Unentangled Bit Commitment and the Clifton-Bub-Halvorson (CBH) Theorem

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Talk given at Pavia mini-workshop on Operational Theories. This was the last talk that I gave before I became ill. It focuses on the results of http://arxiv.org/abs/0707.0620 and http://arxiv.org/abs/0803.1264, and their relation to the CBH theorem.
The section on bit commitment is missing because I did it on the blackboard.

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Unentangled Bit Commitment and the Clifton-Bub-Halvorson (CBH) Theorem

  1. 1. Introduction Framework Unentangled Bit Commitment Broadcasting Comparison to CBH Theorem Conclusions Unentangled Bit Commitment and the Clifton-Bub-Halvorson (CBH) Theorem M. S. Leifer Institute for Quantum Computing University of Waterloo Perimeter Institute for Theoretical Physics Dec. 18th 2007/Pavia Mini-Workshop M. S. Leifer Unentangled Bit Commitment
  2. 2. Introduction Framework Unentangled Bit Commitment Broadcasting Comparison to CBH Theorem Conclusions Outline Introduction: The Brassard-Fuchs Speculation and CBH 1 The Convex Sets Framework 2 Unentangled Bit Commitment 3 Broadcasting 4 Comparison to the CBH Theorem 5 Conclusions 6 M. S. Leifer Unentangled Bit Commitment
  3. 3. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions The Brassard-Fuchs Speculation In ≈2000, Brassard and Fuchs speculated that the basic Hilbert Space structures of quantum theory might be uniquely determined by two cryptographic constraints: The Possibility of Secure Key Distribution The Impossibility of Bit Commitment This was to be viewed as analogous to Einstein’s derivation of the kinematics for special relativity from the two postulates: The laws of physics are invariant under changes of inertial frame. The speed of light in vacuo is constant in all inertial frames. M. S. Leifer Unentangled Bit Commitment
  4. 4. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions The Brassard-Fuchs Speculation In ≈2000, Brassard and Fuchs speculated that the basic Hilbert Space structures of quantum theory might be uniquely determined by two cryptographic constraints: The Possibility of Secure Key Distribution The Impossibility of Bit Commitment This was to be viewed as analogous to Einstein’s derivation of the kinematics for special relativity from the two postulates: The laws of physics are invariant under changes of inertial frame. The speed of light in vacuo is constant in all inertial frames. M. S. Leifer Unentangled Bit Commitment
  5. 5. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions The Brassard-Fuchs Speculation This derivation has to be done within a precise mathematical framework for physical theories, which must be: Narrow enough to convert the axioms into precise mathematical constraints. Broad enough that the work is being done by the postulates rather than the framework assumptions. We are allowed to import definitions and concepts from existing physical frameworks, just as Einstein did. M. S. Leifer Unentangled Bit Commitment
  6. 6. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions The Brassard-Fuchs Speculation This derivation has to be done within a precise mathematical framework for physical theories, which must be: Narrow enough to convert the axioms into precise mathematical constraints. Broad enough that the work is being done by the postulates rather than the framework assumptions. We are allowed to import definitions and concepts from existing physical frameworks, just as Einstein did. M. S. Leifer Unentangled Bit Commitment
  7. 7. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions The Brassard-Fuchs Speculation This derivation has to be done within a precise mathematical framework for physical theories, which must be: Narrow enough to convert the axioms into precise mathematical constraints. Broad enough that the work is being done by the postulates rather than the framework assumptions. We are allowed to import definitions and concepts from existing physical frameworks, just as Einstein did. M. S. Leifer Unentangled Bit Commitment
  8. 8. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions The CBH Theorem In 2003, Clifton, Bub and Halvorson “derived quantum theory” from: The impossibility of superluminal information transfer between two physical systems by performing measurements on one of them. The impossibility of perfectly broadcasting the information contained in an unknown physical state. The impossibility of unconditionally secure bit commitment. The mathematical framework chosen was C ∗ -algebraic theories. M. S. Leifer Unentangled Bit Commitment
  9. 9. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions The CBH Theorem CBH don’t arrive exactly at quantum theory, but intend their theorem to be read as follows: No signalling ⇒ Separate systems correspond to 1 commuting algebras of observables. No broadcasting ⇒ Algebras corresponding to individual 2 systems are nonabelian. No bit commitment ⇒ Bipartite systems can occupy 3 entangled states. There is some debate about whether 3 is independent of 1 and 2. M. S. Leifer Unentangled Bit Commitment
  10. 10. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions The CBH Theorem CBH don’t arrive exactly at quantum theory, but intend their theorem to be read as follows: No signalling ⇒ Separate systems correspond to 1 commuting algebras of observables. No broadcasting ⇒ Algebras corresponding to individual 2 systems are nonabelian. No bit commitment ⇒ Bipartite systems can occupy 3 entangled states. There is some debate about whether 3 is independent of 1 and 2. M. S. Leifer Unentangled Bit Commitment
  11. 11. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions The CBH Theorem CBH don’t arrive exactly at quantum theory, but intend their theorem to be read as follows: No signalling ⇒ Separate systems correspond to 1 commuting algebras of observables. No broadcasting ⇒ Algebras corresponding to individual 2 systems are nonabelian. No bit commitment ⇒ Bipartite systems can occupy 3 entangled states. There is some debate about whether 3 is independent of 1 and 2. M. S. Leifer Unentangled Bit Commitment
  12. 12. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions The CBH Theorem CBH don’t arrive exactly at quantum theory, but intend their theorem to be read as follows: No signalling ⇒ Separate systems correspond to 1 commuting algebras of observables. No broadcasting ⇒ Algebras corresponding to individual 2 systems are nonabelian. No bit commitment ⇒ Bipartite systems can occupy 3 entangled states. There is some debate about whether 3 is independent of 1 and 2. M. S. Leifer Unentangled Bit Commitment
  13. 13. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions Why C ∗ -algebras? We are not in the business of rigorous axiomatization, so CBH say: ...it suffices for present purposes simply to observe that all physical theories that have been found empirically successful – not just phase space and Hilbert space theories but also theories based on a manifold – fall under this framework They should have added: AND THAT’S IT! M. S. Leifer Unentangled Bit Commitment
  14. 14. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions Why C ∗ -algebras? We are not in the business of rigorous axiomatization, so CBH say: ...it suffices for present purposes simply to observe that all physical theories that have been found empirically successful – not just phase space and Hilbert space theories but also theories based on a manifold – fall under this framework They should have added: AND THAT’S IT! M. S. Leifer Unentangled Bit Commitment
  15. 15. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions Why C ∗ -algebras? We are not in the business of rigorous axiomatization, so CBH say: ...it suffices for present purposes simply to observe that all physical theories that have been found empirically successful – not just phase space and Hilbert space theories but also theories based on a manifold – fall under this framework They should have added: AND THAT’S IT! M. S. Leifer Unentangled Bit Commitment
  16. 16. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions C ∗ -algebras: Reasons to be skeptical C ∗ -algebras were invented to do a Hilbert’s 10th job on quantum theory – particularly QFT and quantum stat. mech. Every C ∗ -algebra has a faithful Hilbert space representation (GNS theorem). In finite dimensions we only have classical probability, quantum theory and quantum theory with superselection rules. In infinite dimensions it’s essentially the same story. It is pretty easy to derive quantum theory if you assume quantum theory at the outset. M. S. Leifer Unentangled Bit Commitment
  17. 17. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions C ∗ -algebras: Reasons to be skeptical C ∗ -algebras were invented to do a Hilbert’s 10th job on quantum theory – particularly QFT and quantum stat. mech. Every C ∗ -algebra has a faithful Hilbert space representation (GNS theorem). In finite dimensions we only have classical probability, quantum theory and quantum theory with superselection rules. In infinite dimensions it’s essentially the same story. It is pretty easy to derive quantum theory if you assume quantum theory at the outset. M. S. Leifer Unentangled Bit Commitment
  18. 18. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions C ∗ -algebras: Reasons to be skeptical C ∗ -algebras were invented to do a Hilbert’s 10th job on quantum theory – particularly QFT and quantum stat. mech. Every C ∗ -algebra has a faithful Hilbert space representation (GNS theorem). In finite dimensions we only have classical probability, quantum theory and quantum theory with superselection rules. In infinite dimensions it’s essentially the same story. It is pretty easy to derive quantum theory if you assume quantum theory at the outset. M. S. Leifer Unentangled Bit Commitment
  19. 19. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions C ∗ -algebras: Reasons to be skeptical C ∗ -algebras were invented to do a Hilbert’s 10th job on quantum theory – particularly QFT and quantum stat. mech. Every C ∗ -algebra has a faithful Hilbert space representation (GNS theorem). In finite dimensions we only have classical probability, quantum theory and quantum theory with superselection rules. In infinite dimensions it’s essentially the same story. It is pretty easy to derive quantum theory if you assume quantum theory at the outset. M. S. Leifer Unentangled Bit Commitment
  20. 20. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions C ∗ -algebras: Reasons to be skeptical C ∗ -algebras were invented to do a Hilbert’s 10th job on quantum theory – particularly QFT and quantum stat. mech. Every C ∗ -algebra has a faithful Hilbert space representation (GNS theorem). In finite dimensions we only have classical probability, quantum theory and quantum theory with superselection rules. In infinite dimensions it’s essentially the same story. It is pretty easy to derive quantum theory if you assume quantum theory at the outset. M. S. Leifer Unentangled Bit Commitment
  21. 21. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions Generalized Probabilistic Frameworks C*-algebraic Generic Theories Quantum Classical M. S. Leifer Unentangled Bit Commitment
  22. 22. Introduction Framework The Brassard-Fuchs Speculation Unentangled Bit Commitment The CBH-Theorem C ∗ -algebraic theories Broadcasting Comparison to CBH Theorem Generalizing CBH Conclusions The End Result C*-algebraic Generic Theories Quantum Classical M. S. Leifer Unentangled Bit Commitment
  23. 23. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products The Convex Sets Framework A traditional operational framework. Preparation Transformation Measurement Goal: Predict Prob(outcome|Choice of P, T and M) M. S. Leifer Unentangled Bit Commitment
  24. 24. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Preparations → States Definition The set Ω of normalized states is a compact, closed, convex set. Convex: If ω, µ ∈ Ω and p ∈ [0, 1] then pω + (1 − p)µ ∈ Ω. Extreme points of Ω are called pure states. Note: Every convex subset of a locally convex topological vector space is affinely homeomorphic to the set of all states on a test space (F. W. Shultz, Journal of Combinatorial Theory A 17, 317 (1974)). M. S. Leifer Unentangled Bit Commitment
  25. 25. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Preparations → States Definition The set Ω of normalized states is a compact, closed, convex set. Convex: If ω, µ ∈ Ω and p ∈ [0, 1] then pω + (1 − p)µ ∈ Ω. Extreme points of Ω are called pure states. Note: Every convex subset of a locally convex topological vector space is affinely homeomorphic to the set of all states on a test space (F. W. Shultz, Journal of Combinatorial Theory A 17, 317 (1974)). M. S. Leifer Unentangled Bit Commitment
  26. 26. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Preparations → States Definition The set Ω of normalized states is a compact, closed, convex set. Convex: If ω, µ ∈ Ω and p ∈ [0, 1] then pω + (1 − p)µ ∈ Ω. Extreme points of Ω are called pure states. Note: Every convex subset of a locally convex topological vector space is affinely homeomorphic to the set of all states on a test space (F. W. Shultz, Journal of Combinatorial Theory A 17, 317 (1974)). M. S. Leifer Unentangled Bit Commitment
  27. 27. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Examples Classical: Ω = Probability simplex. Quantum: Ω = {Denisty matrices}. Polyhedral. |0> z y x |1> M. S. Leifer Unentangled Bit Commitment
  28. 28. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Measurement Outcomes → Effects Definition Let A(Ω) be the set of affine functionals Ω → R and V (Ω) be the set of positive affine functionals Ω → R+ . ∀p ∈ [0, 1], f (pω + (1 − p)µ) = pf (ω) + (1 − p)f (µ) A(Ω) is a vector space and V (Ω) is a convex cone. (αf + βg)(ω) = αf (ω) + βg(ω) V (Ω) spans A(Ω). Partial order on A(Ω): f ≤ g iff ∀ω ∈ Ω, f (ω) ≤ g(ω). ˜ ˜ Unit: ∀ω ∈ Ω, 1(ω) = 1. Zero: ∀v ∈ V , 0(v ) = 0. ˜ 1] = {f ∈ V (Ω)|0 ≤ f ≤ 1}. ˜ ˜ ˜ Normalized effects: [0, M. S. Leifer Unentangled Bit Commitment
  29. 29. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Measurement Outcomes → Effects Definition Let A(Ω) be the set of affine functionals Ω → R and V (Ω) be the set of positive affine functionals Ω → R+ . ∀p ∈ [0, 1], f (pω + (1 − p)µ) = pf (ω) + (1 − p)f (µ) A(Ω) is a vector space and V (Ω) is a convex cone. (αf + βg)(ω) = αf (ω) + βg(ω) V (Ω) spans A(Ω). Partial order on A(Ω): f ≤ g iff ∀ω ∈ Ω, f (ω) ≤ g(ω). ˜ ˜ Unit: ∀ω ∈ Ω, 1(ω) = 1. Zero: ∀v ∈ V , 0(v ) = 0. ˜ 1] = {f ∈ V (Ω)|0 ≤ f ≤ 1}. ˜ ˜ ˜ Normalized effects: [0, M. S. Leifer Unentangled Bit Commitment
  30. 30. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Measurement Outcomes → Effects Definition Let A(Ω) be the set of affine functionals Ω → R and V (Ω) be the set of positive affine functionals Ω → R+ . ∀p ∈ [0, 1], f (pω + (1 − p)µ) = pf (ω) + (1 − p)f (µ) A(Ω) is a vector space and V (Ω) is a convex cone. (αf + βg)(ω) = αf (ω) + βg(ω) V (Ω) spans A(Ω). Partial order on A(Ω): f ≤ g iff ∀ω ∈ Ω, f (ω) ≤ g(ω). ˜ ˜ Unit: ∀ω ∈ Ω, 1(ω) = 1. Zero: ∀v ∈ V , 0(v ) = 0. ˜ 1] = {f ∈ V (Ω)|0 ≤ f ≤ 1}. ˜ ˜ ˜ Normalized effects: [0, M. S. Leifer Unentangled Bit Commitment
  31. 31. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Measurement Outcomes → Effects Definition Let A(Ω) be the set of affine functionals Ω → R and V (Ω) be the set of positive affine functionals Ω → R+ . ∀p ∈ [0, 1], f (pω + (1 − p)µ) = pf (ω) + (1 − p)f (µ) A(Ω) is a vector space and V (Ω) is a convex cone. (αf + βg)(ω) = αf (ω) + βg(ω) V (Ω) spans A(Ω). Partial order on A(Ω): f ≤ g iff ∀ω ∈ Ω, f (ω) ≤ g(ω). ˜ ˜ Unit: ∀ω ∈ Ω, 1(ω) = 1. Zero: ∀v ∈ V , 0(v ) = 0. ˜ 1] = {f ∈ V (Ω)|0 ≤ f ≤ 1}. ˜ ˜ ˜ Normalized effects: [0, M. S. Leifer Unentangled Bit Commitment
  32. 32. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Measurement Outcomes → Effects Definition Let A(Ω) be the set of affine functionals Ω → R and V (Ω) be the set of positive affine functionals Ω → R+ . ∀p ∈ [0, 1], f (pω + (1 − p)µ) = pf (ω) + (1 − p)f (µ) A(Ω) is a vector space and V (Ω) is a convex cone. (αf + βg)(ω) = αf (ω) + βg(ω) V (Ω) spans A(Ω). Partial order on A(Ω): f ≤ g iff ∀ω ∈ Ω, f (ω) ≤ g(ω). ˜ ˜ Unit: ∀ω ∈ Ω, 1(ω) = 1. Zero: ∀v ∈ V , 0(v ) = 0. ˜ 1] = {f ∈ V (Ω)|0 ≤ f ≤ 1}. ˜ ˜ ˜ Normalized effects: [0, M. S. Leifer Unentangled Bit Commitment
  33. 33. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Measurement Outcomes → Effects Definition Let A(Ω) be the set of affine functionals Ω → R and V (Ω) be the set of positive affine functionals Ω → R+ . ∀p ∈ [0, 1], f (pω + (1 − p)µ) = pf (ω) + (1 − p)f (µ) A(Ω) is a vector space and V (Ω) is a convex cone. (αf + βg)(ω) = αf (ω) + βg(ω) V (Ω) spans A(Ω). Partial order on A(Ω): f ≤ g iff ∀ω ∈ Ω, f (ω) ≤ g(ω). ˜ ˜ Unit: ∀ω ∈ Ω, 1(ω) = 1. Zero: ∀v ∈ V , 0(v ) = 0. ˜ 1] = {f ∈ V (Ω)|0 ≤ f ≤ 1}. ˜ ˜ ˜ Normalized effects: [0, M. S. Leifer Unentangled Bit Commitment
  34. 34. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products States as vectors Consider the dual space A(Ω)∗ of linear functionals A(Ω) → R and the dual cone of V (Ω)∗ of linear functionals V (Ω) → R+ . V (Ω)∗ can be extended to A(Ω). An element of Ω can be mapped to an element of V (Ω)∗ via ω ∗ (f ) = f (ω). V (Ω)∗ can be thought of as the set of unnormalized states. M. S. Leifer Unentangled Bit Commitment
  35. 35. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products States as vectors Consider the dual space A(Ω)∗ of linear functionals A(Ω) → R and the dual cone of V (Ω)∗ of linear functionals V (Ω) → R+ . V (Ω)∗ can be extended to A(Ω). An element of Ω can be mapped to an element of V (Ω)∗ via ω ∗ (f ) = f (ω). V (Ω)∗ can be thought of as the set of unnormalized states. M. S. Leifer Unentangled Bit Commitment
  36. 36. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products States as vectors Consider the dual space A(Ω)∗ of linear functionals A(Ω) → R and the dual cone of V (Ω)∗ of linear functionals V (Ω) → R+ . V (Ω)∗ can be extended to A(Ω). An element of Ω can be mapped to an element of V (Ω)∗ via ω ∗ (f ) = f (ω). V (Ω)∗ can be thought of as the set of unnormalized states. M. S. Leifer Unentangled Bit Commitment
  37. 37. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products States as vectors Consider the dual space A(Ω)∗ of linear functionals A(Ω) → R and the dual cone of V (Ω)∗ of linear functionals V (Ω) → R+ . V (Ω)∗ can be extended to A(Ω). An element of Ω can be mapped to an element of V (Ω)∗ via ω ∗ (f ) = f (ω). V (Ω)∗ can be thought of as the set of unnormalized states. M. S. Leifer Unentangled Bit Commitment
  38. 38. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Examples Classical: A(Ω) = {functions} V (Ω) = {positive functions} ˜˜ [0, 1] = {Fuzzy indicator functions} V (Ω)∗ = {positive functions} Quantum: A(Ω) ∼ {Hermitian operators} via f (ρ) = Tr(Af ρ) = V (Ω) ∼ {positive operators} = [0, 1] ∼ {POVM elements} ˜˜= V (Ω)∗ ∼ {positive operators} = M. S. Leifer Unentangled Bit Commitment
  39. 39. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Examples Classical: A(Ω) = {functions} V (Ω) = {positive functions} ˜˜ [0, 1] = {Fuzzy indicator functions} V (Ω)∗ = {positive functions} Quantum: A(Ω) ∼ {Hermitian operators} via f (ρ) = Tr(Af ρ) = V (Ω) ∼ {positive operators} = [0, 1] ∼ {POVM elements} ˜˜= V (Ω)∗ ∼ {positive operators} = M. S. Leifer Unentangled Bit Commitment
  40. 40. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Examples Polyhedral: ˜˜ V (Ω) [0, 1] ˜ 1 Ω V (Ω)∗ ˜ 0 M. S. Leifer Unentangled Bit Commitment
  41. 41. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Observables Definition An observable is a finite collection (f1 , f2 , . . . , fN ) of elements of [0, 1] that satisfies N fj = 1. ˜˜ ˜ j=1 Note: Analogous to a POVM in Quantum Theory. Can give more sophisticated measure-theoretic definition. M. S. Leifer Unentangled Bit Commitment
  42. 42. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Observables Definition An observable is a finite collection (f1 , f2 , . . . , fN ) of elements of [0, 1] that satisfies N fj = 1. ˜˜ ˜ j=1 Note: Analogous to a POVM in Quantum Theory. Can give more sophisticated measure-theoretic definition. M. S. Leifer Unentangled Bit Commitment
  43. 43. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Observables Definition An observable is a finite collection (f1 , f2 , . . . , fN ) of elements of [0, 1] that satisfies N fj = 1. ˜˜ ˜ j=1 Note: Analogous to a POVM in Quantum Theory. Can give more sophisticated measure-theoretic definition. M. S. Leifer Unentangled Bit Commitment
  44. 44. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Dynamics Definition The dynamical maps DB|A are a convex subset of the affine maps φ : V (ΩA )∗ → V (ΩB )∗ . ∀α, β ≥ 0, φ(αωA + µB ) = αφ(ωA ) + βφ(µB ) You might want to require other things, e.g. The identity is in DA|A . Maps can be composed. ∀f ∈ V (ΩA ), µB ∈ V (ΩB )∗ , φ(ωA ) = f (ωA )µB is in DB|A . M. S. Leifer Unentangled Bit Commitment
  45. 45. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Dynamics Definition The dynamical maps DB|A are a convex subset of the affine maps φ : V (ΩA )∗ → V (ΩB )∗ . ∀α, β ≥ 0, φ(αωA + µB ) = αφ(ωA ) + βφ(µB ) You might want to require other things, e.g. The identity is in DA|A . Maps can be composed. ∀f ∈ V (ΩA ), µB ∈ V (ΩB )∗ , φ(ωA ) = f (ωA )µB is in DB|A . M. S. Leifer Unentangled Bit Commitment
  46. 46. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Dynamics Definition The dynamical maps DB|A are a convex subset of the affine maps φ : V (ΩA )∗ → V (ΩB )∗ . ∀α, β ≥ 0, φ(αωA + µB ) = αφ(ωA ) + βφ(µB ) You might want to require other things, e.g. The identity is in DA|A . Maps can be composed. ∀f ∈ V (ΩA ), µB ∈ V (ΩB )∗ , φ(ωA ) = f (ωA )µB is in DB|A . M. S. Leifer Unentangled Bit Commitment
  47. 47. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Dynamics Definition The dynamical maps DB|A are a convex subset of the affine maps φ : V (ΩA )∗ → V (ΩB )∗ . ∀α, β ≥ 0, φ(αωA + µB ) = αφ(ωA ) + βφ(µB ) You might want to require other things, e.g. The identity is in DA|A . Maps can be composed. ∀f ∈ V (ΩA ), µB ∈ V (ΩB )∗ , φ(ωA ) = f (ωA )µB is in DB|A . M. S. Leifer Unentangled Bit Commitment
  48. 48. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Combining Systems: Tensor Products Given ΩA and ΩB , what is the joint space ΩAB ? We assume: A joint state must assign joint probabilities to ˜˜ ˜˜ fA ∈ [0A , 1B ], fB ∈ [0A , 1B ]. No-signaling. States are uniquely determined by probability assignments to pairs fA , fB . This does not give a unique tensor product, but a range of possibilities. Direct products: ωA ⊗ ωB (fA , fB ) = ωA (fA )ωB (fB ) M. S. Leifer Unentangled Bit Commitment
  49. 49. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Combining Systems: Tensor Products Given ΩA and ΩB , what is the joint space ΩAB ? We assume: A joint state must assign joint probabilities to ˜˜ ˜˜ fA ∈ [0A , 1B ], fB ∈ [0A , 1B ]. No-signaling. States are uniquely determined by probability assignments to pairs fA , fB . This does not give a unique tensor product, but a range of possibilities. Direct products: ωA ⊗ ωB (fA , fB ) = ωA (fA )ωB (fB ) M. S. Leifer Unentangled Bit Commitment
  50. 50. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Combining Systems: Tensor Products Given ΩA and ΩB , what is the joint space ΩAB ? We assume: A joint state must assign joint probabilities to ˜˜ ˜˜ fA ∈ [0A , 1B ], fB ∈ [0A , 1B ]. No-signaling. States are uniquely determined by probability assignments to pairs fA , fB . This does not give a unique tensor product, but a range of possibilities. Direct products: ωA ⊗ ωB (fA , fB ) = ωA (fA )ωB (fB ) M. S. Leifer Unentangled Bit Commitment
  51. 51. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Combining Systems: Tensor Products Given ΩA and ΩB , what is the joint space ΩAB ? We assume: A joint state must assign joint probabilities to ˜˜ ˜˜ fA ∈ [0A , 1B ], fB ∈ [0A , 1B ]. No-signaling. States are uniquely determined by probability assignments to pairs fA , fB . This does not give a unique tensor product, but a range of possibilities. Direct products: ωA ⊗ ωB (fA , fB ) = ωA (fA )ωB (fB ) M. S. Leifer Unentangled Bit Commitment
  52. 52. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Combining Systems: Tensor Products Given ΩA and ΩB , what is the joint space ΩAB ? We assume: A joint state must assign joint probabilities to ˜˜ ˜˜ fA ∈ [0A , 1B ], fB ∈ [0A , 1B ]. No-signaling. States are uniquely determined by probability assignments to pairs fA , fB . This does not give a unique tensor product, but a range of possibilities. Direct products: ωA ⊗ ωB (fA , fB ) = ωA (fA )ωB (fB ) M. S. Leifer Unentangled Bit Commitment
  53. 53. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Combining Systems: Tensor Products Given ΩA and ΩB , what is the joint space ΩAB ? We assume: A joint state must assign joint probabilities to ˜˜ ˜˜ fA ∈ [0A , 1B ], fB ∈ [0A , 1B ]. No-signaling. States are uniquely determined by probability assignments to pairs fA , fB . This does not give a unique tensor product, but a range of possibilities. Direct products: ωA ⊗ ωB (fA , fB ) = ωA (fA )ωB (fB ) M. S. Leifer Unentangled Bit Commitment
  54. 54. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Combining Systems: Tensor Products Definition Separable TP: V (ΩA )∗ ⊗sep V (ΩB )∗ = conv {ωA ⊗ ωB |ωA ∈ V (ΩA )∗ , ωB ∈ V (ΩB )∗ } Definition Maximal TP: V (ΩA )∗ ⊗max V (ΩB )∗ = (V (ΩA ) ⊗sep V (ΩB ))∗ Definition A tensor product V (ΩA )∗ ⊗ V (ΩB )∗ is a convex cone that satisfies V (ΩA )∗ ⊗sep V (ΩB )∗ ⊆ V (ΩA )∗ ⊗V (ΩB )∗ ⊆ V (ΩA )∗ ⊗max V (ΩB )∗ . M. S. Leifer Unentangled Bit Commitment
  55. 55. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Combining Systems: Tensor Products Definition Separable TP: V (ΩA )∗ ⊗sep V (ΩB )∗ = conv {ωA ⊗ ωB |ωA ∈ V (ΩA )∗ , ωB ∈ V (ΩB )∗ } Definition Maximal TP: V (ΩA )∗ ⊗max V (ΩB )∗ = (V (ΩA ) ⊗sep V (ΩB ))∗ Definition A tensor product V (ΩA )∗ ⊗ V (ΩB )∗ is a convex cone that satisfies V (ΩA )∗ ⊗sep V (ΩB )∗ ⊆ V (ΩA )∗ ⊗V (ΩB )∗ ⊆ V (ΩA )∗ ⊗max V (ΩB )∗ . M. S. Leifer Unentangled Bit Commitment
  56. 56. Introduction States Framework Effects Unentangled Bit Commitment States as vectors Broadcasting Observables Transformations → Affine maps Comparison to CBH Theorem Conclusions Tensor Products Combining Systems: Tensor Products Definition Separable TP: V (ΩA )∗ ⊗sep V (ΩB )∗ = conv {ωA ⊗ ωB |ωA ∈ V (ΩA )∗ , ωB ∈ V (ΩB )∗ } Definition Maximal TP: V (ΩA )∗ ⊗max V (ΩB )∗ = (V (ΩA ) ⊗sep V (ΩB ))∗ Definition A tensor product V (ΩA )∗ ⊗ V (ΩB )∗ is a convex cone that satisfies V (ΩA )∗ ⊗sep V (ΩB )∗ ⊆ V (ΩA )∗ ⊗V (ΩB )∗ ⊆ V (ΩA )∗ ⊗max V (ΩB )∗ . M. S. Leifer Unentangled Bit Commitment
  57. 57. Introduction Framework Unentangled Bit Commitment Distinguishability Broadcasting Broadcasting Comparison to CBH Theorem Conclusions Distinguishability Definition A set of states {ω1 , ω2 , . . . , ωN }, ωj ∈ Ω, is jointly distinguishable if ∃ an observable (f1 , f2 , . . . , fN ) s.t. fj (ωk ) = δjk . Fact The set of pure states of Ω is jointly distinguishable iff Ω is a simplex. M. S. Leifer Unentangled Bit Commitment
  58. 58. Introduction Framework Unentangled Bit Commitment Distinguishability Broadcasting Broadcasting Comparison to CBH Theorem Conclusions Distinguishability Definition A set of states {ω1 , ω2 , . . . , ωN }, ωj ∈ Ω, is jointly distinguishable if ∃ an observable (f1 , f2 , . . . , fN ) s.t. fj (ωk ) = δjk . Fact The set of pure states of Ω is jointly distinguishable iff Ω is a simplex. M. S. Leifer Unentangled Bit Commitment
  59. 59. Introduction Framework Unentangled Bit Commitment Distinguishability Broadcasting Broadcasting Comparison to CBH Theorem Conclusions Reduced States and Maps Definition Given a state vAB ∈ VA ⊗ VB , the marginal state on VA is defined by ˜ ∗ ∀fA ∈ VA , fA (vA ) = fA ⊗ 1B (ωAB ). Definition Given an affine map φBC|A : VA → VB ⊗ VC , the reduced map φ : VA → VB is defined by ˜ ∗ ∀fB ∈ VB , vA ∈ VA , fB (φB|A (vA )) = fB ⊗ 1C φBC|A (vA ) . M. S. Leifer Unentangled Bit Commitment
  60. 60. Introduction Framework Unentangled Bit Commitment Distinguishability Broadcasting Broadcasting Comparison to CBH Theorem Conclusions Reduced States and Maps Definition Given a state vAB ∈ VA ⊗ VB , the marginal state on VA is defined by ˜ ∗ ∀fA ∈ VA , fA (vA ) = fA ⊗ 1B (ωAB ). Definition Given an affine map φBC|A : VA → VB ⊗ VC , the reduced map φ : VA → VB is defined by ˜ ∗ ∀fB ∈ VB , vA ∈ VA , fB (φB|A (vA )) = fB ⊗ 1C φBC|A (vA ) . M. S. Leifer Unentangled Bit Commitment
  61. 61. Introduction Framework Unentangled Bit Commitment Distinguishability Broadcasting Broadcasting Comparison to CBH Theorem Conclusions Broadcasting Definition A state ω ∈ Ω is broadcast by a NPA map φA A |A : VA → VA ⊗ VA if φA |A (ω) = φA |A (ω) = ω. Cloning is a special case where outputs must be uncorrelated. Definition A set of states is co-broadcastable if there exists an NPA map that broadcasts all of them. M. S. Leifer Unentangled Bit Commitment
  62. 62. Introduction Framework Unentangled Bit Commitment Distinguishability Broadcasting Broadcasting Comparison to CBH Theorem Conclusions Broadcasting Definition A state ω ∈ Ω is broadcast by a NPA map φA A |A : VA → VA ⊗ VA if φA |A (ω) = φA |A (ω) = ω. Cloning is a special case where outputs must be uncorrelated. Definition A set of states is co-broadcastable if there exists an NPA map that broadcasts all of them. M. S. Leifer Unentangled Bit Commitment
  63. 63. Introduction Framework Unentangled Bit Commitment Distinguishability Broadcasting Broadcasting Comparison to CBH Theorem Conclusions Broadcasting Definition A state ω ∈ Ω is broadcast by a NPA map φA A |A : VA → VA ⊗ VA if φA |A (ω) = φA |A (ω) = ω. Cloning is a special case where outputs must be uncorrelated. Definition A set of states is co-broadcastable if there exists an NPA map that broadcasts all of them. M. S. Leifer Unentangled Bit Commitment
  64. 64. Introduction Framework Unentangled Bit Commitment Distinguishability Broadcasting Broadcasting Comparison to CBH Theorem Conclusions The No-Broadcasting Theorem Theorem A set of states is co-broadcastable iff it is contained in a simplex that has jointly distinguishable vertices. Quantum theory: states must commute. Universal broadcasting only possible in classical theories. Theorem The set of states broadcast by any affine map is a simplex that has jointly distinguishable vertices. M. S. Leifer Unentangled Bit Commitment
  65. 65. Introduction Framework Unentangled Bit Commitment Distinguishability Broadcasting Broadcasting Comparison to CBH Theorem Conclusions The No-Broadcasting Theorem Theorem A set of states is co-broadcastable iff it is contained in a simplex that has jointly distinguishable vertices. Quantum theory: states must commute. Universal broadcasting only possible in classical theories. Theorem The set of states broadcast by any affine map is a simplex that has jointly distinguishable vertices. M. S. Leifer Unentangled Bit Commitment
  66. 66. Introduction Framework Unentangled Bit Commitment Distinguishability Broadcasting Broadcasting Comparison to CBH Theorem Conclusions The No-Broadcasting Theorem Theorem A set of states is co-broadcastable iff it is contained in a simplex that has jointly distinguishable vertices. Quantum theory: states must commute. Universal broadcasting only possible in classical theories. Theorem The set of states broadcast by any affine map is a simplex that has jointly distinguishable vertices. M. S. Leifer Unentangled Bit Commitment
  67. 67. Introduction Framework Unentangled Bit Commitment Distinguishability Broadcasting Broadcasting Comparison to CBH Theorem Conclusions The No-Broadcasting Theorem distinguishable Ω co-broadcastable M. S. Leifer Unentangled Bit Commitment
  68. 68. Introduction Framework Unentangled Bit Commitment Broadcasting Comparison to CBH Theorem Conclusions Comparison to CBH Theorem Like CBH we have: No broadcasting ⇒ State spaces of individual systems are nonclassical. No bit commitment ⇒ Entangled states must exist. Unlike CBH: No signaling has become a framework assumption. Postulates are genuinely independent. We are not particularly close to quantum theory. M. S. Leifer Unentangled Bit Commitment
  69. 69. Introduction Framework Unentangled Bit Commitment Broadcasting Comparison to CBH Theorem Conclusions Comparison to CBH Theorem Like CBH we have: No broadcasting ⇒ State spaces of individual systems are nonclassical. No bit commitment ⇒ Entangled states must exist. Unlike CBH: No signaling has become a framework assumption. Postulates are genuinely independent. We are not particularly close to quantum theory. M. S. Leifer Unentangled Bit Commitment
  70. 70. Introduction Framework Unentangled Bit Commitment Broadcasting Comparison to CBH Theorem Conclusions Comparison to CBH Theorem Like CBH we have: No broadcasting ⇒ State spaces of individual systems are nonclassical. No bit commitment ⇒ Entangled states must exist. Unlike CBH: No signaling has become a framework assumption. Postulates are genuinely independent. We are not particularly close to quantum theory. M. S. Leifer Unentangled Bit Commitment
  71. 71. Introduction Framework Unentangled Bit Commitment Broadcasting Comparison to CBH Theorem Conclusions Comparison to CBH Theorem Like CBH we have: No broadcasting ⇒ State spaces of individual systems are nonclassical. No bit commitment ⇒ Entangled states must exist. Unlike CBH: No signaling has become a framework assumption. Postulates are genuinely independent. We are not particularly close to quantum theory. M. S. Leifer Unentangled Bit Commitment
  72. 72. Introduction Framework Unentangled Bit Commitment Broadcasting Comparison to CBH Theorem Conclusions Comparison to CBH Theorem Like CBH we have: No broadcasting ⇒ State spaces of individual systems are nonclassical. No bit commitment ⇒ Entangled states must exist. Unlike CBH: No signaling has become a framework assumption. Postulates are genuinely independent. We are not particularly close to quantum theory. M. S. Leifer Unentangled Bit Commitment
  73. 73. Introduction Framework Open Questions Unentangled Bit Commitment Acknowledgments Broadcasting References Comparison to CBH Theorem Conclusions Open Questions Are all qualitative QCrypto results, e.g. key distribution, generic? Can other qinfo constraints, e.g. teleportation, get us closer to quantum theory? Is bit commitment possible in any theories with entanglement? M. S. Leifer Unentangled Bit Commitment
  74. 74. Introduction Framework Open Questions Unentangled Bit Commitment Acknowledgments Broadcasting References Comparison to CBH Theorem Conclusions Open Questions Are all qualitative QCrypto results, e.g. key distribution, generic? Can other qinfo constraints, e.g. teleportation, get us closer to quantum theory? Is bit commitment possible in any theories with entanglement? M. S. Leifer Unentangled Bit Commitment
  75. 75. Introduction Framework Open Questions Unentangled Bit Commitment Acknowledgments Broadcasting References Comparison to CBH Theorem Conclusions Open Questions Are all qualitative QCrypto results, e.g. key distribution, generic? Can other qinfo constraints, e.g. teleportation, get us closer to quantum theory? Is bit commitment possible in any theories with entanglement? M. S. Leifer Unentangled Bit Commitment
  76. 76. Introduction Framework Open Questions Unentangled Bit Commitment Acknowledgments Broadcasting References Comparison to CBH Theorem Conclusions Conjecture Weak version: If the set of joint probabilities that Alice and Bob can obtain in a “prepare and measure” setup is the same as when those they can obtain from making measurements on joint states then bit commitment is impossible Strong version: In all other theories there is a secure bit commitment protocol. Note: Applies to C ∗ -theories, i.e. classical and quantum, due to Choi-Jamiołkowski isomorphism, but it’s weaker than this. But not unentangled nonclassical theories. Implies some sort of isomorphism between ⊗ and DB|A . M. S. Leifer Unentangled Bit Commitment
  77. 77. Introduction Framework Open Questions Unentangled Bit Commitment Acknowledgments Broadcasting References Comparison to CBH Theorem Conclusions Conjecture Weak version: If the set of joint probabilities that Alice and Bob can obtain in a “prepare and measure” setup is the same as when those they can obtain from making measurements on joint states then bit commitment is impossible Strong version: In all other theories there is a secure bit commitment protocol. Note: Applies to C ∗ -theories, i.e. classical and quantum, due to Choi-Jamiołkowski isomorphism, but it’s weaker than this. But not unentangled nonclassical theories. Implies some sort of isomorphism between ⊗ and DB|A . M. S. Leifer Unentangled Bit Commitment
  78. 78. Introduction Framework Open Questions Unentangled Bit Commitment Acknowledgments Broadcasting References Comparison to CBH Theorem Conclusions Conjecture Weak version: If the set of joint probabilities that Alice and Bob can obtain in a “prepare and measure” setup is the same as when those they can obtain from making measurements on joint states then bit commitment is impossible Strong version: In all other theories there is a secure bit commitment protocol. Note: Applies to C ∗ -theories, i.e. classical and quantum, due to Choi-Jamiołkowski isomorphism, but it’s weaker than this. But not unentangled nonclassical theories. Implies some sort of isomorphism between ⊗ and DB|A . M. S. Leifer Unentangled Bit Commitment
  79. 79. Introduction Framework Open Questions Unentangled Bit Commitment Acknowledgments Broadcasting References Comparison to CBH Theorem Conclusions Conjecture Weak version: If the set of joint probabilities that Alice and Bob can obtain in a “prepare and measure” setup is the same as when those they can obtain from making measurements on joint states then bit commitment is impossible Strong version: In all other theories there is a secure bit commitment protocol. Note: Applies to C ∗ -theories, i.e. classical and quantum, due to Choi-Jamiołkowski isomorphism, but it’s weaker than this. But not unentangled nonclassical theories. Implies some sort of isomorphism between ⊗ and DB|A . M. S. Leifer Unentangled Bit Commitment
  80. 80. Introduction Framework Open Questions Unentangled Bit Commitment Acknowledgments Broadcasting References Comparison to CBH Theorem Conclusions Conjecture Weak version: If the set of joint probabilities that Alice and Bob can obtain in a “prepare and measure” setup is the same as when those they can obtain from making measurements on joint states then bit commitment is impossible Strong version: In all other theories there is a secure bit commitment protocol. Note: Applies to C ∗ -theories, i.e. classical and quantum, due to Choi-Jamiołkowski isomorphism, but it’s weaker than this. But not unentangled nonclassical theories. Implies some sort of isomorphism between ⊗ and DB|A . M. S. Leifer Unentangled Bit Commitment
  81. 81. Introduction Framework Open Questions Unentangled Bit Commitment Acknowledgments Broadcasting References Comparison to CBH Theorem Conclusions Acknowledgments This work is supported by: The Foundational Questions Institute (http://www.fqxi.org) MITACS (http://www.mitacs.math.ca) NSERC (http://nserc.ca/) The Province of Ontario: ORDCF/MRI M. S. Leifer Unentangled Bit Commitment
  82. 82. Introduction Framework Open Questions Unentangled Bit Commitment Acknowledgments Broadcasting References Comparison to CBH Theorem Conclusions References H. Barnum, O. Dahlsten, M. Leifer and B. Toner, “Nonclassicality without entanglement enables bit commitment”, arXiv soon. H. Barnum, J. Barrett, M. Leifer and A. Wilce, “Generalized No Broadcasting Theorem”, arXiv:0707.0620. H. Barnum, J. Barrett, M. Leifer and A. Wilce, “Cloning and Broadcasting in Generic Probabilistic Theories”, quant-ph/0611295. M. S. Leifer Unentangled Bit Commitment

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