Abstract
We investigate the entanglement distribution in multi-particle systems in terms of unified (q, s)-entropy. We find that for any tripartite mixed state, the unified (q, s)-entropy entanglement of assistance follows a polygamy relation. This polygamy relation also holds in multi-particle systems. Furthermore, a generalized monogamy relation is provided for unified (q, s)-entropy entanglement in the multi-qubit system.
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Introduction
Quantum entanglement is an important resource in quantum information theory. Different from classical correlations, this restricted shareability of entanglement in multi-particle systems is known as monogamy property. The more entanglement shared between two parties implies the less entanglement shared with the rest of the system. Monogamy property plays a crucial role in quantum cryptography: which restricts the quantity of information captured by an eavesdropper about the secret key to be extracted1,2,3. Monogamy property has also been discussed in the device-independent quantum information processing4, condensed matter physics5 and black-hole physics6, 7.
The study of monogamy property has a long history. The first monogamy relation was found by Coffman et al., who considered a three-qubit system ABC 8, and showed that the amount of entanglement (which is quantified by the squared concurrence) between A and B, plus the amount of entanglement between A and C, cannot be greater than the amount of entanglement between A and the pair BC. Further, Osborne et al. proved the squared concurrence follows a general monogamy inequality for the N-qubit system1. Monogamy inequalities for different entanglement measures have been noted, such as concurrence9,10,11,12, entanglement of formation13, 14, negativity15,16,17,18,19, Rényi entropy entanglement20, 21, and Tsallis entropy entanglement22,23,24. For the other physical resources, such as discord and steering, the monogamy property of them has also been discussed25,26,27,28.
As dual to monogamy property, polygamy property in multi-particle systems has arised many interests by researchers15, 19, 22, 29, 30. Polygamy property was first provided by using the concurrence of assistance to quantify the distributed bipartite entanglement in multi-qubit systems29, 30. Polygamy property has also considered in many entanglement measures, such as Rényi entropy20, Tsallis entropy22, 31 and convex-roof extended negativity19.
Unified (q, s)-entropy is an important entropic measure, which can be used in many areas of quantum information theory. In this paper, we investigate the entanglement distribution in multi-particle systems in terms of unified (q, s)-entropy. We find that for any tripartite mixed state, the unified (q, s)-entropy entanglement of assistance follows a polygamy relation. This polygamy relation also holds in multi-particle systems. Furthermore, a generalized monogamy relation is provided for unified (q, s)-entropy entanglement in the multi-qubit system.
Results
This paper is organized as follows. In the first subsection, we recall the definition of unified (q, s)-entropy and discuss the properties of unified (q, s)-entropy entanglement. In the second subsection, we give our main results. We summarize our results in the third subsection.
Unified (q, s)-entropy entanglement and unified (q, s)-entropy entanglement of assistance
Given a quantum state ρ in the Hilbert space \( {\mathcal H} \). The unified (q, s)-entropy is defined as32
for any q, s ≥ 0 such that \(q\ne 1\) and \(s\ne 0\).
When s tends to 1, the unified (q, s)-entropy converges to Tsallis entropy T q (ρ)33
When s tends to 0, the unified (q, s)-entropy converges to Rényi entropy R q (ρ)34
When q tends to 1, the unified (q, s)-entropy converges to von Neumann entropy S(ρ)35
Because the limits exist in the case of q → 1 and s → 0, we will use q = 1 and s = 1 to represent the limits in this paper. Now, let’s consider the entanglement in terms of the unified (q, s)-entropy. For any pure state \({|\psi \rangle }_{AB}\) in the Hilbert space \({ {\mathcal H} }_{{\mathscr{A}}}\otimes { {\mathcal H} }_{ {\mathcal B} }\) (it’s does not matter for the sizes of subsystem A and B), the unified (q, s)-entropy entanglement is defined as36
for any q, s ≥ 0.
For a mixed state ρ AB , the unified (q, s)-entropy entanglement can be defined via the convex-roof extension
where the minimum is taken over all possible ensembles \(\{{p}_{i},{|{\psi }^{i}\rangle }_{AB}\}\) of ρ AB with \({\sum }_{i}{p}_{i}=1\) and p i ≥ 0. It is straightforward to verify that \({E}_{q,s}({\rho }_{AB}\mathrm{)=0}\) if and only if \({\rho }_{AB}\) is a separable state for \(q,s\ge 0\).
When \(s\) tends to 1, the unified \((q,s)\)-entropy entanglement becomes Tsallis entanglement31. When \(s\) tends to 0, the unified \((q,s)\)-entropy entanglement becomes Rényi entanglement20. Especially, The unified \((q,s)\)-entropy entanglement becomes the entanglement of formation when \(q\) tends to 1. The entanglement of formation is defined as37, 38
where \({E}_{f}(|{\psi }_{AB}^{i}\rangle )=-Tr{\rho }_{A}^{i}\,\mathrm{ln}\,{\rho }_{A}^{i}=-Tr{\rho }_{B}^{i}\,\mathrm{ln}\,{\rho }_{B}^{i}\) is the von Neumann entropy, the minimum is taken over all possible ensembles \(\{{p}_{i},{|{\psi }^{i}\rangle }_{AB}\}\) of ρ AB with \({\sum }_{i}{p}_{i}=1\) and p i ≥ 0.
As a dual quantity to the unified (q, s)-entropy entanglement, the unified (q, s)-entropy entanglement of assistance ((q, s)-EOA) can be defined as
where the maximum is taken over all possible ensembles \(\{{p}_{i},{|{\psi }^{i}\rangle }_{AB}\}\) of ρ AB with \({\sum }_{i}{p}_{i}=1\) and p i ≥ 0. To understand (q, s)-EOA better, consider a tripartite pure state \({|\psi \rangle }_{ABC}\) shared among three parties referred to as Alice, Bob, and Charlie39. The entanglement supplier, Charlie, performs a measurement on his share of the tripartite state, which yields a known bipartite entangled state for Alice and Bob. Tracing over Charlie’s system yields the bipartite mixed state \({\rho }_{AB}=T{r}_{C}({|\psi \rangle }_{ABC}\langle \psi |)\) shared by Alice and Bob. Charlie’s aim is to maximize entanglement for Alice and Bob, and the maximum average entanglement he can create is the (q, s)-EOA.
Concurrence and concurrence of assistance
For any pure state \({|\psi \rangle }_{AB}\) in the Hilbert space \({ {\mathcal H} }_{{\mathscr{A}}}\otimes { {\mathcal H} }_{ {\mathcal B} }\), the concurrence is defined as40
where \({\rho }_{A}=T{r}_{B}({\rho }_{AB})\).
For a mixed state ρ AB , the concurrence can be defined via the convex-roof extension
where the minimum is taken over all possible ensembles \(\{{p}_{i},{|{\psi }^{i}\rangle }_{AB}\}\) of ρ AB with \({\sum }_{i}{p}_{i}=1\) and p i ≥ 0.
As a dual quantity to concurrence, the concurrence of assistance (COA) can be defined as
where the maximum is taken over all possible ensembles \(\{{p}_{i},{|{\psi }^{i}\rangle }_{AB}\}\) of ρ AB with \({\sum }_{i}{p}_{i}=1\) and p i ≥ 0.
Analytical formula for two-qubit states
For a two-qubit mixed state ρ AB , concurrence and COA are known to have analytic formula30, 40
where λ i being the eigenvalues, in decreasing order, of matrix \(\sqrt{{\rho }_{AB}({\sigma }_{y}\otimes {\sigma }_{y}){\rho }_{AB}^{\ast }({\sigma }_{y}\otimes {\sigma }_{y})}\).
In ref. 40, Wootters derived an analytical formula of entanglement of formation for a two-qubit mixed state ρ AB
where \(h(x)=-x\,\mathrm{ln}\,x-(1-x)\,\mathrm{ln}(1-x)\) is the binary entropy.
In ref. 36, Kim found \({E}_{q,s}({\rho }_{AB})\) has an analytical formula for a two-qubit mixed state, which can be expressed as a function of concurrence \({{\mathscr{C}}}_{AB}\) for q ≥ 1, 0 ≤ s ≤ 1 and qs ≤ 3
where the function f q,s (x) has the form
where 0 ≤ x ≤ 1.
Main Results
In this section, we will provide our main results. First, we have following result:
Theorem 1. For any tripartite mixed state ρ ABC in the Hilbert space \({ {\mathcal H} }_{{\mathscr{A}}}\otimes { {\mathcal H} }_{ {\mathcal B} }\otimes { {\mathcal H} }_{{\mathscr{C}}}\), we have
where q ≥ 1 and qs ≥ 1.
Proof: Let \({\rho }_{ABC}=\,{\rm{\max }}\,{\sum }_{i}{p}_{i}{|{\psi }^{i}\rangle }_{A|BC}\langle {\psi }^{i}|\) be an optimal decomposition of \({E}_{q,s}^{a}({\rho }_{A|BC})\). That is
For any bipartite pure state \({|{\psi }^{i}\rangle }_{A|BC}\), the unified (q, s)-entropy entanglement \({E}_{q,s}({|{\psi }^{i}\rangle }_{A|BC})={S}_{q,s}({\rho }_{BC}^{i})\). In ref. 41, Rastegin proved that for any q ≥ 1 and qs ≥ 1, the unified (q, s)-entropy is subadditive, that is
Combining Eq. (18) with Eq. (19), we have
Thus, the proof is completed.
Theorem 1. Shows a simple but interesting polygamy relation of (q, s)-EOA in a tripartite quantum system. The upper bound of (q, s)-EOA of A|BC can’t be greater than the sum of (q, s)-EOA of B|AC and (q, s)-EOA of C|AB. In particular, for a tripartite pure state \({|\psi \rangle }_{A|BC}\), the unified (q, s)-entropy entanglement \({E}_{q,s}({|\psi \rangle }_{A|BC})\le {E}_{q,s}({|\psi \rangle }_{B|AC})+{E}_{q,s}({|\psi \rangle }_{C|AB})\).
We also have the following corollary:
Corollary 1. For any mixed state \({\rho }_{{A}_{1}|{A}_{2}\cdots {A}_{n}}\) in the Hilbert space \({ {\mathcal H} }_{{A}_{1}}\otimes { {\mathcal H} }_{{A}_{2}}\otimes \cdots \otimes { {\mathcal H} }_{{A}_{n}}\), we have
where q ≥ 1 and qs ≥ 1.
Corollary 1. Shows a constrained relationship of (q, s)-EOA in the multi-particle system, and gives an upper bound of (q, s)-EOA of \({A}_{1}|{A}_{2}\cdots {A}_{n}\). In particular, for any pure state \({|\psi \rangle }_{{A}_{1}|{A}_{2}\cdots {A}_{n}}\), the unified (q, s)-entropy entanglement \({E}_{q,s}({|\psi \rangle }_{{A}_{1}|{A}_{2}\cdots {A}_{n}})\le {\sum }_{i=2}^{n}{E}_{q,s}({|\psi \rangle }_{{A}_{i}|{A}_{1}\cdots {A}_{i-1}{A}_{i+1}\cdots {A}_{n}})\).
Example 1: Let’s consider the general GHZ state \(|GHZ\rangle =\alpha {|0\rangle }^{\otimes n}+\beta {|1\rangle }^{\otimes n}\), where |α|2 + |β|2 = 1 and n ≥ 3. It’s easy to show that \({\sum }_{i=2}^{n}{E}_{q,s}^{a}({\rho }_{{A}_{i}|{A}_{1}\cdots {A}_{i-1}{A}_{i+1}\cdots {A}_{n}})-{E}_{q,s}^{a}({|GHZ\rangle }_{{A}_{1}|{A}_{2}\cdots {A}_{n}})\) = \(\frac{n-2}{\mathrm{(1}-q)s}[{({|\alpha |}^{2q}+{|\beta |}^{2q})}^{s}-1]\ge 0\).
Example 2: Consider a four-qubit cluster state \(|{C}_{4}\rangle =\frac{1}{2}(|0000\rangle +|0011\rangle +|1100\rangle -|1111\rangle )\), which is a type of highly entangled state of four-qubit42, 43. The reduced states of \(|{C}_{4}\rangle \) are \({\rho }_{A}={\rho }_{B}={\rho }_{C}={\rho }_{D}=\frac{I}{2}\), thus \({\sum }_{i=2}^{n}{E}_{q,s}^{a}({\rho }_{{A}_{i}|{A}_{1}\cdots {A}_{i-1}{A}_{i+1}\cdots {A}_{n}})-{E}_{q,s}^{a}(|{C}_{4}\rangle )=\frac{2}{\mathrm{(1}-q)s}[\frac{1}{(q-\mathrm{1)}s}-1]\), which is nonnegative for q ≥ 1 and qs ≥ 1.
We note that for any n-qubit mixed state \({\rho }_{A{C}_{1}\cdots {C}_{n}}\), the polygamy relation holds:
for 1 ≤ q ≤ 2 and −q 2 + 4q − 3 ≤ s ≤ 144. Combining Eq. (17) with Eq. (22), we have
Corollary 2. For any multi-qubit mixed state \({\rho }_{AB{C}_{1}\cdots {C}_{n}}\), the following inequality holds
where 1 ≤ q ≤ 2, s = 1.In this case, (q, s)-EOA becomes Tsallis entropy entanglement of assistance which has discussed in ref. 22.
Before our second main result, we have following lemma:
Lemma 1. For q = 2 and \(\tfrac{1}{2}\le s\le 1\), the function f q,s (x) in Eq. (16) satisfies
Proof: For q ≥ 2, 0 ≤ s ≤ 1, and qs ≤ 3, we have \({f}_{q,s}(\sqrt{{x}^{2}+{y}^{2}})\ge {f}_{q,s}(x)+{f}_{q,s}(y)\) 36. On the other hand, for 1 ≤ q ≤ 2 and 0 ≤ s ≤ 1, we have \({f}_{q,s}(\sqrt{{x}^{2}+{y}^{2}})\le {f}_{q,s}(x)+{f}_{q,s}(y)\) 44. The equality holds if and only if q = 2 and \(\tfrac{1}{2}\le s\le 1\). This completes the proofs.
Next, the following result will provide a lower bound of unified (q, s)-entropy entanglement of \({|\psi \rangle }_{AB|{C}_{1}\cdots {C}_{n}}\), with respect to the bipartition between AB and \({C}_{1}\cdots {C}_{n}\):
Theorem 2. For any multi-qubit pure state \({|\psi \rangle }_{AB{C}_{1}\cdots {C}_{n}}\) in the Hilbert space, we have
where q = 2 and \(\frac{1}{2}\le s\le 1\).
Proof: Given a multi-qubit pure state \({|\psi \rangle }_{AB{C}_{1}\cdots {C}_{n}}\), the unified (q, s)-entropy is subadditive for any q ≥ 1 and qs ≥ 1. Thus, the following equality holds
which implies \({S}_{q,s}({\rho }_{{C}_{1}\cdots {C}_{n}})-{S}_{q,s}({\rho }_{A})\le {S}_{q,s}({\rho }_{A{C}_{1}\cdots {C}_{n}})\), and similarly, \({S}_{q,s}({\rho }_{A})-{S}_{q,s}({\rho }_{{C}_{1}\cdots {C}_{n}})\le {S}_{q,s}({\rho }_{A{C}_{1}\cdots {C}_{n}})\). Combine with the two equalities above, one obtain
From the definition of unified (q, s)-entropy entanglement of \({|\psi \rangle }_{AB|{C}_{1}\cdots {C}_{n}}\), with respect to the bipartition between AB and \({C}_{1}\cdots {C}_{n}\), we have
Note that for any pure state \({|\psi \rangle }_{ABC}\) in a \(2\otimes 2\otimes d\) system, the following equality holds45, 46
where ρ AB and ρ AC are the reduced matrices of state \({|\psi \rangle }_{ABC}\) respectively.
For q = 2 and \(\frac{1}{2}\le s\le 1\), we have
where we have used Eq. (29) in the second equality, the third equality holds is due to lemma 1. Therefore,
Compare Eq. (30) with Eq. (31), it’s easy to see that
We also note that
where the first equality holds is due to the monogamy of concurrence1 and f q,s (x) is an increasing function for q ≥ 2, 0 ≤ s ≤ 1, and qs ≤ 336.
On the other hand, we have
Combine Eqs (32) and (33) with Eq. (34), we have
Putting Eq. (35) into Eq. (32), we obtain our result. Similarly, we have
Thus, the proof is completed.
Theorem 2 shows a monogamy relation for a multi-qubit pure state \({|\psi \rangle }_{AB{C}_{1}\cdots {C}_{n}}\). The lower bound of the unified (q, s)-entropy entanglement for \(AB|{C}_{1}\cdots {C}_{n}\) can’t be less than the sum of the two-qubit entanglement between bipartitions of the system. In particular, if \({|\psi \rangle }_{AB{C}_{1}\cdots {C}_{n}}={|\psi \rangle }_{A{C}_{1}\cdots {C}_{n}}\otimes {|\psi \rangle }_{B}\), the entanglement of \(AB|{C}_{1}\cdots {C}_{n}\) is equal to the entanglement of \(A|{C}_{1}\cdots {C}_{n}\). In this case, \({E}_{q,s}({\rho }_{B{C}_{i}})=0\) for \(i=1,2,\ldots ,n\). Theorem 2 becomes \({E}_{q,s}({|\psi \rangle }_{A|{C}_{1}\cdots {C}_{n}})\ge {\sum }_{i=1}^{n}{E}_{q,s}^{a}({\rho }_{A{C}_{i}})\), which is a CKW-type monogamy relation1, 8.
Example 3: Consider a pure state \({|\varphi \rangle }_{AB{C}_{1}{C}_{2}}=\frac{1}{\sqrt{2}}(|0000\rangle +|1001\rangle )\) in the four-qubit system. for the range q = 2 and \(\frac{1}{2}\le s\le 1\), we have \({E}_{q,s}({\rho }_{A{C}_{1}})={E}_{q,s}^{a}({\rho }_{A{C}_{1}})=0\), and \({E}_{q,s}({\rho }_{A{C}_{2}})={E}_{q,s}^{a}({\rho }_{A{C}_{2}})=\frac{1}{s}(1-\frac{1}{{2}^{s}})\). \({E}_{q,s}({\rho }_{B{C}_{i}})={E}_{q,s}^{a}({\rho }_{B{C}_{i}})=0\) where i = 1, 2 and \({E}_{q,s}({|\varphi \rangle }_{AB{C}_{1}{C}_{2}})=\frac{1}{s}(1-\frac{1}{{2}^{s}})\). Therefore, we can see \({|\varphi \rangle }_{AB{C}_{1}{C}_{2}}\) saturates the inequality Eq. (25).
Example 4: Finally, let’s consider a general W state \({|W\rangle }_{{A}_{1}{A}_{2}\mathrm{..}.{A}_{n}}={a}_{1}|00\cdots 01\rangle +{a}_{2}|00\cdots 10\rangle +\cdots +{a}_{n}|10\cdots 00\rangle \) in the n-qubit system, where \({\sum }_{i}^{n}{|{a}_{i}|}^{2}=1\). The reduced state of subsystem A 1 A 2 is
which implies \({E}_{q,s}({|W\rangle }_{{A}_{1}{A}_{2}|{A}_{3}\mathrm{..}.{A}_{n}})\ge 0\). It’s also easy to show that the reduced state \({\rho }_{{A}_{i}{A}_{j}}\) is separable, where \(i,j=\{1,2,\ldots ,n\}\). Thus \({E}_{q,s}({\rho }_{{A}_{1}{A}_{i}})={E}_{q,s}^{a}({\rho }_{{A}_{2}{A}_{i}})={E}_{q,s}({\rho }_{{A}_{2}{A}_{i}})={E}_{q,s}^{a}({\rho }_{{A}_{1}{A}_{i}})=0\). We find that the right side of the inequality Eq. (25) is \({\rm{\max }}\{{\sum }_{i=2}^{n}[{E}_{q,s}({\rho }_{{A}_{1}{A}_{i}})-{E}_{q,s}^{a}({\rho }_{{A}_{2}{A}_{i}})],{\sum }_{i=2}^{n}[{E}_{q,s}({\rho }_{{A}_{2}{A}_{i}})-{E}_{q,s}^{a}({\rho }_{{A}_{1}{A}_{i}})]\}\) = 0. Which mean the inequality Eq. (25) holds for the general W state.
Conclusion
Unified (q, s)-entropy is an important generalized entropy in quantum information theory. Many entropies such as Tsallis entropy, Rényi entropy, and von Neumann entropy can be seen as a special case for unified (q, s)-entropy. In this paper, we have investigated the entanglement distribution in multi-particle systems in terms of unified (q, s)-entropy. We find that for any tripartite mixed state, the (q, s)-EOA follows a polygamy relation for q ≥ 1 and qs ≥ 1. This polygamy relation provides an upper bound for the bipartition A|BC, which also holds in multi-particle systems. Furthermore, for q = 2 and \(\frac{1}{2}\le s\le 1\), a generalized monogamy relation is provided for unified (q, s)-entropy entanglement. This monogamy relation provides a lower bound for the bipartition \(AB|{C}_{1}\cdots {C}_{n}\) in the multi-qubit system. In particular, if \({|\psi \rangle }_{AB{C}_{1}\cdots {C}_{n}}={|\psi \rangle }_{A{C}_{1}\cdots {C}_{n}}\otimes {|\psi \rangle }_{B}\), the generalized monogamy relation becomes a CKW-type monogamy relation.
Both monogamy property and polygamy property are fundamental properties of multipartite entangled states. We have studied the properties above in detail, and provided a two-parameters entropy function to study the entanglement distribution. We believe our result provides a useful methodology to understand the entanglement distribution of multi-particle entanglement.
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Acknowledgements
The authors are grateful to the anonymous referees for their comments and suggestions. This work is supported by the NSFC (Grants No. 11271237, No. 11671244, No. 61303009, No. 61671280, and No. 61673250), the Higher School Doctoral Subject Foundation of Ministry of Education of China (Grant No. 20130202110001), and Fundamental Research Funds for the Central Universities (Grants No. 2016TS060 and No. 2016CBY003).
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Y. Luo performed the calculations and wrote the main manuscript. F.-G. Zhang checked the calculations. Y. Li improved the manuscript. All authors contributed to the discussion and reviewed the manuscript.
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Luo, Y., Zhang, FG. & Li, Y. Entanglement distribution in multi-particle systems in terms of unified entropy. Sci Rep 7, 1122 (2017). https://doi.org/10.1038/s41598-017-01286-2
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DOI: https://doi.org/10.1038/s41598-017-01286-2
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