Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:quant-ph/0606017

Can measuring entanglement be easy?

S. J. van Enk

Published Jun 1, 2006• 3 citations
arXivHTMLPDF

Abstract

I raise some doubts concerning a protocol recently applied in an experiment (Walborn et al, Nature) to measure entanglement. The protocol is much simpler than other known entanglement-verification methods, but, I argue, needs assumptions (namely that the state generated is known and pure) that are too strong to be allowed and that are not justified in most experiments. An extension of the protocol suggested in quant-ph/0605250 is much harder to implement and still relies on assumptions not justified in entanglement-verification protocols, as demonstrated by an explicit example.

DOI
10.48550/arxiv.quant-ph/0606017

Related papers

Experimental procedures for entanglement verification

S. J. van Enk, Norbert Lütkenhaus, Harry J. Kimble

We give an overview of different types of entanglement that can be generated in experiments, as well as of various protocols that can be used to verify or quantify entanglement. We propose several criteria that, we argue, should be applied to experimental entanglement verification procedures. Explicit examples demonstrate that not following these criteria will tend to result in overestimating the amount of entanglement generated in an experiment or in inferring entanglement when there is none. We distinguish protocols meant to refute or eliminate hidden-variable models from those meant to verify entanglement.

arXiv:quant-ph/0611219•Physical Review A•May 15, 2007•101 citations

Experimental determination of entanglement for arbitrary pure states

Shao-Ming Fei, Ming‐Jing Zhao, Kai Chen +1 more

We present a way of experimentally determining the concurrence in terms of the expectation values of local observables for arbitrary multipartite pure states. Instead of the joint measurements on two copies of a state in the experiment for two-qubit systems [S. P. Walborn et al., Nature (London) 440, 20 (2006)], we only need one copy of the state in every measurement for any arbitrary dimensional multipartite systems, avoiding the preparation of twin states or the imperfect copy of the state.

arXiv:0909.4851•Physical Review A•Sep 21, 2009•31 citations

Entanglement verification and its applications in quantum communication

Hauke Häseler

In this thesis, we investigate the uses of entanglement and its verification in quantum communication. Typically, it is the presence of entanglement which allows quantum communication protocols to outperform their classical counterparts, or to achieve tasks which are infeasible with classical physics. Therefore, the verification of entanglement can provide fundamental tests to certify that experimental implementations of such protocols operate quantum mechanically. The main object here is to develop a verification procedure which is adaptable to a wide range of applications, and whose implementation has low requirements on experimental resources. We present such a procedure in the form of the Expectation Value Matrix. The structure of this thesis is as follows: Chapters 1 and 2 give a short introduction and background information on quantum theory and the quantum states of light. In particular, we discuss the basic postulates of quantum mechanics, quantum state discrimination, the description of quantum light and the homodyne detector. Chapter 3 gives a brief introduction to quantum information and in particular to entanglement, and we discuss the basics of quantum key distribution and teleportation. The general framework of the Expectation Value Matrix is introduced. The main matter of this thesis is contained in the subsequent three chapters, which describe different quantum communication protocols and the corresponding adaptation of the entanglement verification method. The subject of Chapter 4 is quantum key distribution, where the detection of entanglement is a means of excluding intercept-resend attacks, and the presence of quantum correlations in the raw data is a necessary precondition for the generation of secret key. We investigate a continuous-variable version of the two-state protocol and develop the Expectation Value Matrix method for such qubit-mode systems. Furthermore, we analyse the role of the phase reference with respect to the security of the protocol and raise awareness of a corresponding security threat. For this, we adapt the verification method to different settings of Stokes operator measurements. In Chapter 5, we investigate quantum memory channels and propose a fundamental bench- mark for these based on the verification of entanglement. After describing some physical effects which can be used for the coherent storage of light, we focus on the storage of squeezed light. This situation requires an extension of our verification procedure to sources of mixed input states. We propose such an extension, and give a detailed analysis of its application to squeezed thermal states, displaced thermal states and mixed qubit states. This is supplemented by finding the optimal entanglement-breaking channels for each of these situations, which provides us with an indication of the strength of the extension to our entanglement criterion. The subject of Chapter 6 is also the benchmarking of quantum memory or teleportation experiments. Considering a number of recently published benchmark criteria, we investigate the question which one is most useful to actual experiments. For this, a criterion must be both strong, i.e., easy to overcome, and it must be implementable with a reasonable amount of experimental resources. We first compare the different criteria for typical settings and sort them according to their resilience to excess noise. Then, we introduce a further improvement to the Expectation Value Matrix method, which results in the desired optimal benchmark criterion. Finally, we investigate naturally occurring phase fluctuations and find them to further simplify the implementation of our criterion. Thus, we formulate the first truly useful way of validating experiments for the quantum storage or transmission of light.

•Jan 1, 2010•0 citations

Quantifying Entanglement

Vlatko Vedral, Martin Bodo Plenio, M. A. Rippin +1 more

We present conditions every measure of entanglement has to satisfy, and construct a whole class of ``good'' entanglement measures. The generalization of our class of entanglement measures to more than two particles is straightforward. We present a measure which has a statistical operational basis that might enable experimental determination of the quantitative degree of entanglement.

arXiv:quant-ph/9702027•Physical Review Letters•Mar 24, 1997•2,021 citations

Verifying Entanglement with Limited Data

Ryan Wilkinson

A new technique efficiently detects quantum entanglement using just a small set of measurements.

Physics•Apr 29, 2026•0 citations

Are superluminal "signals" an acceptable hypothesis? - Difficulties in building a communication protocol with them. (arXiv:quant-ph/0610159v3 UPDATED)

Sofia Wechsler

The local hidden variable assumption was repeatedly proved unable to explain results of experiments in which contextuality is involved. Then, the correlated results of measurements of entangled particles, began to be attributed to a communication between particles through so-called signals. These signals need to possess superluminal velocity or move backward in time. No object that has a rest-mass, not even the photons whose rest mass is zero, behave this way. Still, as the nature of the presumed sinals is not known, people don't reject the idea, despite the conflict with the theory of relativity. For this reason, the present article examines the signals from another point of view: wherever runs a communication, there has also to exist a communication protocol. The article tries to outline a communication protocol between the space-separated, entangled particles, and comes to a contradiction, making the idea of such a communication highly doubtable.

•Oct 14, 2019•0 citations
I raise some doubts concerning a protocol recently applied in an experiment (Walborn et al, Nature) to measure entanglement. The protocol is much simpler than other known entanglement-verification methods, but, I argue, needs assumptions (namely that the state generated is known and pure) that are too strong to be allowed and that are not justified in most experiments. An extension of the protocol suggested in quant-ph/0605250 is much harder to implement and still relies on assumptions not justified in entanglement-verification protocols, as demonstrated by an explicit example.
We give an overview of different types of entanglement that can be generated in experiments, as well as of various protocols that can be used to verify or quantify entanglement. We propose several criteria that, we argue, should be applied to experimental entanglement verification procedures. Explicit examples demonstrate that not following these criteria will tend to result in overestimating the amount of entanglement generated in an experiment or in inferring entanglement when there is none. We distinguish protocols meant to refute or eliminate hidden-variable models from those meant to verify entanglement.
In this thesis, we investigate the uses of entanglement and its verification in quantum communication. Typically, it is the presence of entanglement which allows quantum communication protocols to outperform their classical counterparts, or to achieve tasks which are infeasible with classical physics. Therefore, the verification of entanglement can provide fundamental tests to certify that experimental implementations of such protocols operate quantum mechanically. The main object here is to develop a verification procedure which is adaptable to a wide range of applications, and whose implementation has low requirements on experimental resources. We present such a procedure in the form of the Expectation Value Matrix. The structure of this thesis is as follows: Chapters 1 and 2 give a short introduction and background information on quantum theory and the quantum states of light. In particular, we discuss the basic postulates of quantum mechanics, quantum state discrimination, the description of quantum light and the homodyne detector. Chapter 3 gives a brief introduction to quantum information and in particular to entanglement, and we discuss the basics of quantum key distribution and teleportation. The general framework of the Expectation Value Matrix is introduced. The main matter of this thesis is contained in the subsequent three chapters, which describe different quantum communication protocols and the corresponding adaptation of the entanglement verification method. The subject of Chapter 4 is quantum key distribution, where the detection of entanglement is a means of excluding intercept-resend attacks, and the presence of quantum correlations in the raw data is a necessary precondition for the generation of secret key. We investigate a continuous-variable version of the two-state protocol and develop the Expectation Value Matrix method for such qubit-mode systems. Furthermore, we analyse the role of the phase reference with respect to the security of the protocol and raise awareness of a corresponding security threat. For this, we adapt the verification method to different settings of Stokes operator measurements. In Chapter 5, we investigate quantum memory channels and propose a fundamental bench- mark for these based on the verification of entanglement. After describing some physical effects which can be used for the coherent storage of light, we focus on the storage of squeezed light. This situation requires an extension of our verification procedure to sources of mixed input states. We propose such an extension, and give a detailed analysis of its application to squeezed thermal states, displaced thermal states and mixed qubit states. This is supplemented by finding the optimal entanglement-breaking channels for each of these situations, which provides us with an indication of the strength of the extension to our entanglement criterion. The subject of Chapter 6 is also the benchmarking of quantum memory or teleportation experiments. Considering a number of recently published benchmark criteria, we investigate the question which one is most useful to actual experiments. For this, a criterion must be both strong, i.e., easy to overcome, and it must be implementable with a reasonable amount of experimental resources. We first compare the different criteria for typical settings and sort them according to their resilience to excess noise. Then, we introduce a further improvement to the Expectation Value Matrix method, which results in the desired optimal benchmark criterion. Finally, we investigate naturally occurring phase fluctuations and find them to further simplify the implementation of our criterion. Thus, we formulate the first truly useful way of validating experiments for the quantum storage or transmission of light.
The local hidden variable assumption was repeatedly proved unable to explain results of experiments in which contextuality is involved. Then, the correlated results of measurements of entangled particles, began to be attributed to a communication between particles through so-called signals. These signals need to possess superluminal velocity or move backward in time. No object that has a rest-mass, not even the photons whose rest mass is zero, behave this way. Still, as the nature of the presumed sinals is not known, people don't reject the idea, despite the conflict with the theory of relativity. For this reason, the present article examines the signals from another point of view: wherever runs a communication, there has also to exist a communication protocol. The article tries to outline a communication protocol between the space-separated, entangled particles, and comes to a contradiction, making the idea of such a communication highly doubtable.