Entanglement Decoherence Research Articles

Q-DDCA: Decentralized Dynamic Congestion Avoid Routing in Large-Scale Quantum Networks

The quantum network that allows users to communicate in a quantum way will be available in the foreseeable future. The network capable of distributing Bell state entangled pairs faces many challenges due to entanglement decoherence and limited network performance, especially when the network scale is enormous. Many entanglement distribution protocols have been proposed so far, and most of them are in a centralized and synchronized manner, which may be infeasible in large-scale networks. As such, in this paper, we propose a full spontaneous version of quantum networks in which the quantum nodes autonomously manage multiple entanglement distribution requests. However, one major issue is that quantum nodes have little knowledge about the network, especially the congestion (e.g., some nodes may have no usable quantum memories). We present a routing algorithm to adaptive evaluate the congestion on the neighbor nodes to avoid potential congestion. We use SimQN, the new network layer simulation platform built by our research team, to evaluate our proposed design. The result demonstrates that it can adapt to changes in network resources and reduce the drop rate that eventually leads to a higher entanglement distribution rate but remains fair for multiple requests to use the network resources fairly and achieve a more balanced throughput.

On the Capacity of the Quantum Switch With and Without Entanglement Decoherence

On the Capacity of the Quantum Switch With and Without Entanglement Decoherence

(Best Paper Award) Aluminum Josephson Junction Formation on 200mm Wafers Using Different Oxidation Techniques

For superconducting quantum circuits with a large number of Qubits, reproducible components are crucial for reducing entanglement decoherence. Particularly for reliable industrial manufacturing on full-scale 200 mm' wafers, a very high uniformity level is required to ensure sufficient coherence times. In the present work the special focus was put on manufacturing Al/AlOx/Al Josephson junctions (JJ), which are the most important component of many quantum circuit. Fully Al-based CMOS-compatible JJ’s were produced using a double dry etch process. After patterning the first Al metallization several oxidation processes have been investigated.Static oxidation has been performed by first removing the native AlOx in a multi-chamber system with Ar milling. The final tunneling oxide was controlled by applying a specific pressure in the chamber under a pure O2 atmosphere. Afterwards, without breaking the vacuum, the second Al metallization has been deposited by sputtering. Oxide thicknesses between 1 and 2.5 nm were achieved. A full mapping of the process homogeneity will be given.On the other hand, a dynamic recipe controlled plasma oxidation process was performed, where the native AlOx was first removed by a H2 plasma followed by a defined reoxidation with an oxygen plasma. The resulting oxides had thicknesses up to 10 nm. The second Al metallization was again deposited by sputtering.Both oxidation processes were carefully studied to understand the initial oxidation process of the aluminum surface. Special attention was devoted to the non-destructive removal of the native AlOx with respect to the Al interface. Because the oxide thicknesses varied between 1 and 10 nm, the transition between direct and Fowler-Nordheim tunneling could be investigated. The process-stability on full scale 200 mm wafers and on chip size could be determined via test structures as well as the resistance variation of the Josephson junctions. Furthermore, the electrical properties of the different oxides could be measured and analyzed on wafer level. These studies provide insight into the structure and composition of the aluminum oxides and the applicability for Qubits.

Decoherence of Single‐Excitation Entanglement over Duan‐Lukin‐Cirac‐Zoller Quantum Networks Caused by Slow‐Magnetic‐Field Fluctuations and Protection Approach

AbstractAtomic spin‐wave (SW) memory is a building block for quantum repeater. However, due to decoherence caused by atomic motions and magnetic‐field gradients, SW retrieval efficiency decays with storage time. Slow magnetic‐field fluctuations (SMFFs) lead to shot‐to‐shot phase noises of SWs, but have not been considered in previous repeater protocols. Here, a SW model including a homogeneous phase noise caused by SMFFs is developed, and then revealed that such phase noise induces decoherence of single‐excitation entanglement between two atomic ensembles in a repeater link. For verifying this conclusion, single‐excitation entanglement between two SWs in a cold atomic ensemble is experimentally prepared and observed entanglement decoherence induced by SMFFs. Limited by SMFFs, the evaluated lifetime of the single‐excitation entanglement over a repeater link is ≈135 ms, even if the link uses optical‐lattice atoms as nodes. Then an approach that may overcome such limitations and extend this lifetime to ≈1.7 s is presented.

Analytical method of multiqubit entanglement robustness in correlated quantum channels

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 $W$ state. Our method shows the behavior of entanglement under the noisy channels and characterizes the entanglement robustness of the multiqubit system analytically.

Self-healing of multipartite entanglement in optical quantum networks

Multipartite entanglement serves as an essential resource for constructing quantum networks and makes it possible to realize multi-user quantum information protocols outperforming their classical counterparts. Unfortunately, multipartite entanglement is fragile when distributed in complex environments. Therefore, it is urgent to address the issue of multipartite entanglement decoherence caused by complex environments. Here we demonstrate the self-healing of multipartite continuous-variable (CV) entanglement after an obstruction. In our experiment, the tripartite entanglement destroyed by the obstruction-introduced noise and loss can self-heal after free-space propagation of a certain distance due to the self-healing property of a Bessel–Gaussian (BG) beam. We show that the BG beam provides a more robust mode basis for free-space CV quantum entanglement distribution than the widely used fundamental Gaussian beam under similar conditions. In addition, we also demonstrate the self-healing of five sets of tripartite entanglement and 10 sets of bipartite entanglement in orbital-angular-momentum-multiplexed optical quantum networks. Our results pave the way for constructing obstruction-tolerant high-capacity CV optical quantum networks.

Noiseless linear amplifiers for multimode states

The entanglement structure between different frequency components within broadband quantum light pulses, forged at entanglement creation, represents a promising route to the practical delivery of many multipartite quantum information applications. However, the scalability of such applications is largely limited by the entanglement decoherence caused by photon loss. One promising method to combat such losses is noiseless linear amplification. However, while there have been various procedures that implement noiseless linear amplification on single-mode states, no realization has thus far been proposed for noiseless linear amplification on quantum states carrying a multimode structure. In this work we close this gap, proposing a novel Noiseless Linear Amplifier (NLA) with Photon Catalysis (PC), namely, the PC-NLA. Constructing a multimode version of an existing NLA that uses Quantum Scissors (QS), the QS-NLA, we then show how the PC-NLA is compatible with the QS-NLA, even though the former uses half the physical resources of the latter. We then apply our newly developed multimode NLA frameworks to the problem of Continuous-Variable (CV) entanglement distillation, determining how the multimode structure of the entanglement impacts the performance of the NLAs. Different from single-mode NLA analyses, we find that a multimode NLA is only effective as a CV entanglement distillation strategy when the channel loss is beyond some threshold - a threshold largely dependent on the multimode structure. The results provided here will be valuable for real-world implementations of multipartite quantum information applications that utilize complex entanglement structure within broadband light pulses.

QUANTUM ENTANGLEMENT RECOVERY USING SPEED GRADIENT OVER NOISY QUANTUM CHANNELS

Decoherence of entanglement is one of the main challenges in quantum communication. It is caused by quantum systems interacting with their environment and limiting the distribution of entangled qubits over quantum channels for a few kilo- meters. Preserving and recovering the initial entangled states is a critical issue to permit designed tasks (e.g. quantum key distribution, quantum teleportation, and quantum dense coding) to be fulfilled. This study investigates the adaptive control (speed gradient) method to recover initial entangled state after exposure to decoherence effects. The results show that the entanglement of the initial state can be maximized and recovered over the variant decoherence rates of the quantum channel noise.

Improving fidelity of quantum secret sharing in noisy environments

Quantum secret sharing is a procedure for sharing a secret among a number of participants such that only certain subsets of participants can collaboratively reconstruct it. In this paper, we review a quantum secret sharing scheme to realize a class of access structures. Based on this protocol, we give a concrete example with three participants. Since the noisy channel has a great influence on the shared quantum secret, we analyze the impacts of two kinds of noisy channels on quantum secret sharing and obtain the expression among the fidelity, noisy coefficient and shared quantum state coefficients. In order to enhance the fidelity of the shared secret, we give an optimized strategy through the concrete scheme. Furthermore, we analyze two specific cases, and we can enhance the fidelity through properly adjusting the compensation parameters. Compared with the original way, our method shows an effective influence on the quality decrease of quantum secret sharing schemes due to the entanglement decoherence.

Nanobob: a CubeSat mission concept for quantum communication experiments in an uplink configuration

We present a ground-to-space quantum key distribution (QKD) mission concept and the accompanying feasibility study for the development of the associated low earth orbit nanosatellite payload. The quantum information is carried by single photons with the binary codes represented by polarization states of the photons. Distribution of entangled photons between the ground and the satellite can be used to certify the quantum nature of the link: a guarantee that no eavesdropping can take place. By placing the entangled photon source on the ground, the space segments contains “only” the less complex detection system, enabling its implementation in a compact enclosure, compatible with the 12U CubeSat standard ({sim}12~mbox{dm}^{3}). This reduces the overall cost of the project, making it an ideal choice as a pathfinder for future European quantum communication satellite missions. The space segment is also more versatile than one that contains the source since it is compatible with a multiple of QKD protocols (not restricted to entangled photon schemes) and can be used in quantum physics experiments, such as the investigation of entanglement decoherence. Other possible experiments include atmospheric transmission/turbulence characterization, dark area mapping, fine pointing and tracking, and accurate clock synchronization; all crucial for future global scale quantum communication efforts.

Non-equilibrium quantum phase transition via entanglement decoherence dynamics.

We investigate the decoherence dynamics of continuous variable entanglement as the system-environment coupling strength varies from the weak-coupling to the strong-coupling regimes. Due to the existence of localized modes in the strong-coupling regime, the system cannot approach equilibrium with its environment, which induces a nonequilibrium quantum phase transition. We analytically solve the entanglement decoherence dynamics for an arbitrary spectral density. The nonequilibrium quantum phase transition is demonstrated as the system-environment coupling strength varies for all the Ohmic-type spectral densities. The 3-D entanglement quantum phase diagram is obtained.

Evolutions of two-qubit entangled system in different noisy environments and channels

Quantum information technology is mainly based on quantum entanglement. As an important coherent superposition state, the coherence of quantum entanglement source is easily affected by environment and becomes fragile, which will lead to the failure of the quantum information processing. Thus, it is critical to reveal the evolutions of quantum entanglement source under different noisy environments and different noisy channels. Firstly, we experimentally prepare a high-fidelity two-bit entangled state by several technical methods. The fidelity observed for the state prepared in our experiment is 0.993 and the signal-to-noise ratio can reach up to 299. Then, we simulate the bit-flip noise and phase-shift noise (collective and non-collective) using the all-optical experimental setup. Finally, based on the entanglement qubit state, we experimentally study the evolutions of entanglement characteristic under different noisy environments and the single, double and mixed noisy channels. The experimental results show that for the same type of noise, the entanglement properties disappear fast when entangled qubit passes through dual channel noisy environment. The upper bounds of noise intensity to destroy the entanglement property are 0.25 and 0.26 for the single bit-flip noise and phase-shift noisy channels, respectively. The comparison between the two different kinds of noisy environments shows that the entanglement properties disappear fast when entangled bit passes through non-collective environment. The upper bounds of noise intensity are 0.08 and 0.14 for non-collective bit-flip and phase-shift noise to destroy the entanglement property, while the noise intensities are 0.14 and 0.23 for collective bit-flip and phase-shift noise, respectively. For different kinds of noises, the results show that bit-flip noise is more likely to destroy the entanglement properties than the phase-shift. Our results have great significance for the theoretical and experimental studies of entanglement decoherence and have important application value for quantum information processing technology based on the nonlinear optical system.

Fidelity recovery scheme for quantum teleportation in amplitude damping channel

During the course of quantum teleportation in amplitude damping channel, the quantum Bell entanglement state will suffer a de-coherence, which will lead to the quality decrease of quantum teleportation, or even communication failure. To overcome this influence, we propose a method to compensate for the de-coherence of Bell entanglement state. Based on the parameter estimation of the amplitude damping channel, the compensation is divided into two steps. The first step (called pre-compensation) is carried out before the occurrence of de-coherence; the second step (called match-compensation) is carried out after the quantum entanglement state has experienced the de-coherence in the amplitude damping channel. The former is done at the EPR source device, while the latter is done at the quantum user device. The parameters of pre-compensation and match-compensation are determined by the amplitude damping coefficient. The quantum teleportation is carried out after the entanglement compensation. We will give the theoretical derivation and performance analysis of this method. Compared with the method that has no compensation and the method that the compensation is only done after de-coherence, the method given in this paper has a higher quantum teleportation fidelity which is close to 1, when the compensation parameter is adjusted accurately. Our method shows an effective influence on the teleportation quality decrease due to the entanglement de-coherence.

Entanglement decoherence in a gravitational well according to the event formalism

The event formalism is a nonlinear extension of quantum field theory designed to be compatible with the closed time-like curves that appear in general relativity. Whilst reducing to standard quantum field theory in flat space-time the formalism leads to testably different predictions for entanglement distribution in curved space. In this paper we introduce a more general version of the formalism and use it to analyse the practicality of an experimental test of its predictions in the Earthʼs gravitational well.

Effects of different Dzyaloshinskii-Moriya interaction and magnetic field on entanglement and fidelity intrinsic decoherence in a spin system

Using the Milburn equation, we have studied the properties of the entanglement and fidelity dynamics in a spin system with different Dzyaloshinskii-Moriya interaction and magnetic field in detail. Effects of different Dzyaloshinskii-Moriya interaction, different magnetic fields, and the initial states on the entanglement and fidelity are discussed. Results show that entanglement decoherence can be suppressed by inhomogeneous magnetic fields. Initial state affects greatly the entanglement, and a proper entanglement can be obtained by adjusting the directions of Dzyaloshinskii-Moriya interaction. For a particular initial state, an optimal fidelity is obtained by changing the direction of the Dzyaloshinskii-Moriya interaction. Moreover, no matter how homogeneous or inhomogeneous the magnetic fields are, they cannot enhance the fidelity. The dependence of entanglement and fidelity on the angle of initial state shows periodicity. Hence we can select an optimal initial state for a specific condition according to requirement.

The Correlated Environments Depress Entanglement Decoherence in the Dimer System

The Correlated Environments Depress Entanglement Decoherence in the Dimer System

Decoherence of Entanglement in Markov Approximation in Terms of Rotating Wave Approximation

Decoherence of Entanglement in Markov Approximation in Terms of Rotating Wave Approximation

Influence of decoherence of entanglement on deterministic remote state preparation

We investigate the influence of decoherence of entanglement on the deterministic remote state preparation. Firstly, the determinability and the depletion of entanglement bits and classical bits are obtained. Then two kinds of decoherent channels (depolarizing and dephasing) are analyzed. For the depolarizing channel, the fidelity is related only to the depolarization and the latitude of target quantum bits on the Bloch sphere, but is independent of longitude. For the dephasing channel, the fidelity is not affected by the quantum channel and only related to the latitude of target quantum bits.

Modulation of Entanglement for Coupled Superconducting Qubits Under Non-Markovian Environment

The evolution of entanglement decoherence is investigated for a coupled superconducting qubit under non-Markovian environment by utilizing a commensal entanglement degree. The results show that, owing to the memory feedback effect of environment, the entanglement degree of the coupled qubits at the thermal equilibrium always monotonously tends to zero so that entanglement sudden death occurs briefly in the non-Markovian process. Different from the Markovian process, stronger the dissipation is, faster the entanglement sudden death is. We find that, furthermore, the interaction between the qubits results generally in reduction of entanglement degree in the quantum system. With some special initial states or initial phase angles, however, the influence of the interaction between qubits on the system entanglement degree can be avoided.

Dephasing of two interacting qubits under the influence of thermal reservoirs

A phase relaxation process of interacting two qubits is studied by means of the quantum master equation, where each qubit is influenced by an independent thermal reservoir. The damping operator of the quantum master equation consists of two parts; one includes the effects of the interaction between the qubits and the other does not. It is shown that the interaction significantly affects the phase relaxation process. The decoherence of entanglement is investigated for the two qubits initially prepared in the $X$ state.