Entanglement in Quantum Field Theory

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1 Entanglement in Quantum Field Theory John Cardy University of Oxford DAMTP, December 2013

2 Outline Quantum entanglement in general and its quantification Path integral approach Entanglement entropy in 1+1-dimensional CFT Higher dimensions Mixed states and negativity Work largely carried out with Pasquale Calabrese (Pisa) and Erik Tonni (Trieste)

3 Quantum Entanglement (Bipartite, Pure State) quantum system in a pure state Ψ, density matrix ρ = Ψ Ψ H = H A H B Alice can make unitary transformations and measurements only in A, Bob only in the complement B in general Alice s measurements are entangled with those of Bob example: two spin- 1 2 degrees of freedom ψ = cos θ A B + sin θ A B

4 Measuring bipartite entanglement in pure states Schmidt decomposition: Ψ = j c j ψ j A ψ j B with c j 0, j c2 j = 1, and ψ j A, ψ j B orthonormal. one quantifier of the amount of entanglement is the entropy S A j c j 2 log c j 2 = S B if c 1 = 1, rest zero, S = 0 and Ψ is unentangled if all c j equal, S log min(dimh A, dimh B ) maximal entanglement

5 equivalently, in terms of Alice s reduced density matrix: ρ A Tr B Ψ Ψ S A = Tr A ρ A log ρ A = S B von Neumann entropy similar information is contained in the Rényi entropies S A (n) = (1 n) 1 log Tr A ρ A n S A = lim n 1 S A (n)

6 other measures of entanglement exist, but entropy has several nice properties: additivity, convexity,... it is monotonic under Local Operations and Classical Communication (LOCC) it gives the amount of classical information required to specify ρ A (important for numerical computations) it gives a basis-independent way of identifying and characterising quantum phase transitions in a relativistic QFT the entanglement in the vacuum encodes all the data of the theory (spectrum, anomalous dimensions,...)

7 Entanglement entropy in QFT In this talk we consider the case when: the degrees of freedom are those of a local relativistic QFT in large region R in R d the whole system is in the vacuum state 0 A is the set of degrees of freedom in some large (compact) subset of R, so we can decompose the Hilbert space as H = H A H B in fact this makes sense only in a cut-off QFT (e.g. a lattice), and some of the results will in fact be cut-off dependent How does S A depend on the size and geometry of A and the universal data of the QFT?

8 Rényi entropies from the path integral (d = 1) 0 τ A B _ wave functional Ψ({a}, {b}) conditioned path integral in imaginary time from τ = to τ = 0: Ψ({a}, {b}) = Z 1/2 1 [da(τ)][db(τ)] e (1/ )S[{a(τ)},{b(τ)}] a(0)=a,b(0)=b where S = 0 L( a(τ), b(τ) ) dτ similarly Ψ ({a}, {b}) is given by the path integral from τ = 0 to +

9 ρ A (a 1, a 2 ) = db Ψ(a 1, b)ψ (a 2, b) this is given by the path integral over R 2 cut open along A {τ = 0}, divided by Z 1 : a a 2 1 b

10 Rényi entropies: example n = 2 ρ A (a 1, a 2 ) = db Ψ(a 1, b)ψ (a 2, b) Tr A ρ 2 A = da 1 da 2 db 1 db 2 Ψ(a 1, b 1 )Ψ (a 2, b 1 )Ψ(a 2, b 2 )Ψ (a 1, b 2 ) A B Tr A ρ A 2 = Z (R 2 )/Z 2 1 where Z (R 2 ) is the euclidean path integral (partition function) on an 2-sheeted conifold R 2

11 in general Tr A ρ A n = Z (R n )/Z n 1 where the half-spaces are connected as A B to form R n. conical singularity of opening angle 2πn at the boundary of A and B on τ = 0

12 B u A v B if space is 1d and A is an interval (u, v) (and B is the complement) then Z (R n ) can be thought of as the insertion of twist operators into n copies of the CFT: Z (R n )/Z n 1 = T n(u)t n (v) (CFT ) n these have similar properties to other local operators e.g. in a massless QFT (a CFT) T n (u)t n (v) u v 2 n in a massive QFT, T n m n and T n (u)t n (v) T n 2 e 2m u v

13 main result for d = 1: n = (c/12)(n 1/n) where c is the central charge of the UV CFT consider a cone of radius R and opening angle α = 2πn w = log z maps this into a cylinder of length log R and circumference α Z cone (2πn) Z cone (2π) n = Z cyl(2πn) Z cyl (2π) n eπc log R/12πn (e πc log R/12π ) n R n

14 from this we see for example that for a single interval A of length l [Holzhey, Larsen, Wilczek 1994] S A n l 2 n = (c/3) log(l/ɛ) n=1 note this is much less than the entanglement in a typical state which is O(l) many more universal results, eg finite-temperature cross-over between entanglement and thermodynamic entropy (β = 1/k B T ): S A = (c/3) log ( (β/π) sinh(πl/β) ) (c/3) log l for l β πcl/3β for l β

15 Massive QFT in 1+1 dimensions for 2 intervals A = (, 0) and B = (0, ) S A (c/6) log(m/ɛ) as m 0 the entanglement diverges at a quantum phase transition and gives a basis-independent way of characterising the underlying CFT this is numerically the most accurate way of determining c for a given lattice model

16 Two intervals u v A u v Z (R n )/Z n = T n (u 1 )T n (v 1 )T n (u 2 )T n (v 2 ) in general there is no simple result but for u j v j u 1 u 2 we can use an operator product expansion T n (u)t n (v) = {k j } C {kj }(u v) n j=1 ( Φ 1 kj 2 (u + v) ) j in terms of a complete set of local operators Φ kj this gives the Rényi entropies as an expansion C{k 2 j j } η k j where η = ((u 1 v 1 )(u 2 v 2 ))/((u 1 u 2 )(v 1 v 2 )) {k j } the C {kj } encode all the data of the CFT

17 Higher dimensions d > 1 B A the conifold R n is now {2d conifold} {boundary A} log Z (R n ) Vol( A) ɛ (d 1) this is the area law in 3+1 dimensions [Srednicki 1992] coefficient is non-universal

18 for even d + 1 there are interesting corrections to the area law Vol( A) m 2 log(mɛ), a log(r A /ɛ) whose coefficients are related to curvature anomalies of the CFT and are universal e.g. a is the a-anomaly which is supposed to decrease along RG flows between CFTs [Komargodski-Schwimmer] it would interesting to give an entanglement-based argument for this result [Casini-Huerta]

19 Mutual Information of multiple regions B A2 A1 the non-universal area terms cancel in I (n) (A 1, A 2 ) = S (n) A 1 + S (n) A 2 S (n) A 1 A 2 this mutual Rényi information is expected to be universal depending only on the geometry and the data of the CFT e.g. for a free scalar field in 3+1 dimensions [JC 2013] I (n) (A 1, A 2 ) ( n 4 1 R 1 R 2 15n 3 (n 1) r12 2 ) 2

20 Negativity however, mutual information does not correctly capture the quantum entanglement between A 1 and A 2, e.g. it also includes classical correlations at finite temperature more generally we want a way of quantifying entanglement in a mixed state ρ A1 A 2 one computable measure is negativity [Vidal, Werner 2002] let ρ T 2 A 1 A 2 be the partial transpose: ρ T 2 A 1 A 2 (a 1, a 2 ; a 1, a 2 ) = ρ A 1 A 2 (a 1, a 2 ; a 1, a 2) Tr ρ T 2 A 1 A 2 = 1, but it may now have negative eigenvalues λ k Log-negativity N = log Tr ρ T 2 A 1 A 2 = log k λ k if this is > 0 there are negative eigenvalues. This is an entanglement measure with nice properties, including being an LOCC monotone

21 Negativity in QFT replica trick Tr (ρ T 2 A 1 A 2 ) n = k λ n k = k λ k n if n is even analytically continue to n = 1 to get k λ k (!!) we can compute Tr (ρ T 2 A 1 A 2 ) n by connecting the half-spaces in the opposite order along A 2 : A A 2 1

22 A A 2 1 for ρ A1 A 2 we need T n (u 1 )T n (v 1 )T n (u 2 )T n (v 2 ) But for ρ T 2 A 1 A 2 we need T n (u 1 )T n (v 1 )T n (u 2 )T n (v 2 ) T n T n = Tn (n odd) 1 for n 1 = T n/2 T n/2 (n even) T 1/2 T 1/2 for n 1 so we get a non-trivial result if the intervals are close

23 A 1 A 2 so for example for d > 1 for 2 large regions a finite distance apart N (A 1, A 2 ) Area of common boundary between A 1 and A 2 N appears to decay exponentially with separation of the regions, even in a CFT

24 Other Related Interesting Stuff topological phases in 2 (and higher) spatial dimensions - entanglement entropy distinguishes these in absence of local order parameter [Kitaev/Preskill and many others] entanglement spectrum of the eigenvalues of log ρ A [Haldane] Shannon entropy Tr Ψ 2 log Ψ 2 seems to have interesting properties depute being basis-dependent holographic computation of entanglement using AdS/CFT [Ryu/Takayanagi and many others] time-dependence in particular quantum quenches where the system is prepared in a state ψ which is not an eigenstate of hamiltonian: how do entanglement (and correlation functions) behave?

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