Deriving the Qubit from Entropy Principles Adam Brandenburger and Pierfrancesco La Mura March 17, 2015 The Mathematics of Quantum Mechanics Image: Shutterstock And Beyond From: To: “I don’t believe in Hilbert space anymore.” Quoted by Birkhoff, G., in Dilworth, R. (ed.), Lattice Theory, Proceedings of the Second Symposium in Pure Mathematics of the American Mathematical Society, AMS, 1961 Quantum Quest Can we find physically intelligible principles from which the mathematical structure of quantum mechanics emerges? Image: Nature, 09/12/13, p.154 Axioms for QM “The special theory of relativity, we know, can be deduced in its entirety from two axioms: the equivalence of inertial reference frames, and the constancy of the speed of light. Aharanov {unpublished lecture notes} has proposed such a logical structure for quantum theory.” Popescu, S., and D. Rohrlich, “Quantum Nonlocality as an Axiom,” Foundations of Physics, 24, 1994, 379-385 Heisenberg Uncertainty Principle ∆Q · ∆P ≥ ~/2 We begin with the uncertainty principle and turn it into an axiom Figure from Busch, P., P. Lahti, and R. Werner, “Proof of Heisenberg’s Error-Disturbance Relation,” Physical Review Letters, 111, 2013, 160405 Empirical Model z-direction Outcomes 0 1 Frequencies fz 1 − fz y-direction x-direction Outcomes 0 1 Frequencies fx 1 − fx Outcomes 0 1 Frequencies fy 1 − fy Phase Space Outcome in x-direction Outcome in y-direction Outcome in z-direction q000 0 0 0 q001 0 0 1 q010 0 1 0 q011 0 1 1 q100 1 0 0 q101 1 0 1 q110 1 1 0 q111 1 1 1 q000 + q001 + q010 + q011 = fx q000 + q001 + q100 + q101 = fy q000 + q010 + q100 + q110 = fz Negative Probabilities A phase-space model is a local hidden-variable model (satisfying free choice of measurements) By Bell’s Theorem, some classical feature will need to be relaxed in order to represent quantum systems Abramsky and Brandenburger [2011] prove that the introduction of negative probabilities on phase space makes it possible to reproduce precisely the set of empirical models that obey the no-signaling condition Of course, we insist that all observable events have non-negative probabilities Really?? Or . . . “Negative energies and probabilities should not be considered as nonsense. They are well-defined concepts mathematically, like a negative of money.” – P. Dirac, Proc. Roy. Soc. London A, 1942 Image: http://johncarlosbaez.wordpress.com/2013/07/19/negative-probabilities/ R´enyi Entropy R´enyi entropy is a family of functions measuring information (or uncertainty), axiomatized via additivity over statistically independent systems: Hα (q) = n P 1 qiα ) if α 6= 1 log ( − α−1 2 n i=1 P qi log2 qi − if α = 1 i=1 Requiring a well-behaved entropy function with signed probability distributions yields the following family, for k = 1, 2, 3, . . .: n X 1 H2k (q) = − qi2k ) log2 ( 2k − 1 i=1 Uncertainty Principle We introduce an Uncertainty Principle expressed in entropic terms and formulated in phase space Because R´enyi entropy is a family of measures of information, we state our axiom as holding independently of the value of k (the values of the lower bounds β2k will be determined by the next axiom) An empirical model defined by frequencies fx , fy , fz is allowable iff for every k = 1, 2, 3, . . ., there is a phase-space probability distribution q that represents it and that satisfies H2k (q) ≥ β2k Unbiasedness Principle A set of measurements on a system is called mutually unbiased, or complementary, if complete certainty of the measured value of the outcome of one of them implies maximal uncertainty about the outcomes of the others z-direction Outcomes 0 1 Frequencies 1/2 1/2 y-direction Outcomes Frequencies 0 1 1/2 1/2 x-direction Outcomes 0 1 Frequencies 1 0 Lemma Under the Unbiasedness Principle, the lower bounds in the Uncertainty Principle satisfy β2k = 2 for all k Interim Summary We want to find all empirical probabilities fx , fy , fz that satisfy the Uncertainty and Unbiasedness Principles — i.e., that satisfy the requirement that, for each k, there is a phase-space probability distribution q that represents fx , fy , fz and that has R´enyi entropy H2k (q) ≥ 2 Frequency of outcome 0 Frequency of outcome 1 Measure in x-direction ½(1 + rx) ½(1 − rx) Measure in y-direction ½(1 + ry) ½(1 − ry) Measure in z-direction ½(1 + rz) ½(1 − rz) Re-parameterized Empirical Model Main Mathematical Result Let R2k = {(rx , ry , rz ) ∈ [−1, 1]3 : ∃ q with H2k (q) ≥ 2 which represents (rx , ry , rz )} We want to find ∩k R2k Note that H2k (q) is not monotone decreasing (or increasing) when q is allowed to be a signed probability distribution! Theorem The sets R2k are increasing (set-wise) in k Duality Norm minimization problem: min ||q||2k q∈R8 subject to Aq = b where 1 1 A= 1 1 1 1 0 1 1 0 1 1 1 0 0 1 0 1 1 1 0 1 0 1 0 0 1 1 0 0 , 0 1 (1 + rx )/2 (1 + ry )/2 b= (1 + rz )/2 1 Dual problem: max b T x x∈R4 subject to ||AT x|| 2k 2k−1 ≤1 The Problem in Dual Space b zk yk zk+1 wk yk+1 0 Ck+1 Ck Ck = {x ∈ R4 : ||AT x|| 2k 2k−1 ≤ 1}, Ck+1 = {x ∈ R4 : ||AT x|| 2k+2 ≤ 1} The proof works by showing that ||z k+1 ||2 1 1 ≤ ( ) k(k+1) k 2 ||z ||2 2k+1 The Bloch Sphere +1 +1 −1 −1 The set R2 = ∩k R2k is the unit ball in R3 : R2 = {(rx , ry , rz ) ∈ [−1, 1]3 : rx2 + ry2 + rz2 ≤ 1} Comments I Our approach yields the single qubit — we believe it can be extended to n-qubit systems I We do not claim that our axioms are self-evident — indeed they are mysterious at the macroscopic level I But our axioms are physically intelligible — and evidently true of microscopic systems
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