Shunlong Luo. Academy of Mathematics and Systems Science Chinese Academy of Sciences

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1 Superadditivity of Fisher Information: Classical vs. Quantum Shunlong Luo Academy of Mathematics and Systems Science Chinese Academy of Sciences Information Geometry and its Applications IV in honor of Shun-Ichi Amari on the Occasion of His 80th Birthday June 13-17, 2016, Liblice, Czech

2 A story about a conjecture of more than 50 years old strange difference between classical and quantum statistics Implications for clock synchronization

3 Outline 1. Classical Fisher Information 2. Superadditivity in Classical Case 3. Quantum Fisher Information 4. Superadditivity in Quantum Case 5. Weak Superadditivity in Quantum Case 6. Physical Implications of Superadditivity 7. Problems

4 1. Classical Fisher Information Fisher, 1922, 1925 Fisher information of a probability density p(x) = p(x 1, x 2,, x n ) (with respect to the location parameters) is defined as I F (p) = 4 p(x) 2 dx. R n : gradient : Euclidean norm in R n

5 More generally, the Fisher information matrix of a parametric densities p θ (x) on R n with parameter θ = (θ 1, θ 2,, θ m ) R m is the m m matrix defined as I ij = 4 I F (p θ ) = (I ij ) R n pθ (x) θ i with i, j = 1, 2,, m. pθ (x) dx θ j

6 In particular, if n = m and p θ (x) = p(x θ) is a translation family, then I F (p θ ) = (I ij ) is independent of the parameter θ, and p(x) p(x) I ij = 4 dx. x i x j R n In this case, we may simply denote I F (p θ ) by I F (p). We see that I F (p) = tri F (p).

7 Statistical Origin of Fisher Information Data: n samples x 1, x 2,, x n p θ (x). Aim: Estimate the parameter θ. Cramér-Rao: Unbiased estimate θ θ 1 ni (p θ ). Maximum Likelihood: θ(x 1,, x n ) n( θ θ) N(0, 1/I (pθ )).

8 Fisher Information vs. Shannon Entropy For a probability density p, its Shannon entropy is S(p) = p(x)lnp(x)dx. de Bruijin identity: t S(p g t) = 1 t=0 2 I (p), where g t (x) = 1 2πt e x2 /2t.

9 2. Superadditivity in Classical Case Basic Properties of Fisher Information (a). Fisher information is convex: I F (λ 1 p 1 + λ 2 p 2 ) λ 1 I F (p 1 ) + λ 2 I F (p 2 ). Here p 1 and p 2 are two probability densities and λ 1 + λ 2 = 1, λ j 0, j = 1, 2. Informational meaning: Mixing decreases information.

10 (b). Fisher information is additive: I F (p 1 p 2 ) = I F (p 1 ) + I F (p 2 ). Here p 1 and p 2 are two probability densities, and p 1 p 2 (x, y) := p 1 (x)p 2 (y) is the independent product density (which is a kind of tensor product).

11 (c). Fisher information is invariant under location translation, that is, for any fixed y R n, if we put p y (x) := p(x y), then I F (p y ) = I F (p).

12 Superadditivity (d). Fisher information I F (p) is superadditive: I F (p) I F (p 1 ) + I F (p 2 ). Here p(x) = p(x 1, x 2 ) is a bivariate density with marginal densities p 1 and p 2.

13 Amusing and Remarkable 1925: Fisher information was introduced. 1991: Superadditivity was discovered and proved by Carlen. Statistical meaning: When a composite system is decomposed into two subsystems, the correlation between them is missing, and thus the Fisher information decreases.

14 Superadditivity Analytical Proof E. A. Carlen Superadditivity of Fisher s information and logarithmic Sobolev inequalities Journal of Functional Analysis, 101 (1991),

15 Statistical Proof A. Kagan and Z. Landsman Statistical meaning of Carlen s superadditivity of the Fisher information Statist. Probab. Lett. 32 (1997),

16 3. Quantum Fisher Information Analogy between Classical and Quantum: Probability p θ Density operator (non-negative matrix with unit trace) ρ θ Integral Trace operation tr

17 Quantum Mechanics as a Framework of Calculating Probabilities, a Statistical Theory E. Schrödinger Quantum mechanics began with statistics, and will end with statistics.

18 In classical statistics, probabilities are given a priori: (Ω, F, P). In quantum physics, probabilities are generated from the pairing: (density operators ρ, observable H) p i = trρe i where H = i λ ie i is the spectral decomposition of the self-adjoint operator H.

19 H. Araki, M. M. Yanase Measurement of Quantum Mechanical Operators Phys. Rev. 120, 1960 Wigner-Araki-Yanase Theorem The existence of a conservation law imposes limitation on the measurement of an observable. An operator which does not commute with a conserved quantity cannot be measured exactly.

20 E. P. Wigner and M. M. Yanase Information content of distribution Proc. Nat. Acad. Sci., 49, (1963) Skew information I (ρ, H) = 1 2 tr[ ρ, H] 2 where ρ: density operator H: any self-adjoint operator [, ]: commutator

21 Wigner-Yanase-Dyson information I α (ρ, H) = 1 2 tr[ρα, H][ρ 1 α, H] where α (0, 1).

22 Basic Properties of Skew Information 1 I (ρ, H) ρ H := trρh 2 (trρh) 2. 2 Invariance: I (UρU, H) = I (ρ, H) if unitary U satisfying UH = HU. 3 Additivity 4 Convexity I (ρ 1 ρ 2, H H 2 ) = I (ρ 1, H 1 ) + I (ρ 2, H 2 ). I (λ 1 ρ 1 +λ 2 ρ 2, H) λ 1 I (ρ 1, H)+λ 2 I (ρ 2, H).

23 Four Interpretations of Skew Information As information content of ρ with respect to observable not commuting with H Wigner and Yanase, 1963

24 As a measure of non-commutativity between ρ and H Connes, Stormer, J. Func. Anal. 1978

25 As a kind of quantum Fisher information D. Petz, H. Hasegawa, On the Riemannian metric of α-entropies of density matrices, Lett. Math. Phys S. Luo Phys. Rev. Lett IEEE Trans. Inform. Theory, 2004 Proc. Amer. Math. Soc. 2004

26 As the quantum uncertainty of H in the state ρ S. Luo, Phys. Rev. A, 2005, 2006

27 Skew Information as Quantum Fisher Information Generalizing classical Fisher information ( ) 2 p θ (x) I F (p θ ) := 4 dx θ to the quantum scenario, we may define ( ) 2 ρθ I F (ρ θ ) := 4tr θ as a kind of quantum Fisher information. Here ρ θ is a family of density operators.

28 In particular, if ρ θ satisfies the Landau-von Neumann equation then i ρ θ θ = [H, ρ θ], ρ 0 = ρ I F (ρ θ ) = 4tr[ρ 1/2, H] 2 = 8I (ρ, H) S. Luo, Phys. Rev. Lett. 2003

29 4. Superadditivity in Quantum case Conjecture: For bipartite density operator ρ, I α (ρ, H H 2 ) I α (ρ 1, H 1 )+I α (ρ 2, H 2 ). Here ρ 1 = tr 2 ρ, ρ 2 = tr 1 ρ: marginals of ρ H 1, H 2 : selfadjoint operators over subsystems 1: identity operator : tensor product of operators

30 Comments This conjecture was reviewed by Lieb. The only non-trivial confirmed case is for pure states with α = 1 2. Wigner-Yanase, 1963: Necessary requirement Lieb, 1973: Absolute requirement

31 Disproof F. Hansen, Journal of Statistical Physics, 2007 Numerical counterexample! Counterintuitive! Surprising!

32 L. Cai, N. Li, S. Luo Journal of Physics A, 2008 A Simple Counterexample. Let n > 2 and take ρ = 1 n Then n , H 1 = H 2 = ( I (ρ, H H 2 ) < I (ρ 1, H 1 ) + I (ρ 2, H 2 ) for large n. ).

33 Partial Results S. Luo, Journal of Statistical Physics, 2007 Let H = H H 2. If ρ = Ψ Ψ is a pure state, then superadditivity holds, that is I α (ρ, H) I α (ρ 1, H 1 ) + I α (ρ 2, H 2 ).

34 Let H = H H 2, and ρ be a diagonal density matrix. Then superadditivity holds, that is I α (ρ, H) I α (ρ 1, H 1 ) + I α (ρ 2, H 2 ).

35 Partial Results S. Luo and Q. Zhang Journal of Statistical Physics, 2008 For any classical-quantum state, the superadditivity holds.

36 Tripartite case R. Seiringer Lett. Math. Phys Failure of superadditivity of the Wigner-Yanase skew information for tripartite pure states. The following inequality may be violated by certain pure states: I α (ρ, H) I α (ρ 1, H 1 )+I α (ρ 2, H 2 )+I α (ρ 3, H 3 ) where ρ = Ψ 123 Ψ 123, H = H H H 3.

37 5. Weak Superadditivity in Quantum Case Though neither I (ρ, H H 2 ) I (ρ 1, H 1 ) + I (ρ 2, H 2 ) nor I (ρ, H H 2 ) I (ρ 1, H 1 ) + I (ρ 2, H 2 ) is always true, their sum is true: I (ρ, H H 2 ) + I (ρ, H H 2 ) ( ) 2 I (ρ 1, H 1 ) + I (ρ 2, H 2 ).

38 It holds that I (ρ, H H 2 ) 1 2 ( ) I (ρ 1, H 1 )+I (ρ 2, H 2 ).

39 6. Physical Implications of Superadditivity: Clock Synchronization Classical clock: (p, Q) (Q = i d dx moment observable) p t (x) = e itq p(x). is the Quality: classical Fisher information I F (p). Quantum clock: (ρ, H) ρ t = e ith ρe ith. Quality: quantum Fisher information I α (ρ, H).

40 Clock Synchronization A quantum clock shared by two parties: (ρ, H H 2 ) The violation of the superadditivity means that the sum of the quality of the component clock will be better than the overall quality: I α (ρ 1, H 1 )+I α (ρ 2, H 2 ) > I α (ρ, H H 2 ). Curious property of quantum clock!

41 A Conjecture There does not exit nontrivial quantum clocks such that I α (ρ 1, H 1 ) = I α (ρ 2, H 2 ) = I α (ρ, H H 2 ). Intuition: Otherwise, we could copy quantum timing information.

42 7. Problems 1. Conditions for superadditivity? 2. Intuitive meaning of the failure of superadditivity 3. Difference between classical and quantum from the perspective of Fisher information 4. Quantum logarithmic Sobolev inequalities? Thank you!