Relationship between the n-tangle and the residual entanglement of

Relationship between the n-tangle and the residual entanglement of even n qubits1

arXiv:1003.4774v1 [quant-ph] 24 Mar 2010

a

Xiangrong Lia , Dafa Lib Department of Mathematics, University of California, Irvine, CA 92697-3875, USA b Dept of mathematical sciences, Tsinghua University, Beijing 100084 CHINA

Abstract We show that n-tangle, the generalization of the 3-tangle to even n qubits, is the square of the SLOCC polynomial invariant of degree 2. We find that the n-tangle is not the residual entanglement for any even n ≥ 4 qubits. We give a necessary and sufficient condition for the vanishing of the concurrence C1(2...n) . The condition implies that the concurrence C1(2...n) is always positive for any entangled states while the n-tangle vanishes for some entangled states. We argue that for even n qubits, the concurrence C1(2...n) is equal to or greater than the n-tangle. Further, we reveal that the residual entanglement is a partial measure for product states of any n qubits while the n-tangle is multiplicative for some product states. Keywords: the 3-tangle, the n-tangle of even n qubits, the residual entanglement, SLOCC polynomial invariants PACS numbers: 03.67.Mn, 03.65.Ud

1 The paper was supported by NSFC(Grants No. 10875061,60433050, and 60673034 ) and Tsinghua National Laboratory for Information Science and Technology.

1

1

Introduction

Quantum entanglement is an important physical reτ 1...n = 2|S|, X source in quantum information and computation such S = (aα1 ...αn aβ 1 ...βn aγ 1 ...γn aδ1 ...δn as quantum teleportation, cloning and encryption. En×ǫα1 β 1 ǫα2 β2 ...ǫαn−1 βn−1 tanglement phenomenon distinguishes the quantum world ×ǫγ 1 δ1 ǫγ 2 δ2 ...ǫγ n−1 δn−1 ǫαn γ n ǫβ n δ n ), from the classical world. Considerable attention has been paid in recent years to the quantification and (1.2) classification of entanglement. The concurrence was proposed by Wootters in 1998 to quantify entangle- where |c| is the modulus of the complex number c, αl , ment for bipartite systems [1]. For two qubits, the β l , γ l , and δ l ǫ{0, 1}, and concurrence was defined as C12 = M ax{0, λ1 − λ2 − ǫ00 = ǫ11 = 0 and ǫ01 = −ǫ10 = 1. (1.3) λ3 − λ4 }, where λ2i are the eigenvalues, in decreasing order, of ρ12 ˜ ρ12 . Here, ρ12 is the density matrix The n-tangle of even n qubits is invariant under and ρ ˜12 is the “spin-flipped”density matrix of ρ12 , i.e., permutations of the qubits, and is an entanglement ∗ ρ ˜12 = σ y ⊗ σ y ρ12 σ y ⊗ σ y [2], where the asterisk demonotone [3]. In [3], the n-tangle was proposed as a notes complex conjugation in the standard basis. For the state |ψi of a bipartite system, the concurrence potential entanglement measure. The generalized CKW monogamy inequality for n was also given by [4] qubits was given by [18, 19] q (1.1) C(ψ) = 2(1 − T r(ρ2A )). 2 2 2 . (1.4) C12 + ... + C1n ≤ C1(2...n) The definition of the concurrence in Eq. (1.1) was generalized to multipartite systems [5]. Recently, the concurrence was used to study quantum phase transitions [6]. By means of the concurrence, CKW monogamy inequality for three qubits was established. Namely, 2 2 2 C12 + C13 ≤ C1(23) [2]. Here ρ12 is obtained from the density matrix ρ123 by tracing out over qubit 3, 2 and C1(23) = 4 det ρ1 , where ρ1 = tr23 ρ123 . Note that C1(23) can be called the concurrence between qubit 1 and the pair of qubits 2 and 3 if qubits 2 and 3 are 2 regarded as a single object. The difference (C1(23) − 2 2 (C12 + C13 )) between the two sides of the above CKW monogamy inequality is called “residual entanglement”. The algebraic expression for the residual entanglement is called the 3-tangle (see (20) of [2] for the expression). The expression can also be obtained from Eq. (1.2) by letting n = 3. The 3-tangle is invariant under permutations of all the qubits [2]. The invariance of entanglement measure under permutations of all the qubits represents a collective property of the qubits. The 3-tangle is also an entanglement monotone [7]. Monotonicity for entanglement measure is a natural requirement. The 3-tangle was extended to even n qubits, and the extension was called the n-tangle [3]. Let the state P |ψi = i1 i2 ...in ai1 i2 ...in |i1 i2 ...in i, where i1 , i2 , ..., in ǫ {0, 1}. The n-tangle was defined as [3]

Here ρ12 = tr3...n ρ12...n , i.e., ρ12 is obtained from the density matrix ρ12...n by tracing out over qubits 2 3, ..., and n, and C1(23...n) = 4 det ρ1 , where ρ1 = tr23...n ρ123...n . Note that C1(2...n) can be called the concurrence between qubit 1 and qubits 2, ..., and n if qubits 2, ..., and n are regarded as a single object. The difference between the two sides of CKW monogamy inequality in Eq. (1.4) can be considered as a natural generalization of the residual entanglement of three qubits to n qubits, and was denoted as [18] 2 2 2 + ... + C1n ). − (C12 τ 1(2...n) = C1(2...n)

(1.5)

In this paper, we investigate the relationship between the n-tangle and the residual entanglement for any even n ≥ 4 qubits. This paper is organized as follows. In Sec. 2, we show that the n-tangle is the square of the SLOCC polynomial invariant of degree 2. In Sec. 3, we address the relationship between the n-tangle and the residual entanglement of n qubits. In Sec. 4, we summarize our results and conclusions.

2

The n-tangle is the square of the SLOCC polynomial invariant of degree 2

The SLOCC invariants can be used for SLOCC classification and the entanglement measure [8, 9, 10, 11, 2

Here we take N (l) to be the number of the occurrences of “1” in ln−1 ...l1 l0 , which is a n-bit binary representation of l, i.e., l = ln−1 2n−1 + ... + l1 21 + l0 20 . In [13], it was proven that if |ψi and |ψ ′ i are SLOCC equivalent then

12, 13, 14]. For four qubits, four independent SLOCC polynomial invariants: H, L, M , and Dxt were given in [9], where H is of degree 2, L and M are of degree 4, and Dxt is of degree 6. Very recently, for four and five qubits, SL invariants of degrees 2 (for only four qubits), 4, 6, 8, 10, 12 were studied in [14]. The antilinear operators “combs”, which are invariant under SL(2, C), were constructed in [15]. The geometry of four qubit invariants was investigated in [11]. For any even n qubits, the SLOCC polynomial invariant of degree 2 was given in [13]. The SLOCC invariant of degree 4 of odd n qubits was discussed in [12, 13]. Note that there are no invariants of degree 2 for odd n qubits [9].

2.1

I ∗ (b, n) = I ∗ (a, n) det(A1 )... det(An ),

(2.6)

where I ∗ (b, n) is obtained from I ∗ (a, n) by replacing a in I ∗ (a, n) with b, and I ∗ (a, n) was called the SLOCC polynomial invariant of degree 2 of even n qubits. Note that S0 is just I ∗ (a, n) (see (B) in Appendix A for the proof). By virtue of Eqs. (2.2) and (2.4), we have τ 1...n = (τ ′ (ψ))2 . It then follows from Eqs. (2.4) and (A8) that

Reduction of the n-tangle

The n-tangle in Eq. (1.2) is quartic and the computation of the coefficients takes 3 ∗ 24n multiplications. Denote by αi the complement of αi . That is, αi = 0 when αi = 1. Otherwise, αi = 1. Further, let

τ 1...n

2 2n−1 −1 X = 4 (−1)N (l) a2l a(2n −1)−2l . l=0

(2.7)

In Eq. (2.7), computing the coefficients requires (2n−1 + 2) multiplications. The n-tangle τ 1...n is not considered as the SLOCC polynomial invariant of deα1 ...αn−1 τ 1...n is quartic and satisfies the equation (2.1) gree 4 though ×ǫα1 α1 ǫα2 α2 ...ǫαn−1 αn−1 ). τ 1...n (|ψ ′ i) = τ 1...n (|ψi) det(A1 )... det(An ). However, Note that S0 is of degree 2. Then, S in Eq. (1.2) can the square root of the n-tangle is the SLOCC polybe reduced to S = 2S02 (see (A) of Appendix A for the nomial invariant of degree 2. The square root of the proof). This leads to n-tangle turns out to be τ ′ (ψ). Using the properties of ′ τ 1...n = |2S0 |2 . (2.2) τ (ψ) [13, 16], the square root is also an entanglement monotone, and invariant under permutations of all the qubits. S0

2.2

=

X

(aα1 ...αn−1 0 aα1 ...αn−1 1

The n-tangle is the square of the SLOCC polynomial invariant of de3 gree 2 P2n −1

P2n −1

Let |ψi = i=0 ai |ii and |ψ ′ i = i=0 bi |ii be any states of n qubits. Two states |ψi and |ψ ′ i are SLOCC equivalent if and only if there exist invertible local operators A1 , A2 , ..., An such that [7]

3.1

Relationship between the n-tangle and the residual entanglement The n-tangle is not the residual entanglement for any even n ≥ 4 qubits.

(2.3) To illustrate the relationship between n-tangle and residual entanglement, we consider the following exn amples. For the n-qubit state α1 |0...1i + α2 |0...010i + The entanglement measure of the state |ψi of even n ... + αn |10...0i, equality in Eq. (1.4) holds [2, 20], qubits was proposed as [13, 16] i.e. the residual entanglement τ 1(2...n) = 0. Accord′ ∗ τ (ψ) = 2 |I (a, n)| , (2.4) ing to Eq. (2.7), it is easy to see that the n-tangle τ 1...n = 0. It follows that τ 1...n = τ 1(2...n) . This is where particularly true for the n-qubit state |W i [3]. For the state |GHZi = √12 (|0i⊗n + |1i⊗n ), the residual 2n−2 X−1 ∗ N (l) I (a, n) = [(−1) entanglement τ 1(2...n) = 1 [20], and the n-tangle τ 1...n = l=0 1 [3]. Thus, τ 1...n = τ 1(2...n) for the state |GHZi. Here ×(a2l a(2n −1)−2l − a2l+1 a(2n −2)−2l )]. (2.5) is another example which gives τ 1...n = τ 1(2...n) = |ψ ′ i = A1 ⊗ A2 ⊗ ... ⊗ An |ψi. {z } |

3

2

3.3

4 |αγ| for the state of four qubits: α|0011i+β|0110i+ γ|1100i by utilizing Eq. (2.7). One might wonder if the two generalizations, which are the n-tangle τ 1...n and the residual entanglement τ 1(2...n) , are equal. However, this is not always the case as the following example will show. Consider, for example, the n-qubit symmetric Dicke states with l excitations (1 ≤ l ≤ (n − 1)) [21] X Pi |11 12 ...1l 0l+1 ...0n i, (3.1) |l, ni =

The concurrence C1(2...n) ≥ the ntangle τ 1...n

A closer examination of Eqs. (3.2) and (2.5) reveals that for even n qubits, the concurrence C1(2...n) is equal to or greater than the n-tangle τ 1...n (see Appendix B for the proof). We immediately have the following corollaries: (1). For any state |ψi of even n qubits, if the concurrence C vanishes then, clearly, so does the n-tangle. (2). If the n-tangle τ 1...n of even n qubits is positive, then the concurrence C1(2...n) is also positive.

i

where {Pi } is the set of all the distinct permutations of the qubits. For the Dicke state |(n/2), ni with (n/2) excitations of any even n ≥ 4 qubits, Eq. (2.7) yields 3.4 The residual entanglement is a partial measure for product states the n-tangle τ 1...n = 1. In this case, ρ12 ρ˜12 has only n n−2 2 three nonzero eigenvalues ( 2(n−1) )2 , ( 4(n−1) ) (douIn this section, we show that for product state |ψi1...l ⊗ 1 2 ble). We then get the concurrence C12 = (n−1) 2 . The |φi(l+1)...n of any n qubits, where |ψi is the state of the 2 2 symmetry of the Dicke state leads to C1i = C12 , i = first l qubits, the residual entanglement τ 1(2...n) for the 2 3, ..., n. Calculating C1(2...n) further gives C1(2...n) = product state is reduced to the residual entanglement 1. In light of Eq. (1.5), the residual entanglement τ 1(2...l) for the state |ψi. First we observe that ρ1 (|ψi⊗ |φihψ| ⊗ hφ|) = ρ1 (|ψihψ|). By the definition of the τ 1(2...n) = n−2 n−1 . It says that for the Dicke state |(n/2), ni, the n-tangle τ 1...n is greater than the residual entan- concurrence, 1 . glement τ 1(2...n) and the difference is given by n−1 C1(2...n) (|ψi ⊗ |φi) = C1(2...l) (|ψi). (3.3)

3.2

A necessary and sufficient condi- That is, the concurrence C1(2...n) for the product state tion for the vanishing of the con- |ψi ⊗ |φi is just the concurrence C1(2...l) for the state |ψi. It tells us that the concurrence C1(2...n) only meacurrence C1(2...n)

sures the entanglement of the state |ψi. P n For the state |ψi = 2i=0−1 ai |ii of n qubits, the conLikewise, the concurrence C1k for the state |ψi1...l ⊗ currence C1(2...n) can be written as |φi(l+1)...n is just the concurrence C1k for the state |ψi1...l , k = 2, ..., l. Since qubits 1 and k are not enX tangled, the concurrence C1k for the state |ψi1...l ⊗ 2 =4 C1(2...n) |ai aj+2n−1 − ai+2n−1 aj |2 . |φi(l+1)...n vanishes for k > l. This can be seen as fol0≤i