Squeezed and Entangled States of a Single Spin
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SQUEEZED AND ENTANGLED STATES OF A SINGLE SPIN Barı¸s Oztop,¨ Alexander A. Klyachko and Alexander S. Shumovsky Faculty of Science, Bilkent University Bilkent, Ankara, 06800 Turkey e-mail: [email protected] (Received 23 December 2007; accepted 1 March 2007) Abstract We show correspondence between the notions of spin squeez- ing and spin entanglement. We propose a new measure of spin squeezing. We consider a number of physical examples. Concepts of Physics, Vol. IV, No. 3 (2007) 441 DOI: 10.2478/v10005-007-0020-0 Barı¸s Oztop,¨ Alexander A. Klyachko and Alexander S. Shumovsky It is well known that the concept of squeezed states [1] was orig- inated from the famous work by N.N. Bogoliubov [2] on the super- fluidity of liquid He4 and canonical transformations. Initially, it was developed for the Bose-fields. Later on, it has been extended on spin systems as well. The two main objectives of the present paper are on the one hand to show that the single spin s 1 can be prepared in a squeezed state and on the other hand to demonstrate≥ one-to-one correspondence between the notions of spin squeezing and entanglement. The results are illustrated by physical examples. Spin-coherent states | Historically, the notion of spin-coherent states had been introduced [3] before the notion of spin-squeezed states. In a sense, it just reflected the idea of Glauber [4] about creation of Bose-field coherent states from the vacuum by means of the displacement operator + α field = D(α) vac ;D(α) = exp(αa α∗a); (1) j i j i − where α C is an arbitrary complex parameter and a+; a are the Boson creation2 and annihilation operators. In the case of spin-s system specified by the three spin operators Sx;Sy;Sz that obey the commutation relations [Sj;Sk] = ijk`S`; (2) the lowest spin state s (Sz s = s s ) plays the role of the vacuum state, so thatj − thei spin-coherentj − i − statej − i has the form α = exp(αS+ α∗S ) s (3) j i − − j − i where 1 S = (Sx iSy) (4) ± p2 ± are the spin-ladder operators (compare with Eq. (1)). For extension of the notion of coherence on arbitrary Lie groups we refer to [5]. Since the Hermitian spin operators Sx;Sy;Sz form an orthogonal basis of the su(2) algebra, the spin-displacement operator (exponen- tial in Eq. (3) belongs to the SU(2) group, so that the spin-coherent 442 Concepts of Physics, Vol. IV, No. 3 (2007) Squeezed and entangled states of a single spin state (3) can be considered as the SU(2) rotation of the minimal state s . In other words, there is a direction ~eα in the three-dimensional j− i spin space (new quantization axis) such that the operator Sz;~eα (z- component of the spin vector, which is now directed along the ~eα axis, has the same eigenvalue s in the state α . As well as conventional coherent− states, thej i spin-coherent states (3) can be specified by uncertainties of spin operators measured in those states. In view of the commutation relation (2), the Heisenberg uncertainty relation in an arbitrary state has the form 1 2 V (Sj; )V (Sk; ) S` :; (5) ≥ 4jh j j ij where V (X; ) X2 X 2 ≡ h j j i − h j j i denotes the variance (uncertainty). Thus, in the coherent state (3), the uncertainty of the corresponding z-component V (Sz;~eα ; α) = 0 and α Sz;~e α = s. In turn, the variances of the two spin operators jh j α j ij orthogonal to Sz;~eα are both equal to s=2. An important characteristics of states is provided by the total variance (see Ref. [6] and references therein) X V( ) = V (Sj; ): (6) j=x;y;z By definition, this quantity is invariant under rotations of the basis of the su(2) algebra. Thus, for an arbitrary spin-coherent state V(α) = s. It can be easily seen that s V( ) s(s + 1); (7) ≤ ≤ so that the spin-coherent state corresponds to the minimal total vari- ance. It is also easily seen that any state of spin-1=2 system is a spin-coherent one. Spin-squeezed states | In the case of Bose-fields, the simplest squeezed state is defined by action of the squeezing operator (ζ) on the vacuum state [1] S 1 2 1 +2 ζ field = exp ζ∗a ζa vac (8) j i 2 − 2 j i Concepts of Physics, Vol. IV, No. 3 (2007) 443 Barı¸s Oztop,¨ Alexander A. Klyachko and Alexander S. Shumovsky (for recent review, see [7]). For the two field quadratures Q = (a+ +a) and P = i(a a+) that obey the uncertainty relation − − V (Q; )V (P; ) 1; ≥ in the state (8) we have 2 ζ 2 ζ V (Q; ζ) = e− j j;V (P; ζ) = e j j; so that uncertainty of one of the quadratures can be squeezed with si- multaneous enlargement of uncertainty of the other quadrature under the condition that V (Q; ζ)V (P; ζ) = 1. The attempts to define spin-squeezed states in direct analogy with the Bose-field squeezed states have failed (see critical review in Ref. [8]). An approach has been proposed in Ref. [8] considers the mean- spin vector S¯ with components Sj (j = x; y; z). Its direction again is chosen to be the z-axis. Ah spinj j statei is called to be squeezed whenever the uncertainty in one of the spin components orthogonal to this direction is smaller than s=2. For the spin-squeezed state, the inequality (7) takes the form Vcoh = s < V(ζ) s(s + 1) = Vmax: (9) ≤ Measure of spin squeezing | With the inequality (9) in hands, we can define a measure of squeezing of an arbitrary state of spin 1. It is possible to carry the interval of this inequality into [0; 1] such as V( ) Vcoh 0 < − 1: Vmax Vcoh ≤ − Thus, the measure ν( ) of squeezing becomes V( ) Vcoh ν( ) = − ; (10) Vmax Vcoh − which takes the value 0 for spin-coherent states (no squeezing) and 1 for the maximally squeezed state. Taking into account that X S2 = s(s + 1) h j j j i j=x;y;z 444 Concepts of Physics, Vol. IV, No. 3 (2007) Squeezed and entangled states of a single spin for any state , we can now rewrite Eq. (10) as follows S¯2 ν( ) = 1 ; (11) − s2 ¯2 P 2 where S j Sj is the squared length of the mean-spin vector. ≡ h j j i Entanglement in spin systems | According to the modern theory of quantum entanglement (see [6] and references therein), the com- pletely entangled (CE) states of a given system are defined to be the states with the maximal amount of total uncertainty calculated for the set of basic observables. In the case of a single spin, the basic observables are given by the operators Sx;Sy;Sz, so that V( CE) = max V( ) = s(s + 1); where maximum is taken over all states in the (2s + 1)-dimensional Hilbert space of spin states. This definition assumes the following condition CE Sj CE = 0 j = x; y; z ; (12) h j j i 8 that can be used as the operational definition of complete entan- glement [9] (also see discussion in Appendix A). All other entangled states can be obtained from the completely entangled states by means of SLOCC (stochastic local operations assisted by classical commu- nications) [10]. Thus, by definition, maximally squeezed spin states coincide with the completely entangled spin states. Moreover, it has been shown in Ref. [11] that the square root of the measure of squeezing (11) gives the measure of the amount of entanglement carried by a pure spin state: s ¯2 p S µ( ) ν( ) = 1 : (13) − s2 Let us now note that an organic connection between the notions of entanglement and squeezing is under discussion for some time [12, 13]. In the most of works, a kind of \two-mode" entanglement [14] was considered for the two-qubit (two spin-1=2) systems. In the papers Concepts of Physics, Vol. IV, No. 3 (2007) 445 Barı¸s Oztop,¨ Alexander A. Klyachko and Alexander S. Shumovsky [15, 17, 16], the possibility of entanglement of a single spin-1 object has been shown. It should be stressed that our present consideration shows equivalence of the spin squeezing and spin entanglement for a single object with any s = 1. At s > 1, squeezing implies entangle- ment but not vice versa. Squeezed states of spin-1 | As an illustrative physical example of some considerable interest, examine now squeezing of a single spin-1 object. Stress that hereafter by \spin" we mean an object with the dynamic symmetry given by the SU(2) group, so that it can be a real spin, isospin, angular momentum, etc. A general pure state in the three-dimensional Hilbert 3 space has the form H 1 1 X X 2 = s s ; s = 1: (14) j i j i j j s= 1 s= 1 − − In the basis s , spin operators have the following representation j i Sx = ( + 1 + 1 ) 0 + 0 ( +1 + 1 ); j i j − i h j j i h j h− j Sy = i( + 1 1 ) 0 + i 0 ( +1 1 ); (15) − j i − j − i h j j i h j − h− j Sz = + 1 +1 1 1 : j ih j − j − ih− j Since the spin-displacement operator in Eq. (3) has the form 0 iφ 2iφ 1 1 [1 + cos( α p2)] e sin( α p2) e [1 cos( α p2)] 2 p2 2 −iφ j j j j iφ − j j B e sin( α p2) cos( α p2) e sin( α p2) C B p2 p2 C @ −−2iφ j j −iφ j j j j A e [1 cos( α p2)] e sin( α p2) 1 [1 + cos( α p2)] 2 − j j − p2 j j 2 j j the coherent states for spin 1 have the following general form e2iφ eiφ α = [1 cos( α p2)] + 1 + sin( α p2) 0 j i 2 − j j j i p2 j j j i 1 + [1 + cos( α p2)] 1 : (16) 2 j j j − i Here φ = arg α.