Emergence of the classical world from quantum physics: Schrödinger cats, entanglement, and decoherence

Transcription

1 Emergence of the classical world from quantum physics: Schrödinger cats, entanglement, and decoherence Luiz Davidovich Instituto de Física Universidade Federal do Rio de Janeiro

2 Outline of the talk! Decoherence and the classical limit of the quantum world! Entanglement and decoherence: new experimental results! Multiparticle systems and decoherence 2

3 Schrödinger on the classical limit! 1926: At first sight it appears very strange to try to describe a process, which we previously regarded as belonging to particle mechanics, by a system of such proper vibrations.'' Demonstrates that a group of proper vibrations of high quantum number $n$ and of relatively small quantum number differences may represent a particle executing the motion expected from usual mechanics, i. e. oscillating with a constant frequency. 3

4 Schrödinger on the classical limit! 1935: An uncertainty originally restricted to the atomic domain has become transformed into a macroscopic uncertainty, which can be resolved through direct observation... This inhibits us from accepting in a naive way a `blurred model' as an image of reality...there is a difference between a shaky or not sharply focused photograph and a 4 photograph of clouds and fogbanks.

5 Quantum measurement

6 Quantum measurement

7 Quantum measurement

8 Quantum measurement

9 Quantum measurement Linear evolution:

10 Quantum measurement Linear evolution:

11 Quantum measurement Linear evolution:

12 Quantum measurement Linear evolution:

13 Quantum physics and localization! Let Ψ 1 and Ψ 2 be two solutions of the same Schrödinger equation. Then Ψ = Ψ 1 +Ψ 2 also represents a solution of the Schrödinger equation, with equal claim to describe a possible real state. When the system is a macrosystem, and when Ψ 1 and Ψ 2 are `narrow with respect to the macro-coordinates, then in by far the greater number of cases, this is no longer true for Ψ. Narrowness in regard to macro-coordinates is a requirement which is not only independent of the principles of quantum mechanics, but, moreover, incompatible with them. Letter from Einstein to Born, January 1, 1954

14 Why interference cannot be seen?

15 Why interference cannot be seen?! Decoherence: entanglement with the environment - same process by which quantum computers become classical computers!

16 Why interference cannot be seen?! Decoherence: entanglement with the environment - same process by which quantum computers become classical computers!! Dynamics of decoherence: related to elusive boundary between quantum and classical world

17 Decoherence dynamics 8

18 Decoherence dynamics 8

19 Decoherence dynamics Exponential decay: 8

20 Dynamics of entanglement! Multiparticle system, initially entangled, with individual couplings of particles to independent environments: each particle undergoes decay, dephasing, diffusion.

21 Dynamics of entanglement! Multiparticle system, initially entangled, with individual couplings of particles to independent environments: each particle undergoes decay, dephasing, diffusion.! How is local dynamics related to nonlocal loss of entanglement?

22 Dynamics of entanglement! Multiparticle system, initially entangled, with individual couplings of particles to independent environments: each particle undergoes decay, dephasing, diffusion.! How is local dynamics related to nonlocal loss of entanglement?! How does loss of entanglement scale with number of particles?

23 Dynamics of entanglement! Multiparticle system, initially entangled, with individual couplings of particles to independent environments: each particle undergoes decay, dephasing, diffusion.! How is local dynamics related to nonlocal loss of entanglement?! How does loss of entanglement scale with number of particles?! Need measure of entanglement!

24 Entangled and separable states! Separable states: Pure states: Mixed states (R. F. Werner, PRA, 1989):! Entangled state: non-separable

25 Entangled and separable states! Separable states: Pure states: Mixed states (R. F. Werner, PRA, 1989):! Entangled state: non-separable Bell states - Maximally entangled states: complete ignorance on each qubit

26 Entangled and separable states! Separable states: Pure states: Mixed states (R. F. Werner, PRA, 1989):! Entangled state: non-separable Bell states - Maximally entangled states: complete ignorance on each qubit ρ A,B =

27 Schrödinger on Entanglement Naturwissenschaften 23, 807 (1935) This is the reason that knowledge of the individual systems can decline to the scantiest, even zero, while that of the combined system remains continually maximal. Best possible knowledge of a whole does not include best possible knowledge of its parts and that is what keeps coming back to 11 haunt us.

28 Measures of entanglement for pure states Von Neumann entropy Linear entropy reduced density matrix of A or B Separable state (two qubits): S(ρ r ) = 0 Maximally entangled state: 12

29 A mathematical interlude: partial transposition of a matrix Transposition: a positive map T : Does not change eigenvalues! T Matrix in computational basis: 00, 01, 10, 11 { }

30 A mathematical interlude: partial transposition of a matrix Transposition: a positive map T : Does not change eigenvalues! T Matrix in computational basis: 00, 01, 10, 11 { } Partial transposition:1 A T B : T B Example: 1 2 ± Φ = 1 ± = ( 1 ( 00 ± ± 11 ) ) ± ± ±1 0 0 ±

31 A mathematical interlude: partial transposition of a matrix Transposition: a positive map T : Does not change eigenvalues! T Matrix in computational basis: 00, 01, 10, 11 { } Partial transposition:1 A T B : T B Example: 1 2 ± Φ = 1 ± = ( 1 ( 00 ± ± 11 ) ) ± ± Negative eigenvalue! ±1 0 0 ±

32 Mixed states: Separability criterium! If ρ is separable, then the partially transposed matrix is positive (Asher Peres, PRL, 1996):! For 2X2 and 2X3 systems, ρ is separable iff it remains a density operator under the operation of partial transposition (Horodecki family 1996) - that is, it has a partial positive transpose (PPT)

33 Negativity as a measure of entanglement N ( AB ) 2 i i Negative eigenvalues of partially transposed matrix 15

34 Negativity as a measure of entanglement N ( AB ) 2 Negative eigenvalues of partially transposed matrix =1 for a Bell state i i 15

35 Negativity as a measure of entanglement N ( AB ) 2 Negative eigenvalues of partially transposed matrix =1 for a Bell state i i Dimensions higher than 6: =0 does not imply separability! 15

36 Mixed states: Concurrence! (Wootters, 1998) - for two qubits: eingenvalues (positive), in decreasing order, of (conjugation in the computational basis) maximally entangled separable

37 Mixed states: Concurrence! (Wootters, 1998) - for two qubits: eingenvalues (positive), in decreasing order, of (conjugation in the computational basis) maximally entangled Pure states: C = 2 1 Trρ r 2 ( ) separable

38 Mixed states: Concurrence! (Wootters, 1998) - for two qubits: C 2! Tangle eingenvalues (positive), in decreasing order, of (conjugation in the computational basis) maximally entangled Pure states: C = 2 1 Trρ r 2 ( ) separable

39 Relation between concurrence and negativity Two qubits! F. Verstraete et al., J. Phys. A: Math. Gen. 34, (2001) Boundary of separability: independent of the entanglement measure Negativity Concurrence

40 A paradigmatic example: Atomic decay M. P. Almeida et al., Science 316, 579 (2007) Qubit states: Amplitude channel : Weisskopf and Wigner (1930)! Usual master equation for decay of two-level atom, upon tracing on environment (Markovian approximation)

41 A paradigmatic example: Atomic decay M. P. Almeida et al., Science 316, 579 (2007) Qubit states: Amplitude channel : Our strategy: follow evolution as a function of p, not t Weisskopf and Wigner (1930)! Usual master equation for decay of two-level atom, upon tracing on environment (Markovian approximation)

42 A paradigmatic example: Atomic decay M. P. Almeida et al., Science 316, 579 (2007) Qubit states: Amplitude channel : Our strategy: follow evolution as a function of p, not t Usual master equation for decay of two-level atom, upon tracing on environment Weisskopf and Wigner (1930)! (Markovian approximation) Apply evolution to two qubits, take trace with respect to environment degrees of freedom, find evolution of two-qubit reduced density matrix, calculate entanglement

43 Realization of amplitude map with photons g 0 g 0 e 0 1 p e 0 + p g 1 H V V H 0 E Sagnac-like interferometer 1 E

44 Realization of amplitude map with photons g 0 g 0 e 0 1 p e 0 + p g 1 H V V H 0 E Sagnac-like interferometer 1 E

45 Realization of amplitude map with photons g 0 g 0 e 0 1 p e 0 + p g 1 H V V H 0 E Sagnac-like interferometer 1 E

46 Realization of amplitude map with photons g 0 g 0 e 0 1 p e 0 + p g 1 H V V H 0 E Sagnac-like interferometer 1 E

47 Realization of amplitude map with photons g 0 g 0 e 0 1 p e 0 + p g 1 p =sin 2 (2 ) H V V H 0 E Sagnac-like interferometer 1 E

48 Realization of amplitude map with photons g 0 g 0 e 0 1 p e 0 + p g 1 p =sin 2 (2 ) H V V H 0 E Sagnac-like interferometer 1 E

49 Investigating the dynamics of entanglement

50 Sudden death of entanglement N (p = 0) = 2 Λ

51 Sudden death of entanglement N (p = 0) = 2 Λ Entanglement Sudden Death (Yu and Eberly)

52 Role of environment! Usually one traces out environment, and one looks at irreversible evolution of system! As entanglement decays and eventually disappears, what is its imprint onto the environment? 22

53 Measuring the environment?

54 Role of environment! Usually one traces out environment, and one looks at irreversible evolution of system! Our setup allows in principle access to environment! Is it useful to watch the environment? 24

55 Quantum Darwinism describes the proliferation, in the environment, of multiple records of selected states of a quantum system! pointer states. Detailed study of the environment uncovers essential trait of the classical world! 25

56 Simple model S 1 E 1 Amplitude channels S 2 E 2 O. Jiménez Farías et al., PRL 109, (2012) G. H. Aguilar et al., PRL 113, (2014) G. H. Aguilar et al., PRA 89, (2014)

57 0.3 Tangles C 2 S 1 S 2 C 2 E 1 E p

58 Fidelity p=0.27 p= p Fidelity with respect to state Di =1/ p 6( 0000i i i i i i) Dicke state with second and fourth qubits flipped 28

59 Fidelity p=0.27 p= p Genuine multipartite! W. Wieczorek et al., Phys. Rev. Lett. 101, (2008) Fidelity with respect to state Di =1/ p 6( 0000i i i i i i) Dicke state with second and fourth qubits flipped 28

60 Decay of entanglement for N qubits, other environments?! Independent individual environments

61 Results for amplitude damping! Bipartitions k : N k! Critical transition probability for which negativity vanishes (same for all partitions): p AD c (k) = / 2/N State fully separable at this point! / < 1! Smaller than 1 if finite-time disappearance of entanglement! Critical value approaches 1 when N! Does entanglement become more robust when number of qubits increases? 30

62 Does entanglement become more robust with increasing N? 31

63 Is ESD relevant for many particles? 32

64 Is ESD relevant for many particles? Λ p ( ) exp pn ( )Λ 0 ( ) 32

65 Generalization: Graph states! At each vertex, place a qubit in the state ( ) / 2! Apply control-z gate between two connected vertices: ( ) / ( ) / 2 ( ) / ! Universal states for measurementbased quantum computation (Raussendorf and Briegel) R. Raussendorf and H.J. Briegel, Phys. Rev. Lett. 86, 5188 (2001)

66 Generalization: Graph states Any convex bi- or multi-partite entanglement quantifier (no increase under LOCC) For a large class of quantum channels, and any partition, total entanglement is determined by entanglement of boundary qubits (connected by gray lines) Lower bounds for entanglement depend only on number of boundary qubits

67 35

68 1935

69 Collaborators: entanglement dynamics Leandro Aolita Fernando de Melo Rafel Chaves Malena Hor-Meyll Alejo Salles Osvaldo Jiménez-Farías Gabriel Aguillar Marcelo P. de Almeida Andrea Valdés-Hernandéz Paulo Souto Ribeiro Stephen Walborn Daniel Cavalcanti Antonio Acín Joe Eberly Xiao-Feng Qian 37

70 Collaborators: quantum metrology Gabriel Bié Marcio Taddei Camille Latune Bruno Escher Nicim Zagury Ruynet Matos Filho

71 THANKS!