy channel. Particularly, it was found an exactly formula which relates the entanglement of states in the input and output of the channel, when the input state is pure and the channel is modeled by a canonical unitary operation.
Present-day quantum processors are open systems in which dissipation and decoherence degrade the information carried by a quantum state as it propagates through a circuit. We study this degradation in dissipative random quantum circuits, modeling each two-qubit gate as a diluted unitary that interpolates between the intended unitary operation and a random dissipative quantum channel. Using the fidelity between the ideal and noisy output states as a diagnostic, we show that sufficiently random unitary gates or dissipative Kraus operators lead to a universal decay of the average fidelity. This decay is determined only by the dissipation strength, system size, and circuit depth, while microscopic details of the gates and noise, including the Kraus rank, appear only in subleading corrections to higher moments. We derive a closed-form expression for the average fidelity in terms of three physically meaningful error parameters, dissipation strength, coherent two-qubit gate-error strength, and connectivity-error probability. Finally, we identify the cases when dissipative effects can be reproduced by an unitary-noise model and when dissipation-induced decoherence remains distinguishable from coherent noise at the level of the average fidelity.