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Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum Computing Perimeter Institute for Theoretical Physics Waterloo, November 2004 – p.1
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Page 1: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Emergence of classical reality fromwithin quantum theory

Oral Thesis presentation

David Poulin

Institute for Quantum Computing

Perimeter Institute for Theoretical Physics

Waterloo, November 2004 – p.1

Page 2: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

I like getting into trouble...H. Ollivier, DP, and W.H. Zurek, “Emergence of objective properties from subjectivequantum states: Environment as a witness”, to appear in PRL

DP, R. Blume-Kohout, R. Laflamme, and H. Ollivier, “Exponential speed-up with asingle bit of quantum information: Measuring the average fidelity decay”, PRL 04

J.-C. Boileau, D. Gottesman, R. Laflamme, DP, R.W. Spekkens, “Robustpolarization-based quantum key distribution over collective-noise channel”, PRL 04.

DP, “Macroscopic observables", to appear in PRA.

J.A. Holbrook, D.W. Kribs, R. Laflamme, and DP, “Noiseless Subsystems for CollectiveRotation Channels in Quantum Information Theory”, Int. Eqs. & Oper. Theory.

J. Emerson, S. Lloyd, DP, and D. Cory, “Estimation of the Local Density of States on aQuantum Computer”, PRA 04.

D. Poulin, R. Laflamme, G.J. Milburn, and J.P. Paz, “Testing integrability with a singlebit of quantum information”, PRA 03.

DP and R. Blume-Kohout, “Compatibility of quantum states”, PRA 03.

H. Ollivier, DP, and W. H. Zurek, “Environment as a witness: selective proliferation ofinformation and emergence of objectivity", submitted to PRA.

H. Ollivier, DP, and W.H. Zurek, “Emergence of objective properties from subjectivequantum states: Environment as a witness”, to appear in PRL

DP, R. Blume-Kohout, R. Laflamme, and H. Ollivier, “Exponential speed-up with asingle bit of quantum information: Measuring the average fidelity decay”, PRL 04

J.-C. Boileau, D. Gottesman, R. Laflamme, DP, R.W. Spekkens, “Robustpolarization-based quantum key distribution over collective-noise channel”, PRL 04.

DP, “Macroscopic observables", to appear in PRA.

J.A. Holbrook, D.W. Kribs, R. Laflamme, and DP, “Noiseless Subsystems for CollectiveRotation Channels in Quantum Information Theory”, Int. Eqs. & Oper. Theory.

J. Emerson, S. Lloyd, DP, and D. Cory, “Estimation of the Local Density of States on aQuantum Computer”, PRA 04.

D. Poulin, R. Laflamme, G.J. Milburn, and J.P. Paz, “Testing integrability with a singlebit of quantum information”, PRA 03.

DP and R. Blume-Kohout, “Compatibility of quantum states”, PRA 03.

H. Ollivier, DP, and W. H. Zurek, “Environment as a witness: selective proliferation ofinformation and emergence of objectivity", submitted to PRA.

Conventional / Different

Waterloo, November 2004 – p.2

Page 3: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

I like getting into trouble...H. Ollivier, DP, and W.H. Zurek, “Emergence of objective properties from subjectivequantum states: Environment as a witness”, to appear in PRL

DP, R. Blume-Kohout, R. Laflamme, and H. Ollivier, “Exponential speed-up with asingle bit of quantum information: Measuring the average fidelity decay”, PRL 04

J.-C. Boileau, D. Gottesman, R. Laflamme, DP, R.W. Spekkens, “Robustpolarization-based quantum key distribution over collective-noise channel”, PRL 04.

DP, “Macroscopic observables", to appear in PRA.

J.A. Holbrook, D.W. Kribs, R. Laflamme, and DP, “Noiseless Subsystems for CollectiveRotation Channels in Quantum Information Theory”, Int. Eqs. & Oper. Theory.

J. Emerson, S. Lloyd, DP, and D. Cory, “Estimation of the Local Density of States on aQuantum Computer”, PRA 04.

D. Poulin, R. Laflamme, G.J. Milburn, and J.P. Paz, “Testing integrability with a singlebit of quantum information”, PRA 03.

DP and R. Blume-Kohout, “Compatibility of quantum states”, PRA 03.

H. Ollivier, DP, and W. H. Zurek, “Environment as a witness: selective proliferation ofinformation and emergence of objectivity", submitted to PRA.

Conventional / Different Waterloo, November 2004 – p.2

Page 4: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Outline

Motivation

Environment as a witness

Macroscopic observables

Conclusion

Waterloo, November 2004 – p.3

Page 5: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Epistemic view

Classical Quantum

Classical QuantumP (x,p),

xpP (x,p) = 1 ρ, Tr{ρ} = 1

P = {H,P} ρ = −i[H, ρ]P (x,p|D) = P (D|x,p)P (x,p)

P (D) ρ|Q = QρQP (Q)

P (x,p) =∑

D P (x,p|D)P (D) ρ 6= ∑

Q ρQP (Q)

States of complete knowledge (δ) No such statesP (x,p|D) = P (x,p)

The absence of such states of complete knowledge isresponsible for the discomfort with quantum theory.

Waterloo, November 2004 – p.4

Page 6: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Epistemic view

Classical QuantumP (x,p),

xpP (x,p) = 1 ρ, Tr{ρ} = 1

Classical QuantumP (x,p),

xpP (x,p) = 1 ρ, Tr{ρ} = 1

P = {H,P} ρ = −i[H, ρ]P (x,p|D) = P (D|x,p)P (x,p)

P (D) ρ|Q = QρQP (Q)

P (x,p) =∑

D P (x,p|D)P (D) ρ 6= ∑

Q ρQP (Q)

States of complete knowledge (δ) No such statesP (x,p|D) = P (x,p)

The absence of such states of complete knowledge isresponsible for the discomfort with quantum theory.

Waterloo, November 2004 – p.4

Page 7: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Epistemic view

Classical QuantumP (x,p),

xpP (x,p) = 1 ρ, Tr{ρ} = 1

P = {H,P} ρ = −i[H, ρ]

Classical QuantumP (x,p),

xpP (x,p) = 1 ρ, Tr{ρ} = 1

P = {H,P} ρ = −i[H, ρ]P (x,p|D) = P (D|x,p)P (x,p)

P (D) ρ|Q = QρQP (Q)

P (x,p) =∑

D P (x,p|D)P (D) ρ 6= ∑

Q ρQP (Q)

States of complete knowledge (δ) No such statesP (x,p|D) = P (x,p)

The absence of such states of complete knowledge isresponsible for the discomfort with quantum theory.

Waterloo, November 2004 – p.4

Page 8: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Epistemic view

Classical QuantumP (x,p),

xpP (x,p) = 1 ρ, Tr{ρ} = 1

P = {H,P} ρ = −i[H, ρ]P (x,p|D) = P (D|x,p)P (x,p)

P (D) ρ|Q = QρQP (Q)

Classical QuantumP (x,p),

xpP (x,p) = 1 ρ, Tr{ρ} = 1

P = {H,P} ρ = −i[H, ρ]P (x,p|D) = P (D|x,p)P (x,p)

P (D) ρ|Q = QρQP (Q)

P (x,p) =∑

D P (x,p|D)P (D) ρ 6= ∑

Q ρQP (Q)

States of complete knowledge (δ) No such statesP (x,p|D) = P (x,p)

The absence of such states of complete knowledge isresponsible for the discomfort with quantum theory.

Waterloo, November 2004 – p.4

Page 9: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Epistemic view

Classical QuantumP (x,p),

xpP (x,p) = 1 ρ, Tr{ρ} = 1

P = {H,P} ρ = −i[H, ρ]P (x,p|D) = P (D|x,p)P (x,p)

P (D) ρ|Q = QρQP (Q)

P (x,p) =∑

D P (x,p|D)P (D) ρ 6= ∑

Q ρQP (Q)

Classical QuantumP (x,p),

xpP (x,p) = 1 ρ, Tr{ρ} = 1

P = {H,P} ρ = −i[H, ρ]P (x,p|D) = P (D|x,p)P (x,p)

P (D) ρ|Q = QρQP (Q)

P (x,p) =∑

D P (x,p|D)P (D) ρ 6= ∑

Q ρQP (Q)

States of complete knowledge (δ) No such statesP (x,p|D) = P (x,p)

The absence of such states of complete knowledge isresponsible for the discomfort with quantum theory.

Waterloo, November 2004 – p.4

Page 10: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Epistemic view

Classical QuantumP (x,p),

xpP (x,p) = 1 ρ, Tr{ρ} = 1

P = {H,P} ρ = −i[H, ρ]P (x,p|D) = P (D|x,p)P (x,p)

P (D) ρ|Q = QρQP (Q)

P (x,p) =∑

D P (x,p|D)P (D) ρ 6= ∑

Q ρQP (Q)

States of complete knowledge (δ) No such statesP (x,p|D) = P (x,p)

The absence of such states of complete knowledge isresponsible for the discomfort with quantum theory.

Waterloo, November 2004 – p.4

Page 11: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Epistemic view

Classical QuantumP (x,p),

xpP (x,p) = 1 ρ, Tr{ρ} = 1

P = {H,P} ρ = −i[H, ρ]P (x,p|D) = P (D|x,p)P (x,p)

P (D) ρ|Q = QρQP (Q)

P (x,p) =∑

D P (x,p|D)P (D) ρ 6= ∑

Q ρQP (Q)

States of complete knowledge (δ) No such statesP (x,p|D) = P (x,p)

The absence of such states of complete knowledge isresponsible for the discomfort with quantum theory.

Waterloo, November 2004 – p.4

Page 12: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Epistemic view

Properties of classical systems can be discoveredby initially ignorant observers.

How can the classical objective reality emergefrom the underlying subjective quantum theory?

Waterloo, November 2004 – p.5

Page 13: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Epistemic view

How can the classical objective reality emergefrom the underlying subjective quantum theory?

Waterloo, November 2004 – p.5

Page 14: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Epistemic view

How can the classical objective reality emergefrom the underlying subjective quantum theory?

Waterloo, November 2004 – p.5

Page 15: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

|e〉|Catalive〉 → (|e〉+ |g〉)|Catalive〉→ |e〉|Catalive〉+ |g〉|Catdead〉

Quantum theory allows superpositionof macroscopic objects.

Such superpositions however are not observed.

Waterloo, November 2004 – p.6

Page 16: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

|e〉|Catalive〉 → (|e〉+ |g〉)|Catalive〉→ |e〉|Catalive〉+ |g〉|Catdead〉

Quantum theory allows superpositionof macroscopic objects.

Such superpositions however are not observed.

Waterloo, November 2004 – p.6

Page 17: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

Environment induced superselection (einselection):

(|e〉|Catalive〉+ |g〉|Catdead〉)|Mousealive〉→ |e〉|Catalive〉|Mousedead〉+ |g〉|Catdead〉|Mousealive〉= |Ψ〉

If the mouse is not a controlable degree of freedom

ρAtom+Cat = TrMouse|Ψ〉〈Ψ|

=1

2|e〉〈e| ⊗ |Catalive〉〈Catalive|

+1

2|g〉〈g| ⊗ |Catdead〉〈Catdead|

Waterloo, November 2004 – p.7

Page 18: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

Environment induced superselection (einselection):

(|e〉|Catalive〉+ |g〉|Catdead〉)|Mousealive〉

(|e〉|Catalive〉+ |g〉|Catdead〉)|Mousealive〉→ |e〉|Catalive〉|Mousedead〉+ |g〉|Catdead〉|Mousealive〉= |Ψ〉

If the mouse is not a controlable degree of freedom

ρAtom+Cat = TrMouse|Ψ〉〈Ψ|

=1

2|e〉〈e| ⊗ |Catalive〉〈Catalive|

+1

2|g〉〈g| ⊗ |Catdead〉〈Catdead|

Waterloo, November 2004 – p.7

Page 19: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

Environment induced superselection (einselection):

(|e〉|Catalive〉+ |g〉|Catdead〉)|Mousealive〉→ |e〉|Catalive〉|Mousedead〉+ |g〉|Catdead〉|Mousealive〉= |Ψ〉

If the mouse is not a controlable degree of freedom

ρAtom+Cat = TrMouse|Ψ〉〈Ψ|

=1

2|e〉〈e| ⊗ |Catalive〉〈Catalive|

+1

2|g〉〈g| ⊗ |Catdead〉〈Catdead|

Waterloo, November 2004 – p.7

Page 20: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

Environment induced superselection (einselection):

(|e〉|Catalive〉+ |g〉|Catdead〉)|Mousealive〉→ |e〉|Catalive〉|Mousedead〉+ |g〉|Catdead〉|Mousealive〉= |Ψ〉

If the mouse is not a controlable degree of freedom

ρAtom+Cat = TrMouse|Ψ〉〈Ψ|

=1

2|e〉〈e| ⊗ |Catalive〉〈Catalive|

+1

2|g〉〈g| ⊗ |Catdead〉〈Catdead|

Waterloo, November 2004 – p.7

Page 21: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

Environment induced superselection (einselection):

(|e〉|Catalive〉+ |g〉|Catdead〉)|Mousealive〉→ |e〉|Catalive〉|Mousedead〉+ |g〉|Catdead〉|Mousealive〉= |Ψ〉

If the mouse is not a controlable degree of freedom

ρAtom+Cat = TrMouse|Ψ〉〈Ψ|

ρAtom+Cat = TrMouse|Ψ〉〈Ψ|

=1

2|e〉〈e| ⊗ |Catalive〉〈Catalive|

+1

2|g〉〈g| ⊗ |Catdead〉〈Catdead|

Waterloo, November 2004 – p.7

Page 22: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

Environment induced superselection (einselection):

(|e〉|Catalive〉+ |g〉|Catdead〉)|Mousealive〉→ |e〉|Catalive〉|Mousedead〉+ |g〉|Catdead〉|Mousealive〉= |Ψ〉

If the mouse is not a controlable degree of freedom

ρAtom+Cat = TrMouse|Ψ〉〈Ψ|

=1

2|e〉〈e| ⊗ |Catalive〉〈Catalive|

+1

2|g〉〈g| ⊗ |Catdead〉〈Catdead|

Waterloo, November 2004 – p.7

Page 23: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

The description of the quantum systems of interest (Atom +Cat) is a classical mixture of “e alive” and “g dead”.

Operationally, the interaction with an environmentexplains why we only experience statistical mixtures

as opposed to coherent superpositions.

Basis ambiguity: mixture of what?

Does not restrict my freedom of measurements, so∑

ρ|QP (Q) 6= ρ.

Indirect information acquisition (Part I).A pre-agreed set of measurements (Part II).

Waterloo, November 2004 – p.8

Page 24: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

The description of the quantum systems of interest (Atom +Cat) is a classical mixture of “e alive” and “g dead”.

Operationally, the interaction with an environmentexplains why we only experience statistical mixtures

as opposed to coherent superpositions.

Basis ambiguity: mixture of what?

Does not restrict my freedom of measurements, so∑

ρ|QP (Q) 6= ρ.

Indirect information acquisition (Part I).A pre-agreed set of measurements (Part II).

Waterloo, November 2004 – p.8

Page 25: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

The description of the quantum systems of interest (Atom +Cat) is a classical mixture of “e alive” and “g dead”.

Operationally, the interaction with an environmentexplains why we only experience statistical mixtures

as opposed to coherent superpositions.

Basis ambiguity: mixture of what?

Does not restrict my freedom of measurements, so∑

ρ|QP (Q) 6= ρ.

Indirect information acquisition (Part I).A pre-agreed set of measurements (Part II).

Waterloo, November 2004 – p.8

Page 26: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

The description of the quantum systems of interest (Atom +Cat) is a classical mixture of “e alive” and “g dead”.

Operationally, the interaction with an environmentexplains why we only experience statistical mixtures

as opposed to coherent superpositions.

Basis ambiguity: mixture of what?

Does not restrict my freedom of measurements, so∑

ρ|QP (Q) 6= ρ.

Indirect information acquisition (Part I).

A pre-agreed set of measurements (Part II).

Waterloo, November 2004 – p.8

Page 27: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Decoherence

The description of the quantum systems of interest (Atom +Cat) is a classical mixture of “e alive” and “g dead”.

Operationally, the interaction with an environmentexplains why we only experience statistical mixtures

as opposed to coherent superpositions.

Basis ambiguity: mixture of what?

Does not restrict my freedom of measurements, so∑

ρ|QP (Q) 6= ρ.

Indirect information acquisition (Part I).A pre-agreed set of measurements (Part II).

Waterloo, November 2004 – p.8

Page 28: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Part I: Environment as a witness

David Poulin, Harold Ollivier, Wojciech Zurek

Waterloo, November 2004 – p.9

Page 29: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Environment as a witness

Switch focus of attention from S to E .What states of S are maximally stable?What properties of S can be learned frominterrogating E?

How is this information displayed in E?Delocalized in E (require a joint measurement)?Single or multiple copies? (Redundant?)

Role of E : reservoir destroying coherence ⇒selective amplifier proliferating information about S.

Waterloo, November 2004 – p.10

Page 30: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Environment as a witness

Switch focus of attention from S to E .What states of S are maximally stable?What properties of S can be learned frominterrogating E?

How is this information displayed in E?Delocalized in E (require a joint measurement)?Single or multiple copies? (Redundant?)

Role of E : reservoir destroying coherence ⇒selective amplifier proliferating information about S.

Waterloo, November 2004 – p.10

Page 31: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Environment as a witness

Switch focus of attention from S to E .What states of S are maximally stable?What properties of S can be learned frominterrogating E?

How is this information displayed in E?Delocalized in E (require a joint measurement)?Single or multiple copies? (Redundant?)

Role of E : reservoir destroying coherence ⇒selective amplifier proliferating information about S.

Waterloo, November 2004 – p.10

Page 32: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Information theory

H(X|Y ) I(X : Y ) H(Y |X)

H(X) H(Y )

H(X) = −∑

x P (x) lnP (x),H(X|Y ) =

y P (y)∑

x P (x|y) lnP (x|y)Entropy = # bits required to specify the value of X.

Waterloo, November 2004 – p.11

Page 33: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Information theory

Observable σ on S: σ = {σi}Observable τ on E : τ = {τk}

Given the state of S + E , the joint probability is given byBorn’s rule: P (σi, τk) = Tr{ρSE(σi ⊗ τk)}

H(σ) unpredictability of the value of σ of S.

H(σ|τ) remaining unpredictability about σ after having“peeked at the environment” through τ .

I(σ : τ) information learned about the property σ of Sgiven the value of τ of E .

Waterloo, November 2004 – p.12

Page 34: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Information theory

Observable σ on S: σ = {σi}Observable τ on E : τ = {τk}

Given the state of S + E , the joint probability is given byBorn’s rule: P (σi, τk) = Tr{ρSE(σi ⊗ τk)}

H(σ) unpredictability of the value of σ of S.

H(σ|τ) remaining unpredictability about σ after having“peeked at the environment” through τ .

I(σ : τ) information learned about the property σ of Sgiven the value of τ of E .

Waterloo, November 2004 – p.12

Page 35: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Information theory

Observable σ on S: σ = {σi}Observable τ on E : τ = {τk}

Given the state of S + E , the joint probability is given byBorn’s rule: P (σi, τk) = Tr{ρSE(σi ⊗ τk)}

H(σ) unpredictability of the value of σ of S.

H(σ|τ) remaining unpredictability about σ after having“peeked at the environment” through τ .

I(σ : τ) information learned about the property σ of Sgiven the value of τ of E .

Waterloo, November 2004 – p.12

Page 36: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Information theory

Observable σ on S: σ = {σi}Observable τ on E : τ = {τk}

Given the state of S + E , the joint probability is given byBorn’s rule: P (σi, τk) = Tr{ρSE(σi ⊗ τk)}

H(σ) unpredictability of the value of σ of S.

H(σ|τ) remaining unpredictability about σ after having“peeked at the environment” through τ .

I(σ : τ) information learned about the property σ of Sgiven the value of τ of E .

Waterloo, November 2004 – p.12

Page 37: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Information theory

Observable σ on S: σ = {σi}Observable τ on E : τ = {τk}

Given the state of S + E , the joint probability is given byBorn’s rule: P (σi, τk) = Tr{ρSE(σi ⊗ τk)}

H(σ) unpredictability of the value of σ of S.

H(σ|τ) remaining unpredictability about σ after having“peeked at the environment” through τ .

I(σ : τ) information learned about the property σ of Sgiven the value of τ of E .

Waterloo, November 2004 – p.12

Page 38: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Information in the environment

I(σ) = maxτ I(σ : τ).

I(σ) ≈ H(σ) ⇒It is possible to learn about σ indirectly.

There exists a measurement τ on E such thatH(σ|τ) ≈ 0.

a

µ

R0.1(σ

)

0

10

20

30

40

50

0π/2

π/4

0

π/4

π/8a

π/4µ

0.6

0.8

π/4

π/2 0π/8

1.0

0.4

0.2

00

I N(σ

)

m

0.4

0.8

1.0

π/2 0 10 20 3040 50

µ

0.6

0.2

00

π/4

I(σ

:e)

µ = 0.23

a) b) c)

Not a selective criterion.

Waterloo, November 2004 – p.13

Page 39: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Information in the environment

I(σ) = maxτ I(σ : τ). I(σ) ≈ H(σ) ⇒

It is possible to learn about σ indirectly.

There exists a measurement τ on E such thatH(σ|τ) ≈ 0.

a

µ

R0.1(σ

)

0

10

20

30

40

50

0π/2

π/4

0

π/4

π/8a

π/4µ

0.6

0.8

π/4

π/2 0π/8

1.0

0.4

0.2

00

I N(σ

)

m

0.4

0.8

1.0

π/2 0 10 20 3040 50

µ

0.6

0.2

00

π/4

I(σ

:e)

µ = 0.23

a) b) c)

Not a selective criterion.

Waterloo, November 2004 – p.13

Page 40: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Information in the environment

I(σ) = maxτ I(σ : τ). I(σ) ≈ H(σ) ⇒It is possible to learn about σ indirectly.

There exists a measurement τ on E such thatH(σ|τ) ≈ 0.

a

µ

R0.1(σ

)

0

10

20

30

40

50

0π/2

π/4

0

π/4

π/8a

π/4µ

0.6

0.8

π/4

π/2 0π/8

1.0

0.4

0.2

00

I N(σ

)

m

0.4

0.8

1.0

π/2 0 10 20 3040 50

µ

0.6

0.2

00

π/4

I(σ

:e)

µ = 0.23

a) b) c)

Not a selective criterion.

Waterloo, November 2004 – p.13

Page 41: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Information in the environment

I(σ) = maxτ I(σ : τ). I(σ) ≈ H(σ) ⇒It is possible to learn about σ indirectly.

There exists a measurement τ on E such thatH(σ|τ) ≈ 0.

a

µ

R0.1(σ

)

0

10

20

30

40

50

0π/2

π/4

0

π/4

π/8a

π/4µ

0.6

0.8

π/4

π/2 0π/8

1.0

0.4

0.2

00

I N(σ

)

m

0.4

0.8

1.0

π/2 0 10 20 3040 50

µ

0.6

0.2

00

π/4

I(σ

:e)

µ = 0.23

a) b) c)

Not a selective criterion.

Waterloo, November 2004 – p.13

Page 42: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Information in the environment

I(σ) = maxτ I(σ : τ). I(σ) ≈ H(σ) ⇒It is possible to learn about σ indirectly.

There exists a measurement τ on E such thatH(σ|τ) ≈ 0.

a

µ

R0.1(σ

)

0

10

20

30

40

50

0π/2

π/4

0

π/4

π/8a

π/4µ

0.6

0.8

π/4

π/2 0π/8

1.0

0.4

0.2

00

I N(σ

)

m

0.4

0.8

1.0

π/2 0 10 20 3040 50

µ

0.6

0.2

00

π/4

I(σ

:e)

µ = 0.23

a) b) c)

Not a selective criterion.

Waterloo, November 2004 – p.13

Page 43: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Information in the environment

I(σ) = maxτ I(σ : τ). I(σ) ≈ H(σ) ⇒It is possible to learn about σ indirectly.

There exists a measurement τ on E such thatH(σ|τ) ≈ 0.

a

µ

R0.1(σ

)

0

10

20

30

40

50

0π/2

π/4

0

π/4

π/8a

π/4µ

0.6

0.8

π/4

π/2 0π/8

1.0

0.4

0.2

00

I N(σ

)

m

0.4

0.8

1.0

π/2 0 10 20 3040 50

µ

0.6

0.2

00

π/4

I(σ

:e)

µ = 0.23

a) b) c)

Not a selective criterion.

Waterloo, November 2004 – p.13

Page 44: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Redundancy of information

This is a manifestation of “basis ambiguity”.

The solution is obvious when we look at real models.

Assume that the environment is composed ofsubsystems:

E =⊗

k

Ek

Denote the number of disjoint fragments of E — i.e.F ⊂ {Ek} — which contain a copy of this informationR(σ).

R(σ) � 1 is a prerequisitefor the objective existence of σ.

Waterloo, November 2004 – p.14

Page 45: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Redundancy of information

This is a manifestation of “basis ambiguity”.

The solution is obvious when we look at real models.

Assume that the environment is composed ofsubsystems:

E =⊗

k

Ek

Denote the number of disjoint fragments of E — i.e.F ⊂ {Ek} — which contain a copy of this informationR(σ).

R(σ) � 1 is a prerequisitefor the objective existence of σ.

Waterloo, November 2004 – p.14

Page 46: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Redundancy of information

This is a manifestation of “basis ambiguity”.

The solution is obvious when we look at real models.

Assume that the environment is composed ofsubsystems:

E =⊗

k

Ek

Denote the number of disjoint fragments of E — i.e.F ⊂ {Ek} — which contain a copy of this informationR(σ).

R(σ) � 1 is a prerequisitefor the objective existence of σ.

Waterloo, November 2004 – p.14

Page 47: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Redundancy of information

This is a manifestation of “basis ambiguity”.

The solution is obvious when we look at real models.

Assume that the environment is composed ofsubsystems:

E =⊗

k

Ek

Denote the number of disjoint fragments of E — i.e.F ⊂ {Ek} — which contain a copy of this informationR(σ).

R(σ) � 1 is a prerequisitefor the objective existence of σ.

Waterloo, November 2004 – p.14

Page 48: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Redundancy of information

This is a manifestation of “basis ambiguity”.

The solution is obvious when we look at real models.

Assume that the environment is composed ofsubsystems:

E =⊗

k

Ek

Denote the number of disjoint fragments of E — i.e.F ⊂ {Ek} — which contain a copy of this informationR(σ).

R(σ) � 1 is a prerequisitefor the objective existence of σ.

Waterloo, November 2004 – p.14

Page 49: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Redundancy of information

R(σ) � 1 ⇒

There are many copies of the information about σ in theenvironment.

Many independent observers can learn about σ withoutinvalidating each other’s information.

a

µ

R0.1(σ

)

0

10

20

30

40

50

0π/2

π/4

0

π/4

π/8a

π/4µ

0.6

0.8

π/4

π/2 0π/8

1.0

0.4

0.2

00

I N(σ

)

m

0.4

0.8

1.0

π/2 0 10 20 3040 50

µ

0.6

0.2

00

π/4

I(σ

:e)

µ = 0.23

a) b) c)

R(σ) � 1 and IN (σ) ≈ H(σ)

implies a unique observable.

Waterloo, November 2004 – p.15

Page 50: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Redundancy of information

R(σ) � 1 ⇒There are many copies of the information about σ in theenvironment.

Many independent observers can learn about σ withoutinvalidating each other’s information.

a

µ

R0.1(σ

)

0

10

20

30

40

50

0π/2

π/4

0

π/4

π/8a

π/4µ

0.6

0.8

π/4

π/2 0π/8

1.0

0.4

0.2

00

I N(σ

)

m

0.4

0.8

1.0

π/2 0 10 20 3040 50

µ

0.6

0.2

00

π/4

I(σ

:e)

µ = 0.23

a) b) c)

R(σ) � 1 and IN (σ) ≈ H(σ)

implies a unique observable.

Waterloo, November 2004 – p.15

Page 51: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Redundancy of information

R(σ) � 1 ⇒There are many copies of the information about σ in theenvironment.

Many independent observers can learn about σ withoutinvalidating each other’s information.

a

µ

R0.1(σ

)

0

10

20

30

40

50

0π/2

π/4

0

π/4

π/8a

π/4µ

0.6

0.8

π/4

π/2 0π/8

1.0

0.4

0.2

00

I N(σ

)

m

0.4

0.8

1.0

π/2 0 10 20 3040 50

µ

0.6

0.2

00

π/4

I(σ

:e)

µ = 0.23

a) b) c)

R(σ) � 1 and IN (σ) ≈ H(σ)

implies a unique observable.

Waterloo, November 2004 – p.15

Page 52: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Redundancy of information

R(σ) � 1 ⇒There are many copies of the information about σ in theenvironment.

Many independent observers can learn about σ withoutinvalidating each other’s information.

a

µ

R0.1(σ

)

0

10

20

30

40

50

0π/2

π/4

0

π/4

π/8a

π/4µ

0.6

0.8

π/4

π/2 0π/8

1.0

0.4

0.2

00

I N(σ

)

m

0.4

0.8

1.0

π/2 0 10 20 3040 50

µ

0.6

0.2

00

π/4

I(σ

:e)

µ = 0.23

a) b) c)

R(σ) � 1 and IN (σ) ≈ H(σ)

implies a unique observable.

Waterloo, November 2004 – p.15

Page 53: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Redundancy of information

R(σ) � 1 ⇒There are many copies of the information about σ in theenvironment.

Many independent observers can learn about σ withoutinvalidating each other’s information.

a

µ

R0.1(σ

)

0

10

20

30

40

50

0π/2

π/4

0

π/4

π/8a

π/4µ

0.6

0.8

π/4

π/2 0π/8

1.0

0.4

0.2

00

I N(σ

)

m

0.4

0.8

1.0

π/2 0 10 20 3040 50

µ

0.6

0.2

00

π/4

I(σ

:e)

µ = 0.23

a) b) c)

R(σ) � 1 and IN (σ) ≈ H(σ)

implies a unique observable.

Waterloo, November 2004 – p.15

Page 54: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Consequences of redundancy

R(σ) � 1 and I(σ) ≈ H(σ) ⇒

Many independent observers can learn about σ frommonitoring disjoint fragments of E .

Reach a consensus about the properties of S.

Information available from fragments of E is only aboutthe preferred system observable.

The information about S extractable from fragmentsof E follows classical update rules.

Waterloo, November 2004 – p.16

Page 55: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Consequences of redundancy

R(σ) � 1 and I(σ) ≈ H(σ) ⇒Many independent observers can learn about σ frommonitoring disjoint fragments of E .

Reach a consensus about the properties of S.

Information available from fragments of E is only aboutthe preferred system observable.

The information about S extractable from fragmentsof E follows classical update rules.

Waterloo, November 2004 – p.16

Page 56: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Consequences of redundancy

R(σ) � 1 and I(σ) ≈ H(σ) ⇒Many independent observers can learn about σ frommonitoring disjoint fragments of E .

Reach a consensus about the properties of S.

Information available from fragments of E is only aboutthe preferred system observable.

The information about S extractable from fragmentsof E follows classical update rules.

Waterloo, November 2004 – p.16

Page 57: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Summary of Part I

Decoherence and Einselection ; Consensus amongindependent observers.

Arbitrary measurements.

Information through E ; Consensus amongindependent observers.

Basis ambiguity.

Redundant broadcasting of information ⇒ Consensusamong independent observers.

Redundant spreading ⇒ selection of preferredsystem observable.

Waterloo, November 2004 – p.17

Page 58: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Summary of Part I

Decoherence and Einselection ; Consensus amongindependent observers.

Arbitrary measurements.

Information through E ; Consensus amongindependent observers.

Basis ambiguity.

Redundant broadcasting of information ⇒ Consensusamong independent observers.

Redundant spreading ⇒ selection of preferredsystem observable.

Waterloo, November 2004 – p.17

Page 59: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Summary of Part I

Decoherence and Einselection ; Consensus amongindependent observers.

Arbitrary measurements.

Information through E ; Consensus amongindependent observers.

Basis ambiguity.

Redundant broadcasting of information ⇒ Consensusamong independent observers.

Redundant spreading ⇒ selection of preferredsystem observable.

Waterloo, November 2004 – p.17

Page 60: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Part II: Macroscopic observables

What is the observers only have access to macroscopicobservables?

a: a|0〉 = λ0|0〉 and a|1〉 = λ1|1〉.

Ensemble of N systems: AN =∑N

k=1 a(k).

Basis |X〉 = |b1〉 ⊗ |b2〉 ⊗ . . .⊗ |bN 〉:AN |X〉 = Λh(X)|X〉

Hamming weight: h(X) = #1′s in XN

Eigenvalues: Λh = N [(1− h)λ0 + hλ1]

Spectral decomposition AN =∑

h ΛhQh.Qh =

X:h(X)=h |X〉〈X|.

Waterloo, November 2004 – p.18

Page 61: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Part II: Macroscopic observables

What is the observers only have access to macroscopicobservables?

a: a|0〉 = λ0|0〉 and a|1〉 = λ1|1〉.

Ensemble of N systems: AN =∑N

k=1 a(k).

Basis |X〉 = |b1〉 ⊗ |b2〉 ⊗ . . .⊗ |bN 〉:AN |X〉 = Λh(X)|X〉

Hamming weight: h(X) = #1′s in XN

Eigenvalues: Λh = N [(1− h)λ0 + hλ1]

Spectral decomposition AN =∑

h ΛhQh.Qh =

X:h(X)=h |X〉〈X|.

Waterloo, November 2004 – p.18

Page 62: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Part II: Macroscopic observables

What is the observers only have access to macroscopicobservables?

a: a|0〉 = λ0|0〉 and a|1〉 = λ1|1〉.

Ensemble of N systems: AN =∑N

k=1 a(k).

Basis |X〉 = |b1〉 ⊗ |b2〉 ⊗ . . .⊗ |bN 〉:AN |X〉 = Λh(X)|X〉

Hamming weight: h(X) = #1′s in XN

Eigenvalues: Λh = N [(1− h)λ0 + hλ1]

Spectral decomposition AN =∑

h ΛhQh.Qh =

X:h(X)=h |X〉〈X|.

Waterloo, November 2004 – p.18

Page 63: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Part II: Macroscopic observables

What is the observers only have access to macroscopicobservables?

a: a|0〉 = λ0|0〉 and a|1〉 = λ1|1〉.

Ensemble of N systems: AN =∑N

k=1 a(k).

Basis |X〉 = |b1〉 ⊗ |b2〉 ⊗ . . .⊗ |bN 〉:AN |X〉 = Λh(X)|X〉

Hamming weight: h(X) = #1′s in XN

Eigenvalues: Λh = N [(1− h)λ0 + hλ1]

Spectral decomposition AN =∑

h ΛhQh.Qh =

X:h(X)=h |X〉〈X|.

Waterloo, November 2004 – p.18

Page 64: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Part II: Macroscopic observables

What is the observers only have access to macroscopicobservables?

a: a|0〉 = λ0|0〉 and a|1〉 = λ1|1〉.

Ensemble of N systems: AN =∑N

k=1 a(k).

Basis |X〉 = |b1〉 ⊗ |b2〉 ⊗ . . .⊗ |bN 〉:AN |X〉 = Λh(X)|X〉

Hamming weight: h(X) = #1′s in XN

Eigenvalues: Λh = N [(1− h)λ0 + hλ1]

Spectral decomposition AN =∑

h ΛhQh.Qh =

X:h(X)=h |X〉〈X|.

Waterloo, November 2004 – p.18

Page 65: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

von Neumann measurement theory

Measuring average value of a over an ensemble of Nsystems.

⇔Making a von Neumann measurement {Qh} on the

collection of systems.

From state ρN = |ψ〉〈ψ|⊗N and ψ = α|0〉+ β|1〉, we expect

〈a〉 = Tr{a|ψ〉〈ψ|} = |α|2λ0 + |β|2λ1.

State after measurement ≈ ρN (as N becomes large).

Finkelstein, Hartle, Graham, FGG,etc.

limN→∞

∣AN |ψ〉⊗N − 〈a〉|ψ〉⊗N

∣= 0

Waterloo, November 2004 – p.19

Page 66: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

von Neumann measurement theory

Measuring average value of a over an ensemble of Nsystems.

⇔Making a von Neumann measurement {Qh} on the

collection of systems.From state ρN = |ψ〉〈ψ|⊗N and ψ = α|0〉+ β|1〉, we expect

〈a〉 = Tr{a|ψ〉〈ψ|} = |α|2λ0 + |β|2λ1.

State after measurement ≈ ρN (as N becomes large).

Finkelstein, Hartle, Graham, FGG,etc.

limN→∞

∣AN |ψ〉⊗N − 〈a〉|ψ〉⊗N

∣= 0

Waterloo, November 2004 – p.19

Page 67: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

von Neumann measurement theory

Measuring average value of a over an ensemble of Nsystems.

⇔Making a von Neumann measurement {Qh} on the

collection of systems.From state ρN = |ψ〉〈ψ|⊗N and ψ = α|0〉+ β|1〉, we expect

〈a〉 = Tr{a|ψ〉〈ψ|} = |α|2λ0 + |β|2λ1.

State after measurement ≈ ρN (as N becomes large).

Finkelstein, Hartle, Graham, FGG,etc.

limN→∞

∣AN |ψ〉⊗N − 〈a〉|ψ〉⊗N

∣= 0

Waterloo, November 2004 – p.19

Page 68: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

von Neumann measurement theory

Measuring average value of a over an ensemble of Nsystems.

⇔Making a von Neumann measurement {Qh} on the

collection of systems.From state ρN = |ψ〉〈ψ|⊗N and ψ = α|0〉+ β|1〉, we expect

〈a〉 = Tr{a|ψ〉〈ψ|} = |α|2λ0 + |β|2λ1.

State after measurement ≈ ρN (as N becomes large).

Finkelstein, Hartle, Graham, FGG,etc.

limN→∞

∣AN |ψ〉⊗N − 〈a〉|ψ〉⊗N

∣= 0

Waterloo, November 2004 – p.19

Page 69: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

von Neumann measurement theory

Measuring average value of a over an ensemble of Nsystems.

⇔Making a von Neumann measurement {Qh} on the

collection of systems.From state ρN = |ψ〉〈ψ|⊗N and ψ = α|0〉+ β|1〉, we expect

〈a〉 = Tr{a|ψ〉〈ψ|} = |α|2λ0 + |β|2λ1.

State after measurement ≈ ρN (as N becomes large).

Finkelstein, Hartle, Graham, FGG,etc.

limN→∞

∣AN |ψ〉⊗N − 〈a〉|ψ〉⊗N

∣= 0

Waterloo, November 2004 – p.19

Page 70: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

von Neumann measurement theory

So for N finite but large...

Conditional post-measurement state:

ρNh−→ ρN |h = QhρNQh

P (Qh) .

Average post-measurement state:ρN → ρ′N =

h ρN |hP (Qh) =∑

hQhρNQh.

F (ρN , ρ′N ) = 〈ψ|⊗Nρ′N |ψ〉⊗N ≤ 1

2π√N |α| · |β|

Fidelity between pre- and post-measurement states goes to zero.

Waterloo, November 2004 – p.20

Page 71: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

von Neumann measurement theory

So for N finite but large...

Conditional post-measurement state:

ρNh−→ ρN |h = QhρNQh

P (Qh) .

Average post-measurement state:ρN → ρ′N =

h ρN |hP (Qh) =∑

hQhρNQh.

F (ρN , ρ′N ) = 〈ψ|⊗Nρ′N |ψ〉⊗N ≤ 1

2π√N |α| · |β|

Fidelity between pre- and post-measurement states goes to zero.

Waterloo, November 2004 – p.20

Page 72: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

von Neumann measurement theory

So for N finite but large...

Conditional post-measurement state:

ρNh−→ ρN |h = QhρNQh

P (Qh) .

Average post-measurement state:ρN → ρ′N =

h ρN |hP (Qh) =∑

hQhρNQh.

F (ρN , ρ′N ) = 〈ψ|⊗Nρ′N |ψ〉⊗N ≤ 1

2π√N |α| · |β|

Fidelity between pre- and post-measurement states goes to zero.

Waterloo, November 2004 – p.20

Page 73: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

von Neumann measurement theory

So for N finite but large...

Conditional post-measurement state:

ρNh−→ ρN |h = QhρNQh

P (Qh) .

Average post-measurement state:ρN → ρ′N =

h ρN |hP (Qh) =∑

hQhρNQh.

F (ρN , ρ′N ) = 〈ψ|⊗Nρ′N |ψ〉⊗N ≤ 1

2π√N |α| · |β|

Fidelity between pre- and post-measurement states goes to zero.

Waterloo, November 2004 – p.20

Page 74: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Realistic measurement model

Coupling between S and A: HSA = AN ⊗ P ,where [P,R] = i.

ShiftR0

|h〉 |h〉

h

R0

Subsequent measurement of R⇔ measurement ofQh =

h′

qh′(h)Qh′.

The precision of the “measurement” depends on thecoherence length of the measurement apparatus.

Waterloo, November 2004 – p.21

Page 75: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Realistic measurement model

Coupling between S and A: HSA = AN ⊗ P ,where [P,R] = i.

ShiftR0

|h〉 |h〉

h

R0

Subsequent measurement of R⇔ measurement ofQh =

h′

qh′(h)Qh′.

The precision of the “measurement” depends on thecoherence length of the measurement apparatus.

Waterloo, November 2004 – p.21

Page 76: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Realistic measurement model

Coupling between S and A: HSA = AN ⊗ P ,where [P,R] = i.

ShiftR0

|h〉 |h〉

h

R0

Subsequent measurement of R⇔ measurement ofQh.

Subsequent measurement of R⇔ measurement ofQh =

h′

qh′(h)Qh′.

The precision of the “measurement” depends on thecoherence length of the measurement apparatus.

Waterloo, November 2004 – p.21

Page 77: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Realistic measurement model

Coupling between S and A: HSA = AN ⊗ P ,where [P,R] = i.

ShiftR0

|h〉 |h〉

h

R0

Subsequent measurement of R⇔ measurement ofQh =

h′

qh′(h)Qh′.

The precision of the “measurement” depends on thecoherence length of the measurement apparatus.

Waterloo, November 2004 – p.21

Page 78: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Realistic measurement model

Coupling between S and A: HSA = AN ⊗ P ,where [P,R] = i.

ShiftR0

|h〉 |h〉

h

R0

Subsequent measurement of R⇔ measurement ofQh =

h′

qh′(h)Qh′.

The precision of the “measurement” depends on thecoherence length of the measurement apparatus.

Waterloo, November 2004 – p.21

Page 79: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Smooth POVM’s

Qh =1

(2π)1/4√σ

h′

exp

{

−(h− h′)2

4σ2

}

Qh′

The measurement result will be h ≈ |β|2 within themeasurement accuracy σ:

P (|h− |β|2| ≥ ε) ≈ exp{

− ε2

2σ2+1/N

}

The state disturbance is negligeable:

F (ρN , ρ′N ) ≥ 1− 1+ln(4Nσ2)

2Nσ2

F (ρN , ρN |h) ≥ (more complicated but same scaling)

Desired behaviour (N = ∞):measurement coarseness σ � 1/

√N .

Waterloo, November 2004 – p.22

Page 80: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Smooth POVM’s

Qh =1

(2π)1/4√σ

h′

exp

{

−(h− h′)2

4σ2

}

Qh′

The measurement result will be h ≈ |β|2 within themeasurement accuracy σ:

P (|h− |β|2| ≥ ε) ≈ exp{

− ε2

2σ2+1/N

}

The state disturbance is negligeable:

F (ρN , ρ′N ) ≥ 1− 1+ln(4Nσ2)

2Nσ2

F (ρN , ρN |h) ≥ (more complicated but same scaling)

Desired behaviour (N = ∞):measurement coarseness σ � 1/

√N .

Waterloo, November 2004 – p.22

Page 81: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Smooth POVM’s

Qh =1

(2π)1/4√σ

h′

exp

{

−(h− h′)2

4σ2

}

Qh′

The measurement result will be h ≈ |β|2 within themeasurement accuracy σ:

P (|h− |β|2| ≥ ε) ≈ exp{

− ε2

2σ2+1/N

}

The state disturbance is negligeable:

F (ρN , ρ′N ) ≥ 1− 1+ln(4Nσ2)

2Nσ2

F (ρN , ρN |h) ≥ (more complicated but same scaling)

Desired behaviour (N = ∞):measurement coarseness σ � 1/

√N .

Waterloo, November 2004 – p.22

Page 82: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Smooth POVM’s

Qh =1

(2π)1/4√σ

h′

exp

{

−(h− h′)2

4σ2

}

Qh′

The measurement result will be h ≈ |β|2 within themeasurement accuracy σ:

P (|h− |β|2| ≥ ε) ≈ exp{

− ε2

2σ2+1/N

}

The state disturbance is negligeable:

F (ρN , ρ′N ) ≥ 1− 1+ln(4Nσ2)

2Nσ2

F (ρN , ρN |h) ≥ (more complicated but same scaling)

Desired behaviour (N = ∞):measurement coarseness σ � 1/

√N .

Waterloo, November 2004 – p.22

Page 83: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Classicality

Exchangeable state: ρN =∫

P (ρ)ρ⊗Ndρ

ρN |h =QhρN Qh

P (Qh)

=

P (ρ)

P (Qh)Qhρ

⊗N Qhdρ

≈∫

P (ρ)P (Qh|ρ)P (Qh)

ρ⊗Ndρ

This is like Bayesian updating:

P (ρ)h−→ P (ρ|h) = P (ρ)P (Qh|ρ)/P (Qh)

Waterloo, November 2004 – p.23

Page 84: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Classicality

Exchangeable state: ρN =∫

P (ρ)ρ⊗Ndρ

ρN |h =QhρN Qh

P (Qh)

ρN |h =QhρN Qh

P (Qh)

=

P (ρ)

P (Qh)Qhρ

⊗N Qhdρ

≈∫

P (ρ)P (Qh|ρ)P (Qh)

ρ⊗Ndρ

This is like Bayesian updating:

P (ρ)h−→ P (ρ|h) = P (ρ)P (Qh|ρ)/P (Qh)

Waterloo, November 2004 – p.23

Page 85: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Classicality

Exchangeable state: ρN =∫

P (ρ)ρ⊗Ndρ

ρN |h =QhρN Qh

P (Qh)

=

P (ρ)

P (Qh)Qhρ

⊗N Qhdρ

ρN |h =QhρN Qh

P (Qh)

=

P (ρ)

P (Qh)Qhρ

⊗N Qhdρ

≈∫

P (ρ)P (Qh|ρ)P (Qh)

ρ⊗Ndρ

This is like Bayesian updating:

P (ρ)h−→ P (ρ|h) = P (ρ)P (Qh|ρ)/P (Qh)

Waterloo, November 2004 – p.23

Page 86: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Classicality

Exchangeable state: ρN =∫

P (ρ)ρ⊗Ndρ

ρN |h =QhρN Qh

P (Qh)

=

P (ρ)

P (Qh)Qhρ

⊗N Qhdρ

≈∫

P (ρ)P (Qh|ρ)P (Qh)

ρ⊗Ndρ

This is like Bayesian updating:

P (ρ)h−→ P (ρ|h) = P (ρ)P (Qh|ρ)/P (Qh)

Waterloo, November 2004 – p.23

Page 87: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Classicality

Exchangeable state: ρN =∫

P (ρ)ρ⊗Ndρ

ρN |h =QhρN Qh

P (Qh)

=

P (ρ)

P (Qh)Qhρ

⊗N Qhdρ

≈∫

P (ρ)P (Qh|ρ)P (Qh)

ρ⊗Ndρ

This is like Bayesian updating:

P (ρ)h−→ P (ρ|h) = P (ρ)P (Qh|ρ)/P (Qh)

Waterloo, November 2004 – p.23

Page 88: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Classicality

For coarse macroscopic observables, the averagestates ρ of an ensemble behave as objective elementsof reality.

They can be “discovered without being disturbed”.Upon measurements, their update rule is Bayesian,just like for phase space coordinates of classicalsystems.

The argument can be extended to entangled states; thecondition becomes σ �

ξ/N .Macroscopic measurements behave classicallywhen coarse on the quantum correlation lengthscale ξ of the ensemble.

Waterloo, November 2004 – p.24

Page 89: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Classicality

For coarse macroscopic observables, the averagestates ρ of an ensemble behave as objective elementsof reality.

They can be “discovered without being disturbed”.Upon measurements, their update rule is Bayesian,just like for phase space coordinates of classicalsystems.

The argument can be extended to entangled states; thecondition becomes σ �

ξ/N .Macroscopic measurements behave classicallywhen coarse on the quantum correlation lengthscale ξ of the ensemble.

Waterloo, November 2004 – p.24

Page 90: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Summary of Part II

von Neumann measurement (textbook) ⇒ Importantdisturbance.

Coarse grained POVMs ⇒ No disturbance for large N .

Macroscopic limit requires large N and coarse graining.

Under these conditions, the average state of an ensemblebehaves as an objective element of reality.

Waterloo, November 2004 – p.25

Page 91: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Summary of Part II

von Neumann measurement (textbook) ⇒ Importantdisturbance.

Coarse grained POVMs ⇒ No disturbance for large N .

Macroscopic limit requires large N and coarse graining.

Under these conditions, the average state of an ensemblebehaves as an objective element of reality.

Waterloo, November 2004 – p.25

Page 92: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Summary of Part II

von Neumann measurement (textbook) ⇒ Importantdisturbance.

Coarse grained POVMs ⇒ No disturbance for large N .

Macroscopic limit requires large N and coarse graining.

Under these conditions, the average state of an ensemblebehaves as an objective element of reality.

Waterloo, November 2004 – p.25

Page 93: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Summary of Part II

von Neumann measurement (textbook) ⇒ Importantdisturbance.

Coarse grained POVMs ⇒ No disturbance for large N .

Macroscopic limit requires large N and coarse graining.

Under these conditions, the average state of an ensemblebehaves as an objective element of reality.

Waterloo, November 2004 – p.25

Page 94: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Conclusion

Distinction between quantum and classical: lack of anobjective reality.

Distinction in the update rule.

An objective reality is operationally recovered when1. The information is acquired by probing a fragment of

the environment, or2. The information is about macroscopic quantities.

Under realistic assumptions, classical realityemerges from the underlying quantum theory.

Waterloo, November 2004 – p.26

Page 95: Emergence of classical reality from within quantum theory · Emergence of classical reality from within quantum theory Oral Thesis presentation David Poulin Institute for Quantum

Acknowledgments

Special thanks to Raymond Laflamme, Harold Ollivier, and WojciechZurek.

Howard Barnum, Jonathan Baugh, Charlie Bennett, Alexandre Blais, Robin Blume-Kohout,

Jean-Christian Boileau, Gilles Brassard, Carl Caves, David Cory, Fay Dowker, Artur Ekert,

Joseph Emerson, Marie Ericsson, Chris Fuchs, Ernesto Galvão, Florian Girelli, Daniel

Gottesman, Patrick Hayden, Lucien Hardy, Dominik Janzing, Alexei Kitaev, Manny Knill,

David Kribs, Frédéric Leblond, Etera Livine, Fotini Markopoulou, Gerard Milburn, Mike

Mosca, Casey Myers, Camille Negrevergne, Juan Pablo Paz, Ruediger Schack, Lee Smolin,

Rolando Somma, Raphael Sorkin, Rob Spekkens, Alain Tapp, Hugo Touchette, Antony

Valentini, Guifre Vidal, Lorenza Viola, Christof Zalka, Paolo Zanardi, all other members and

staff of IQC and PI!

NSERC, FCAR, CIAR, CRC, ORDC, and Big Mike.

Merci à ma famille, à mes amis, et à Isabelle.

Waterloo, November 2004 – p.27


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