Maximal Bell's inequality violation for non-maximal entanglement

Maximal Bell's inequality violation for non-maximal entanglement

Physics Letters A 323 (2004) 190–193 Maximal Bell’s inequality violation for non-maximal entanglement M. Kobayashi a,b , ...

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Physics Letters A 323 (2004) 190–193

Maximal Bell’s inequality violation for non-maximal entanglement M. Kobayashi a,b , F. Khanna b , A. Mann c , M. Revzen b,c,∗ , A. Santana d a Department of Physics, Gifu University, Gifu, Japan b Physics Department, University of Alberta, Edmonton, Alberta, T6G 2J1 Canada c Department of Physics, Technion—Israel Institute of Technology, Haifa 32000, Israel d Instituto de Física, Universidade Federal da Bahia, Salvador, Bahia, Brazil

Received 19 November 2003; received in revised form 18 December 2003; accepted 3 February 2004 Communicated by P.R. Holland

Abstract Bell’s inequality violation (BIQV) for correlations of polarization is studied for a product state of two two-mode squeezed vacuum (TMSV) states. The violation allowed is shown to attain its maximal limit for all values of the squeezing parameter, ζ . We show via an explicit example that a state whose entanglement is not maximal allow maximal BIQV. The Wigner function of the state is non-negative and the average value of either polarization is nil.  2004 Elsevier B.V. All rights reserved. PACS: 03.65.Bz Keywords: Bell’s inequality; Entanglement; Polarization; Parity

Two-mode squeezed vacuum (TMSV) states generated via non-degenerate parametric amplifiers are of interest, following the pioneering study of Grangier et al. [1] that exhibited Bell’s inequality violations (BIQV) [2,3] via intensity correlation measurements [4] for such states. (The Bell inequality we consider in this Letter is the so-called CHSH inequality, [5].) These BIQV studies, aside from their intrinsic interest, are valuable for elucidation of the applicability of Bell’s inequalities for continuous variables problems in general, and to the original Einstein, Podolsky and Rosen (EPR) [6] state in particular. The latter’s per-

* Corresponding author.

E-mail address: [email protected] (M. Revzen). 0375-9601/$ – see front matter  2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2004.02.005

tinency to BIQV was discussed by Bell [7]. The reason for this general interest is associated with the fact that the Wigner function [8] for the EPR state (and, in general, for the TMSV state) is non-negative and thus has some attributes of a probability density in phase space, i.e., the problem relates to the fundamental question as to whether the quantum behaviour of these states can be underpinned with a theory based on local hidden variables. Bell [7] considered the problem and discussed correlations among the particles’ positions. He suggested [7] that such correlations within the EPR state (whose Wigner function is non-negative) will not allow BIQV. Wodkiewicz and Banaszek [9] noted that, intrinsically, [10–12], it is the parity correlation that is expressed by the Wigner function. They [9] then showed that the TMVS allow BIQV via parity

M. Kobayashi et al. / Physics Letters A 323 (2004) 190–193

correlations measurements. This analysis was greatly advanced by explicit definitions of “rotation” in parity space in [13,14], where it is shown that BIQV can achieve its Cirelson’s limit [15] for maximal entanglement, i.e., ζ → ∞, where ζ is the (positive) squeezing parameter. The above underscores the importance of specifying the operators (i.e., the dynamical variables) involved in the definition of the Bell operator [16], whose correlation values are bounded by Bell’s inequality. In the present Letter we consider polarization as our dynamical variable (the representative operator is given below—above Eq. (17)). We show that the state under study: |ζ , a product of two TMSV states, whose Wigner function is non-negative, exhibits the remarkable property of allowing maximal BIQV, i.e., attaining the Cirelson limit, for all values of the squeezing parameter ζ . The state under study, |ζ , is given by ([17]) |ζ  = exp(ζ Kx )|0,


+ + + + b+ + a− b− − a+ b+ − a− b− . Kx = a+


Here the +/− subscripts denote the polarization relative to some chosen axis, common to A and B, with A and B labeling the two different channels toward which the two beams head—the operators + a and b refer to the respective channels: e.g., a+ designates the creation operator for horizontally, i.e., “x-polarized” [18] photon headed into the A channel, etc. |ζ  is a product of two two-mode squeezed vacua—one pertaining to x-polarized and one to ypolarized. The Wigner function W (αA , αB ) for the state is thus, likewise, a product of two functions each of the form [19] (α = q + ip; we delete the polarization subscripts to reduce notational cluttering) W (αA , αB )    4 = 2 exp −2 cosh(2ζ ) |αA |2 + |αB |2 π  + 2 sinh(2ζ )(αA αB + c.c.) .


The total Wigner function, being a product of two such Gaussian functions, is clearly non-negative. We further recall the symmetry operator [17],  +  + + + a− − a− a+ + b+ b− − b− b+ . K0 = i a+ (4) Since [K0 , Kx ]− = 0,



and K0 |0 = 0,


we have |ζ  ≡ exp(−iδK0 )|ζ  = |ζ ,


i.e., exp(−iδK0 ) is a symmetry operator: it leaves the state invariant. Note that K0 is made of, additively, two parts   + + a− − a− a+ , and K0A = i a+   + + K0B = i b+ (8) b− − b− b+ . Each of these parts acts on a distinct channel—which we take to be at a different locale, but K0 = K0A + K0B ,


is a symmetry operator for the system as a whole. On the other hand, the operator exp(iδA K0A ) is a symmetry breaking operator—it breaks the K0 symmetry. This operator does not commute with Kx . + Rotating the polarizations, [18], i.e., e.g., a+ → + aδA , is accomplished via     aα+ (δA ) = exp iδA K0A aα+ exp −iδA K0A , α = ±,     bβ (δB ) = exp iδB K0B bβ exp −iδB K0B , β = ±, and their Hermitian adjoints. We shall study the correlations between polarizations of the two channels. To this end our normalizer (i.e., reference correlation) is,  +   + + + a+ − a− a− b+ b+ − b− b− |ζ . C(A, B) = ζ | a+ (10) It is shown in the Appendix of [17] that the correlation function   C αβ (δA , δB ) = ζ | exp iδA K0A + iδB K0B aα+ aα bβ+ bβ   × exp −iδA K0A − iδB K0B |ζ , α, β = ±, is a function of δ = δA − δB . Hence the operator, exp(−iδK0 ) implies   C A(δ), B(δ)  +  +  = ζ | aδ+ aδ − aδ+ ¯ aδ¯ bδ bδ − bδ¯ bδ¯ |ζ    = C A(0), B(0) ≡ C(A, B), (11)


M. Kobayashi et al. / Physics Letters A 323 (2004) 190–193

with δ¯ = δ ± π/2 (cf. [18]). The polarization along δ¯ is orthogonal to the one along δ, i.e., the correlation is invariant under equal (common) rotation of the two channels. Direct calculation gives   C A(δ), B(δ ) = C ++ (δ, δ ) + C −− (δ, δ ) − C +− (δ, δ ) − C −+ (δ, δ ),


thus, C(A, B) = 2 cosh2 ζ sinh2 ζ.


Similarly, direct calculation yields   C A(δ), B(δ ) = C(A, B) cos 2(δ − δ ).


Define the normalized expectation value of the polarization correlation by C(A(δ), B(δ )) E(δ, δ ) = √ C(A(δ), B(δ))C(A(δ ), B(δ )) = cos 2(δ − δ ).


Our expression for the Bell inequality [4] is, thus  E(δA , δB ) + E(δA , δ ) B  + E(δ , δB ) − E(δ , δ )  2. (16) A



Using Eq. (15), the left-hand side can be written as  cos 2(δA − δB ) + cos 2(δA − δ ) + cos 2(δ − δB ) B A  − cos 2(δ − δ )  2. (17) A


= π/4, δ = π/8, δ = Clearly the choice δA = 0, δA B B √ −π/8, gives for the LHS 2 2, i.e., maximal violation of the inequality. This result is independent of ζ . We recall that for each polarization, ±, the entanglement of the state is maximized only for ζ → ∞ [9,13, 14], i.e., not only is the state under study a product of two states, but also both component states are not maximally entangled (for ζ < ∞). However, BIQV discussed in these references are for parity as the relevant dynamical variable. In our study the relevant dynamical variable is the polarization in each channel, + + a+ − a− a− ). For this variable the amount viz: (a+ of BIQV can be estimated by evaluating its average value:  +  + ζ | a+ (18) a+ − a− a− |ζ  = 0,

which holds for all values of ζ for the pure state |ζ  (Eq. (1)), and for both channels variables. This value,

(nil) in Eq. (18), is analogous to the two-spin state case, such as, e.g. (the numerical superscripts label the particles),     |ψ = α ↑1 ↑2 + β ↓1 ↓2 , where ψ|σz(1) |ψ = ψ|σz(2) |ψ = 0 implies allowance of maximal BIQV and, in the spin case, |α| = |β|, i.e., maximal entanglement. For comparison, we evaluate (just for one polarization, +) the expectation value of the parity operator [13,14] as the relevant dynamical variable,  ∞  (1 − tanh2 ζ ) −1n+ |n+ n+ | |ζ  = . ζ | (19) (1 + tanh2 ζ ) n=0 Hence, with parity as the relevant variable the state allows maximal BIQV only at ζ → ∞—only at this limit does average value (of the relevant dynamical variable for either channel) vanishes. We have shown thus that correlations among polarizations as dynamical variables (with expectation values between −1 and 1) allow maximal Bell inequality violation for a product of two two-mode squeezed vacuum states. The Wigner function of the state is nonnegative. This same state for parity as the dynamical variable allows maximal violation of Bell’s inequality only in the limit of the squeezing parameter ζ → ∞, i.e., only when the state’s expectation value for either parity is nil. Hence we have demonstrated that the violation incurred in Bell’s inequality is strongly related not only to the entanglement (of the state) and the particular dynamical variables—i.e., the variables involved in the correlations considered in the Bell operator but that maximal entanglement is not necessary for maximal Bell’s inequality violation.

Acknowledgements The research of F.K. is supported by NSERCC and the research of A.S. is supported by CNPq.

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