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

Physical Review Letters

Experimental Direct Observation of Mixed State Entanglement

Christian Schmid, Nikolai Kiesel, Witlef Wieczorek, Harald Weinfurter, Florian Mintert, Andreas Buchleitner

Published Dec 31, 2008• 60 citations
Publisher

Abstract

We report on the direct estimation of concurrence for mixed quantum states. The used method relies on joint measurements on two copies of an entangled state. In the experimental demonstration two polarization-entangled photon pairs emitted from spontaneous parametric down-conversion are analyzed together using a linear optics controlled phase gate. We demonstrate that the measured data, without need for further numerical processing, directly yield reliable estimates, despite experimental imperfections.

DOI
10.1103/physrevlett.101.260505

Related papers

Experimental Measurement of Lower and Upper Bounds of Concurrence for Mixed Quantum States

Xiao-Ling Niu, Yan-Xiao Gong, Jian Li +5 more

We experimentally measure the lower and upper bounds of concurrence for a set of two-qubit mixed quantum states using photonic systems. The measured concurrence bounds are in agreement with the results evaluated from the density matrices reconstructed through quantum state tomography. In our experiment, we propose and demonstrate a simple method to provide two faithful copies of a two-photon mixed state required for parity measurements: Two photon pairs generated by two neighboring pump laser pulses through optical parametric down conversion processes represent two identical copies. This method can be conveniently generalized for entanglement estimation of multi-photon mixed states.

arXiv:0808.0776•Aug 6, 2008•0 citations

Experimental measurement of lower and upper bounds of concurrence for mixed quantum states

Yun‐Feng Huang, Xiao-Ling Niu, Yan-Xiao Gong +5 more

We experimentally measure the lower and upper bounds of concurrence for a set of two-qubit mixed quantum states using photonic systems. The measured concurrence bounds are in agreement with the results evaluated from the density matrices reconstructed through quantum state tomography. In our experiment, we propose and demonstrate a simple method for providing two faithful copies of a two-photon mixed state required for parity measurements: two photon pairs generated by two neighboring pump-laser pulses through optical parametric down-conversion processes represent two identical copies. This method can be conveniently generalized for entanglement estimation of multiphoton mixed states.

Physical Review A•May 27, 2009•18 citations

Direct measurement of the concurrence of two-photon polarization-entangled states

Li-Hua Zhang, Qing Yang, Ming Yang +2 more

We present three protocols for directly measuring the concurrence of two-photon polarization-entangled states, including pure states and mixed states. Two signal photon pairs in the same polarization-entangled states are needed in each detection round, and the signal photons will pass through polarization-independent beam splitters simultaneously. The concurrence of the photon pairs will be encoded in the total probability of picking up the balanced states in the two output modes of the beam splitter, which can be directly measured by single-photon detectors. The photon detection efficiency can be further enhanced with the help of the weak cross-Kerr nonlinearity, and the signal photons will not be absorbed. The third scheme using the bilateral nondemolition measurements can measure the concurrence of both pure and mixed states with high efficiency and high fidelity and is robust against the copy error of the state to be measured. The sophisticated controlled-not operation of the previous schemes is not required here, so our protocols are more feasible within the current experimental technology.

Physical Review A•Dec 31, 2013•34 citations

Experimental measurement of multidimensional entanglement via equivalent symmetric projection

Fang‐Wen Sun, Jianming Cai, Jin‐Shi Xu +5 more

We construct a linear optics measurement process to determine the entanglement measure, named $I$ concurrence, of a set of $4\ifmmode\times\else\texttimes\fi{}4$-dimensional two-photon entangled pure states produced in the optical parametric down conversion process. In our experiment, an equivalent symmetric projection for the twofold copy of a single subsystem [presented by L. Aolita and F. Mintert, Phys. Rev. Lett. 97, 050501 (2006)] can be realized by observing the one-side two-photon coincidence without any triggering detection on the other subsystem. Here, for the first time, we realize the measurement for entanglement contained in biphoton pure states by taking advantage of the indistinguishability and the bunching effect of photons. Our method can determine the $I$ concurrence of generic high-dimensional bipartite pure states produced in the parametric down conversion process.

arXiv:0706.0935•Physical Review A•Nov 5, 2007•17 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

Estimation of entanglement negativity of a two-qubit quantum system with two measurements

Satyabrata Adhikari

Numerous work had been done to quantify the entanglement of a two-qubit quantum state, but it can be seen that previous works were based on joint measurements on two copies or more than two copies of the quantum state under consideration. In this work, we show that a single copy and two measurements are enough to estimate the entanglement quantifier like entanglement negativity and concurrence. To achieve our goal, we establish a relationship between the entanglement negativity and the minimum eigenvalue of the structural physical approximation of the partial transpose of an arbitrary two-qubit state. The derived relation makes it possible to estimate entanglement negativity experimentally by Hong-Ou-Mandel interferometry with only two detectors. Also, we derive the upper bound of the concurrence of an arbitrary two-qubit state and have shown that the upper bound can be realized in experiment. We will further show that the concurrence of i) an arbitrary pure two-qubit state and ii) a particular class of mixed states, namely, rank-2 quasi-distillable mixed states, can be exactly estimated with two measurements.

arXiv:1808.09745•Europhysics Letters (EPL)•Dec 14, 2018•3 citations
We report on the direct estimation of concurrence for mixed quantum states. The used method relies on joint measurements on two copies of an entangled state. In the experimental demonstration two polarization-entangled photon pairs emitted from spontaneous parametric down-conversion are analyzed together using a linear optics controlled phase gate. We demonstrate that the measured data, without need for further numerical processing, directly yield reliable estimates, despite experimental imperfections.
We experimentally measure the lower and upper bounds of concurrence for a set of two-qubit mixed quantum states using photonic systems. The measured concurrence bounds are in agreement with the results evaluated from the density matrices reconstructed through quantum state tomography. In our experiment, we propose and demonstrate a simple method to provide two faithful copies of a two-photon mixed state required for parity measurements: Two photon pairs generated by two neighboring pump laser pulses through optical parametric down conversion processes represent two identical copies. This method can be conveniently generalized for entanglement estimation of multi-photon mixed states.
We experimentally measure the lower and upper bounds of concurrence for a set of two-qubit mixed quantum states using photonic systems. The measured concurrence bounds are in agreement with the results evaluated from the density matrices reconstructed through quantum state tomography. In our experiment, we propose and demonstrate a simple method for providing two faithful copies of a two-photon mixed state required for parity measurements: two photon pairs generated by two neighboring pump-laser pulses through optical parametric down-conversion processes represent two identical copies. This method can be conveniently generalized for entanglement estimation of multiphoton mixed states.
We present three protocols for directly measuring the concurrence of two-photon polarization-entangled states, including pure states and mixed states. Two signal photon pairs in the same polarization-entangled states are needed in each detection round, and the signal photons will pass through polarization-independent beam splitters simultaneously. The concurrence of the photon pairs will be encoded in the total probability of picking up the balanced states in the two output modes of the beam splitter, which can be directly measured by single-photon detectors. The photon detection efficiency can be further enhanced with the help of the weak cross-Kerr nonlinearity, and the signal photons will not be absorbed. The third scheme using the bilateral nondemolition measurements can measure the concurrence of both pure and mixed states with high efficiency and high fidelity and is robust against the copy error of the state to be measured. The sophisticated controlled-not operation of the previous schemes is not required here, so our protocols are more feasible within the current experimental technology.
We construct a linear optics measurement process to determine the entanglement measure, named III concurrence, of a set of $4\ifmmode\times\else\texttimes\fi{}4$-dimensional two-photon entangled pure states produced in the optical parametric down conversion process. In our experiment, an equivalent symmetric projection for the twofold copy of a single subsystem [presented by L. Aolita and F. Mintert, Phys. Rev. Lett. 97, 050501 (2006)] can be realized by observing the one-side two-photon coincidence without any triggering detection on the other subsystem. Here, for the first time, we realize the measurement for entanglement contained in biphoton pure states by taking advantage of the indistinguishability and the bunching effect of photons. Our method can determine the III concurrence of generic high-dimensional bipartite pure states produced in the parametric down conversion process.
Numerous work had been done to quantify the entanglement of a two-qubit quantum state, but it can be seen that previous works were based on joint measurements on two copies or more than two copies of the quantum state under consideration. In this work, we show that a single copy and two measurements are enough to estimate the entanglement quantifier like entanglement negativity and concurrence. To achieve our goal, we establish a relationship between the entanglement negativity and the minimum eigenvalue of the structural physical approximation of the partial transpose of an arbitrary two-qubit state. The derived relation makes it possible to estimate entanglement negativity experimentally by Hong-Ou-Mandel interferometry with only two detectors. Also, we derive the upper bound of the concurrence of an arbitrary two-qubit state and have shown that the upper bound can be realized in experiment. We will further show that the concurrence of i) an arbitrary pure two-qubit state and ii) a particular class of mixed states, namely, rank-2 quasi-distillable mixed states, can be exactly estimated with two measurements.