Abstract
1. INTRODUCTION
Quantum entanglement, which lies at the heart of the foundations of quantum mechanics, is a crucial resource for quantum information science [1,2]. For a given state, how to determine whether it is entangled or not is a fundamental question in quantum entanglement theory. In the past two decades since the definition of entanglement was clarified in Ref. [3], tons of research have been reported related to this subject, such as the well-known positive partial transpose criterion [4], the computable cross norm criterion (or matrix realignment criterion) [5,6], the permutation separability criterion [7,8], and entanglement witnesses [9–17]. Entanglement witnesses accomplish this task without requiring full state tomography. Thus, several types of entanglement witnesses have been defined and studied theoretically [18–32] and have been demonstrated in various physical systems [33–39].
Unlike the other criteria in which it is assumed that the state density matrix is preknown, entanglement witnesses are Hermitian operators and designed directly for detection of entanglement for states. Rather, an operator that acts on a bipartite system is an entanglement witness if and only if for all separable states and for, at least, one entangled state . Entanglement witnesses completely characterize separable states and allow to detect entanglement experimentally [40]. Moreover, a witness is optimal if for any an operator is no longer a witness [41]. Optimal entanglement witnesses are the best entanglement detectors, that is, is optimal if and only if there is no other witness that detects more quantum entangled states than does. A witness, which is not optimal, may be optimized via a suitable optimization procedure [41]. It is, therefore, clear that knowing all optimal entanglement witnesses one is able to detect all entangled states. In a recent work, Riccardi
In this paper, we experimentally demonstrated the method in Ref. [42] for detecting entanglement of both pure and mixed two-qubit states. The mixed states we select are motivated by realistic experimental conditions, which emerge from four typical single-qubit entanglement breaking channels, i.e., Pauli channels, dephasing channels, depolarizing channels, and amplitude damping channels. The features of requiring low measurement resources and no
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2. THEORETICAL SCENARIOS
A two-qubit entanglement witness can be decomposed as
It is proven in Ref. [42] that, there are six families of rank-1 projectors within the class of Eq. (2),
In return, six families of entanglement witness that are of the form as Eq. (3) can be derived:
Then, for a given two-qubit state , to identify whether it is entangled or not, we perform all the local measurements of after which is evaluated for each . To find a value of below zero, we minimize the value over and for each . If a negative value is obtained, then, the state is identified as entangled, and at the same time, a corresponding optimal witness for this state is defined. Such a postprocess implies that the method requires no
3. EXPERIMENTAL DEMONSTRATION
In the experiment, we test the performance of the procedure with pure entangled two-qubit states of the form of Eq. (7) and multiple families of mixed entangled two-qubit states, which are generated by sending one party of the maximally entangled state through four types of single-qubit entanglement breaking channels, i.e., Pauli channels, dephasing channels, depolarizing channels, and amplitude damping channels, acting on one of the qubits. The experimental setup is illustrated in Fig. 1(a).
Figure 1.Experimental setup. (a) Optical structure for the experiments. The entangled photon pairs are produced via the type-I spontaneous parametric down-conversion (SPDC) process by pumping two adjacent nonlinear crystals of BBO with a 405-nm laser diode. Two
In our experiment, polarizations of the photon pairs are generated in a pure entangled state,
The mixed two-qubit entangled states emerge from four typical types of quantum noisy channels, i.e., Pauli channels, dephasing channels, depolarizing channels, and amplitude damping channels, acting on one of the qubits. These noisy channels are selected to show the validity of the method when applying it in a realistic experimental environment. The initial state is prepared in . Quantum noisy channels are completely positive trace-preserving maps acting on a quantum state in a -dimensional Hilbert space , which can be mathematically described as
Taking a single-qubit Pauli channel as an example (see Appendix A for details on realizations of other noisy channels), the Kraus operators are given by , , , and with . The transformation of a single-qubit state under the Pauli channel is
To verify the validity of the method for experimental witnessing for entangled states with limited local measurements, we compare the witness values and the logarithmic negativity of the states [47,48]. The logarithmic negativity is a notion of entanglement measure. Here, denotes the trace norm, and denotes the partial transpose. It varies between 0 and 1, indicating the separated and maximally entangled states, respectively, and can be evaluated with the states reconstructed via a full state tomography.
In the following, we show explicitly the performance of the witnesses on a family of pure states [Eq. (7)] and mixed states [Eq. (8)], which emerge from several single-qubit quantum noisy channels.
In Fig. 2, we show the experimental results of the witness and negativity for both pure and mixed states. For pure states, the experimental results of is 0 only when , which indicates the states are separated. As expected, for and 1, the experimental results of the entanglement witnesses are and , respectively. For the rest states, are always less than 0, which proves the method is effective in detecting entangled two-qubit pure states.
Figure 2.Experimental results for two-qubit systems. (a) Entanglement witness value as a function of state parameter
For the mixed states, which emerge from the Pauli channel, negativities are always larger than 0 (from to ) for . As expected, the experimental results of the entanglement witnesses are less than 0 (from to ). Entanglement is detected for . For the dephasing channel, the experimental results of the negativities decrease from to (the theoretical prediction is 0 for ) and, then, increase to . The experimental results of the entanglement witness increase from to for and decrease to for . For the depolarizing channel with , the experimental results of the negativity and the entanglement witness decrease from to 0 and increase from to , respectively. For , the negativity is theoretically predicted to be 0, and the experimental results of the witness are larger than 0. This indicates the entanglement is detected for , and no entanglement is detected for . For the amplitude damping channel, the experimental results of the negativity are always larger than 0 for . The experimental results of the entanglement witness are always less than 0. Entanglement is detected for . Thus, the method of the construction and optimization of entanglement witnesses is valid for the family of the mixed states emerging from the noisy channels.
4. EXTENSIONS OF ENTANGLEMENT WITNESS TO HIGHER-DIMENSIONAL BIPARTITE SYSTEMS
Finally, we show the demonstration of extensions of entanglement witness to photonic higher-dimensional bipartite states [49–51], which are constructed with the limited local measurements. A straightforward generalization consists of the following:
Another possible construction is of the so-called diagonal-type entanglement witness [52],
We consider and
In our experiment, a family of three-dimensional bipartite states is used to demonstrate the above entanglement witnesses,
The experimental results of and are shown in Figs. 3(a) and 3(b), respectively. The negativities of the states in Fig. 3(c) increase from to with ; the values of and decrease from to and from to , respectively. This indicates the validity of the proposed extensions of the entanglement witnesses to higher-dimensional bipartite systems.
Figure 3.Experimental results for higher-dimensional bipartite systems. (a) Values of the entanglement witness
5. CONCLUSION
We have reported an experimental demonstration of a method for construction of entanglement witnesses from a limited fixed set of local measurements (). The method required a classical optimization on the statistics of the results after performing the measurements within and can be executed without knowing
Acknowledgment
Acknowledgment. G. Z. performed the experiments with contributions from C. Z., K. W., and L. X.; P. X. designed the experiments, analyzed the results, and wrote the paper.
APPENDIX A: EXPERIMENTALLY REALIZING NOISY CHANNELS
The Kraus operators for a single-qubit dephasing channel are given by and . The transformation of a single-qubit state under the dephasing channel is
For a single-qubit amplitude damping channel, the Kraus operators are given by and . The amplitude damping channel is experimentally realized by a dual interferometer setup [
APPENDIX B: IMPERFECTIONS IN OUR EXPERIMENT
The systematic shift between experimental results and theoretical predictions is caused by imperfection of state preparation, whereas the error bars are estimated through the statistical uncertainty of the photon numbers. Ideally, we prepare pure two-qubit entangled states and mixed two-qubit entangled states. Mixed entangled states emerge from four types of quantum noisy channels, i.e., Pauli channels, dephasing channels, depolarizing channels, and amplitude damping channels. However, in our experiment, the state preparation is not perfect, and the fidelity of the initial pure entangled state is about 97%, which is obtained by the state tomography.
In Fig.
Figure 4.Concurrence and negativity of the states versus the noisy parameter
Figure 5.(a) Entanglement witness value as a function of the state parameter
APPENDIX C: HIGHER-DIMENSIONAL BIPARTITE SYSTEMS
Below, we discuss the possible generalizations to higher-dimensional bipartite systems. Similarly, the goal is to detect entanglement of bipartite qudit systems when only a limited fixed set of local measurements can be performed.
Any Hermitian operator in a -dimensional system can be represented as
Then, any Hermitian operator in a system can be represented as
Similar to the scenario of qubit systems of Eq. (
As a straightforward generalization of the projections in Eq. (
One has, therefore, the analog of Eq. (
Another possible construction is of the so-called diagonal-type entanglement witness [
We consider and
The procedure for detection of the entanglement for a given bipartite qudit system is similar to that of a two-qubit system introduced earlier in this paper. For experimental demonstration, we consider the situation of , that is, entangled two-qutrit states, which are prepared by employing the spatial and polarization modes of the photons. Specifically, the bases are encoded by the horizontal polarization of the photon in the lower mode and the vertical polarization and the horizontal polarization in the upper mode, respectively. The corresponding state preparation setup is shown in Fig.
Figure 6.Experimental setup for two-qutrit systems. The two-qutrit states are generated in the state preparation module, consisting of a set of SPDC entangled photon sources, two BDs, and two HWPs by employing the spatial and polarization modes of the photons. The local measurements and the state tomographic measurements are executed by the measurement modules, consisting of a sequence of HWPs, QWPs, a BD, and a PBS.
References
[1] P. Xue, Y.-F. Xiao. Universal quantum computation in decoherence-free subspace with neutral atoms. Phys. Rev. Lett., 97, 140501(2006).
[2] Z. Bian, J. Li, H. Qin, X. Zhan, R. Zhang, B. C. Sanders, P. Xue. Realization of single-qubit positive-operator-valued measurement via a one-dimensional photonic quantum walk. Phys. Rev. Lett., 114, 203602(2015).
[3] R. F. Werner. Quantum states with Einstein-Podolsky-Rosen correlations admitting a hidden-variable model. Phys. Rev. A, 40, 4277-4281(1989).
[4] A. Peres. Separability criterion for density matrices. Phys. Rev. Lett., 77, 1413-1415(1996).
[5] O. Rudolph. On the cross norm criterion for separability. J. Phys. A, 36, 5825(2003).
[6] K. Chen, L.-A. Wu. A matrix realignment method for recognizing entanglement(2002).
[7] P. Wocjan, M. Horodecki. Characterization of combinatorially independent permutation separability criteria. Open Syst. Inf. Dyn., 12, 331-345(2005).
[8] M. Horodecki, P. Horodecki, R. Horodecki. Separability of mixed quantum states: linear contractions and permutation criteria. Open Syst. Inf. Dyn., 13, 103-111(2006).
[9] M. Horodecki, P. Horodecki, R. Horodecki. Separability of mixed states: necessary and sufficient conditions. Phys. Lett. A, 223, 1-8(1996).
[10] B. M. Terhal. Bell inequalities and the separability criterion. Phys. Lett. A, 271, 319-326(2000).
[11] B. M. Terhal. A family of indecomposable positive linear maps based on entangled quantum states. Linear Algebra Appl., 323, 61-73(2001).
[12] B. M. Terhal. Detecting quantum entanglement. Theor. Comput. Sci., 287, 313-335(2002).
[13] G. Tóth, O. Gühne. Detecting genuine multipartite entanglement with two local measurements. Phys. Rev. Lett., 94, 060501(2005).
[14] F. A. Bovino, G. Castagnoli, A. Ekert, P. Horodecki, C. M. Alves, A. V. Sergienko. Direct measurement of nonlinear properties of bipartite quantum states. Phys. Rev. Lett., 95, 240407(2005).
[15] O. Gühne, N. Lütkenhaus. Nonlinear entanglement witnesses. Phys. Rev. Lett., 96, 170502(2006).
[16] R. Augusiak, M. Demianowicz, P. Horodecki. Universal observable detecting all two-qubit entanglement and determinant-based separability tests. Phys. Rev. A, 77, 030301(2008).
[17] O. Gühne, G. Tóth. Entanglement detection. Phys. Rep., 474, 1-75(2009).
[18] O. Gühne, P. Hyllus, D. Bruß, A. Ekert, M. Lewenstein, C. Macchiavello, A. Sanpera. Detection of entanglement with few local measurements. Phys. Rev. A, 66, 062305(2002).
[19] O. Gühne, P. Hyllus. Investigating three qubit entanglement with local measurements. Int. J. Theor. Phys., 42, 1001-1013(2003).
[20] O. Gühne, P. Hyllus, D. Bruss, A. Ekert, M. Lewenstein, C. Macchiavello, A. Sanpera. Experimental detection of entanglement via witness operators and local measurements. J. Mod. Opt., 50, 1079-1102(2003).
[21] C.-J. Zhang, Y.-S. Zhang, S. Zhang, G.-C. Guo. Optimal entanglement witnesses based on local orthogonal observables. Phys. Rev. A, 76, 012334(2007).
[22] B. Jungnitsch, T. Moroder, O. Gühne. Taming multiparticle entanglement. Phys. Rev. Lett., 106, 190502(2011).
[23] M. Mozrzymas, A. Rutkowski, M. Studziński. Using non-positive maps to characterize entanglement witnesses. J. Phys. A: Math. Theor., 48, 395302(2015).
[24] F. Shahandeh, M. Ringbauer, J. C. Loredo, T. C. Ralph. Ultrafine entanglement witnessing. Phys. Rev. Lett., 118, 110502(2017).
[25] M. Gachechiladze, N. Wyderka, O. Gühne. The structure of ultrafine entanglement witnesses. J. Phys. A: Math. Theor., 51, 365307(2018).
[26] S. Gerke, W. Vogel, J. Sperling. Numerical construction of multipartite entanglement witnesses. Phys. Rev. X, 8, 031047(2018).
[27] D. Chruściński, G. Sarbicki, F. Wudarski. Entanglement witnesses from mutually unbiased bases. Phys. Rev. A, 97, 032318(2018).
[28] S.-Q. Shen, T.-R. Xu, S.-M. Fei, X. Li-Jost, M. Li. Optimization of ultrafine entanglement witnesses. Phys. Rev. A, 97, 032343(2018).
[29] T. Simnacher, N. Wyderka, R. Schwonnek, O. Gühne. Entanglement detection with scrambled data. Phys. Rev. A, 99, 062339(2019).
[30] J. Bae, D. Chruściński, B. C. Hiesmayr. Mirrored entanglement witnesses. npj Quantum Inf., 6, 15(2020).
[31] S.-Q. Shen, J.-M. Liang, M. Li, J. Yu, S.-M. Fei. Nonlinear improvement of qubit-qudit entanglement witnesses. Phys. Rev. A, 101, 012312(2020).
[32] T. Li, L.-M. Lai, D.-F. Liang, S.-M. Fei, Z.-X. Wang. Entanglement witnesses based on symmetric informationally complete measurements. Int. J. Theor. Phys., 59, 3549-3557(2020).
[33] M. Barbieri, F. De Martini, G. Di Nepi, P. Mataloni, G. M. D’Ariano, C. Macchiavello. Detection of entanglement with polarized photons: experimental realization of an entanglement witness. Phys. Rev. Lett., 91, 227901(2003).
[34] G. Lima, E. S. Gómez, A. Vargas, R. O. Vianna, C. Saavedra. Fast entanglement detection for unknown states of two spatial qutrits. Phys. Rev. A, 82, 012302(2010).
[35] J. Dai, Y. L. Len, Y. S. Teo, B.-G. Englert, L. A. Krivitsky. Experimental detection of entanglement with optimal-witness families. Phys. Rev. Lett., 113, 170402(2014).
[36] F. Brange, O. Malkoc, P. Samuelsson. Minimal entanglement witness from electrical current correlations. Phys. Rev. Lett., 118, 036804(2017).
[37] N. Friis, G. Vitagliano, M. Malik, M. Huber. Entanglement certification from theory to experiment. Nat. Rev. Phys., 1, 72-87(2019).
[38] B. Dirkse, M. Pompili, R. Hanson, M. Walter, S. Wehner. Witnessing entanglement in experiments with correlated noise. Quantum Sci. Technol., 5, 035007(2020).
[39] G. Zhu, D. Dilley, K. Wang, L. Xiao, E. Chitambar, P. Xue. Less entanglement exhibiting more nonlocality with noisy measurements. npj Quantum Inf., 7, 166(2021).
[40] R. Horodecki, P. Horodecki, M. Horodecki, K. Horodecki. Quantum entanglement. Rev. Mod. Phys., 81, 865-942(2009).
[41] M. Lewenstein, B. Kraus, J. I. Cirac, P. Horodecki. Optimization of entanglement witnesses. Phys. Rev. A, 62, 052310(2000).
[42] A. Riccardi, D. Chruściński, C. Macchiavello. Optimal entanglement witnesses from limited local measurements. Phys. Rev. A, 101, 062319(2020).
[43] R. Jozsa. Fidelity for mixed quantum states. J. Mod. Opt., 41, 2315-2323(1994).
[44] J. Fiurášek. Maximum-likelihood estimation of quantum measurement. Phys. Rev. A, 64, 024102(2001).
[45] A. Chiuri, V. Rosati, G. Vallone, S. Pádua, H. Imai, S. Giacomini, C. Macchiavello, P. Mataloni. Experimental realization of optimal noise estimation for a general Pauli channel. Phys. Rev. Lett., 107, 253602(2011).
[46] A. Orieux, L. Sansoni, M. Persechino, P. Mataloni, M. Rossi, C. Macchiavello. Experimental detection of quantum channels. Phys. Rev. Lett., 111, 220501(2013).
[47] K. Życzkowski, P. Horodecki, A. Sanpera, M. Lewenstein. Volume of the set of separable states. Phys. Rev. A, 58, 883-892(1998).
[48] G. Vidal, R. F. Werner. Computable measure of entanglement. Phys. Rev. A, 65, 032314(2002).
[49] G. Zhu, O. Kálmán, K. Wang, L. Xiao, D. Qu, X. Zhan, Z. Bian, T. Kiss, P. Xue. Experimental orthogonalization of highly overlapping quantum states with single photons. Phys. Rev. A, 100, 052307(2019).
[50] Z. Bian, L. Xiao, K. Wang, F. A. Onanga, F. Ruzicka, W. Yi, Y. N. Joglekar, P. Xue. Quantum information dynamics in a high-dimensional parity-time-symmetric system. Phys. Rev. A, 102, 030201(2020).
[51] X. Zhan, X. Zhang, J. Li, Y. Zhang, B. C. Sanders, P. Xue. Realization of the contextuality-nonlocality tradeoff with a qubit-qutrit photon pair. Phys. Rev. Lett., 116, 090401(2016).
[52] D. Chruściński, G. Sarbicki. Entanglement witnesses: construction, analysis and classification. J. Phys. A, 47, 483001(2014).
[53] K.-C. Ha. Atomic positive linear maps in matrix algebras. Publ. Res. Inst. Math. Sci., 34, 591-599(1998).
[54] L. Xiao, T. Deng, K. Wang, G. Zhu, Z. Wang, W. Yi, P. Xue. Non-Hermitian bulk–boundary correspondence in quantum dynamics. Nat. Phys., 16, 761-766(2020).
[55] L. Xiao, K. Wang, X. Zhan, Z. Bian, K. Kawabata, M. Ueda, W. Yi, P. Xue. Observation of critical phenomena in parity-time-symmetric quantum dynamics. Phys. Rev. Lett., 123, 230401(2019).
[56] P. Xue, R. Zhang, H. Qin, X. Zhan, Z. H. Bian, J. Li, B. C. Sanders. Experimental quantum-walk revival with a time-dependent coin. Phys. Rev. Lett., 114, 140502(2015).
[57] K. K. Wang, X. Qiu, L. Xiao, X. Zhan, Z. Bian, B. C. Sanders, W. Yi, P. Xue. Observation of emergent momentum-time skyrmions in parity-time-symmetric non-unitary quench dynamics. Nat. Commun., 10, 2293(2019).
[58] L. Xiao, X. Zhan, Z. Bian, K. Wang, X. Wang, J. Li, K. Mochizuki, D. Kim, N. Kawakami, W. Yi, H. Obuse, B. C. Sanders, P. Xue. Observation of topological edge states in parity-time-symmetric quantum walks. Nat. Phys., 13, 1117-1123(2017).
[59] K. Wang, X. Qiu, L. Xiao, X. Zhan, Z. Bian, W. Yi, P. Xue. Simulating dynamic quantum phase transitions in photonic quantum walks. Phys. Rev. Lett., 122, 020501(2019).
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