Foray into Relativistic Quantum Information Science:

1. Foray into Relativistic Quantum Information Science: & Foray into Microsoft Powerpoint presentation Wigner Rotations and Bell States Chopin Soo Laboratory for Quantum Information Science (LQIS) (http://www.phys.ncku.edu.tw/~QIS/) Physics Dept., NCKU ref : quant-ph/0307107 seminar: Inst. of Phys. Acad. Sinica (Sept. 26, 2003)

2. Apology:

3. Motivations for investigating Relativistic(Lorentz Invariant) QIS: Applications: e.g. quantum cryptography, entanglement-enhanced communication, high precision clock synchronization based upon shared entanglement, quantum-enhanced positioning, quantum teleportation,… Need: careful analysis of properties of entangled particles under Lorentz transformations, & construction of meaningful measures of entanglement (key concept and primary resource in QIS) Issues: Lorentz invariance of entanglement (?) Possible modifications to Bell Inequality violations => alter efficiency of eavesdropper detection, compromise security of quantum protocols. Quantum teleportation: Realizable, and compatible with QFT ?

4. Conceptual/consistency issues: e.g. LOCC (local operation and classical communication) is often invoked (e.g. in quantum teleportation) in non-relativistic QIS, but quantum-classical interface not sharply defined. Bell Inequality violation: => Not compatible with local, non-superluminal hidden variable theory. “Compatible” with QM, and no faster-than-light communication. But non-rel. QM not fully consistent (!) with Lorentz invariance and causal structure of spacetime. OR (a better formulation(?)) violation is consequence of, and fully compatible with, quantum theory which is local, Lorentz invariant & causal => (QFT).

6. In Non-Relativistic Quantum Mechanics ([x,p] =1): <x2|exp[-iH(x2- x1)0/] |x1>  0 (x2)0 > (x1)0 Even if (s)2 = [(x2- x1)0]2 - [(x2- x1)].[(x2- x1)]< 0 (space-like) “faster-than-light” x2 x1 If(s)2 < 0,  Lorentz trans. : (x2’)0 < (x1’)0 (reversal of temporal order) In Quantum Field Theory microcausality is ensured as [i(x2 ),k(x1)]± = 0 (s)2 < 0

7. Quantum Mechanics: Wavefunction (“state”)  does not transform unitarily under Lorentz trans. Quantum Field Theory:  = field operator Physical states |> are unitary (albeit infinite-dimensional) representation spaces of Lorentz group Lorentz group: non-compact, nofinite-dimensional unitary rep. => Questions regarding the validity of “fundamental 2-state qubit” of non-rel QIS (?) and “fundamental entangled(Bell) spin-up spin-down states” Of non-rel QIS with 1-ebit (?)

8. Book : Quantum Theory of Fields, Vol. I. Steven Weinberg Preface:

9. Massive classified by momentum and spin To evaluate: : L = Pure Lorentz Boost (Eq. A)

10. Wigner Transformation : (W. k = k) D[W] is a unitary representation of the Little Group of k

11. => Little Group of k = SO(3) (Wigner Rotation) Note: consistently produces no rotation in spin space (c.f. Eq. A) for this special case

13. Infinitesimal Wigner angle: In absence of boost: Wigner rotations = ordinary rotations

14. Explicit Unitary Representation: Writing

15. Lie Algebra of Lorentz Grp : Note: Explicit infinite-dimensionalunitary representation with Hermitian generators for non-compact Lorentz group!

16. Finite Wigner rotations: => =>Not as easy to write finite expression in closed form using infinite products of infinitesimal transformations for generic Lorentz trans => Complete Wigner rotation :

17. For spin ½ particles: Specialize to & Under Lorentz trans.:

18. Two-particle states: n1,2 = species label Notes: => But : => Hence

19. suggests combining rotational “singlet”(1) and “triplet”(3) Bell states as the 4 . c.f.Conventional assignment (see e.g. Nielsen and Chuang)

20. Under arbitrary Lorentz transformations: =>Complete behaviour of Bell states under Lorentz trans. is : Under pure rotations

21. Reduced Density Matrices and Identical Particles Reduced ( ) density matrices m-particle operator equivalent to Yang’s definition => Reduced Density Matrices are therefore defined as partial traces of higher particle no. matrices

24. Lorentz Invariance of von Neumann Entropies of Reduced Density Matrices => von Neumann entropy Lorentz Invariant! =>

25. Worked example: System of two identical fermions “Diagonalization” : 1-particle reduced density matrix: Note: for total system

26. But => Entropy of reduced density matrix Maximizing and minimizing, subject to => (c.f. for bosons) e.g. “Unentangled” 2-particle state : “entanglement entropy” (lowest value)

27. Consider “Entangled” Bell state: => than lowest value True for Results are Lorentz invariant!

28. Entropy: In general, divergent in QFT e.g. Von Neumann Entropy => Generalized Zeta Function =>

29. Alternative and generalization:

30. Summary: Modest results/observations from our foray: 1. Computation of explicit Wigner rotations for massive particles 2. Explicit unitary rep. of Lorentz group and its generators 3. Definition, and behaviour of Bell States under arbitrary Lorentz trans. 4. Definition, and applications of Lorentz covariant reduced density matrices to identical particle systems. 5. Lorentz-invariant characterization of entanglement. 6. Relation betn. von Neumann entropy and generalized zeta function => towards Relativistic(Lorentz invariant) QIS <=> (founded upon QFT) => towards General Relativistic QIS <=> QG(?)

31. Real Life • Give an example or real life anecdote • Add a strong statement that summarizes how you feel or think about this topic

33. Glimm’s vector Physics QIS & QC ? Truth Mathematics Engineering

34. The End.That’s all folks!