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**Wojciech Hubert Zurek Theory Division, Los Alamos**

Quantum Physics Relative States and the Environment (Everett ‘57) Wojciech Hubert Zurek Theory Division, Los Alamos “BEYOND DECOHERENCE” The general goal -- whether & how can one understand the Universe around us in terms of quantum theory. Colloquium -- a bit less detailed, “general audience”. Will try to be as rigorous as time permits. Invite questions -- will stop twice & ask for q’s before the end

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**Textbook Quantum Theory**

0. State of a composite system is a vector in the tensor product of the constituent Hilbert spaces. (“Complexity”) Quantum states of a system are represented by vectors in its Hilbert space. (“Quantum Superposition Principle”) Evolutions are unitary (e.g. generated by Schroedinger equation). (“Unitarity”) Immediate repetition of a measurement yields the same outcome. (“Predictability”) Outcomes restricted to orthonormal states {|sk>} (eigenstates of the measured observable). Just one outcome is seen each time. (“Collapse Postulate”) Probability of finding an outcome |sk> given states |ƒ> is pk=|<sk|ƒ>|2. (“Born’s Rule”) Note: 3 “on the border”. Not controversial: Need it to even have a concept of a state. 4(&5?) outside of “just quantum” -- require classical component to the Universe. Bohr, Dirac, “Copenhagen” -- 4&5 require “classical apparatus”, etc. Everett: Relative states -- no need for explicit collapse! But need a preferred basis!!!

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**EINSELECTION*, POINTER BASIS,**

AND DECOHERENCE S E REDUCED DENSITY MATRIX EINSELECTION* leads to POINTER STATES (same states appear on the diagonal of for times long compared to the decoherence time; pointer states are effectively classical!) Pointer states left unperturbed by the “environmental monitoring”. *Environment INduced superSELECTION

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**DECOHERENCE AND EINSELECTION**

Thesis: Quantum theory can explain emergence of the classical. Principle of superposition loses its validity in “open” systems, that is, systems interacting with their environments. Decoherence restricts stable states (states that can persist, and, therefore, “exist”) to the exceptional… Pointer states that exist or evolve predictably in spite of the immersion of the system in the environment. Predictability sieve can be used to ‘sift’ through the Hilbert space of any open quantum system in search of these pointer states. EINSELECTION (or Environment INduced superSELECTION) is the process of selection of these preferred pointer states. For macroscopic systems, decoherence and einselection can be very effective, enforcing ban on Schroedinger cats. Einselection enforces an effective border that divides quantum from classical, making a point of view similar to Bohr’s Copenhagen Interpretation possible, although starting from a rather different standpoint (i. e., no ab initio classical domain of the universe). (Zeh, Joos, Paz, Caldeira, Leggett, Kiefer, Gell-Mann, Hartle, Omnes, Dalvit, Dziarmaga, Cucchietti … Haroche, Raimond, Brune, Zeilinger, Arndt, Hasselbach…)

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Goal Justify axioms 4&5 using the noncontroversial 0-3. PLAN OF THE TALK: Why are the measurement outcomes limited to an orthogonal subset of all the possible states in the Hilbert states? (as in “Collapse”) Why does “Born’s rule” yield probabilities? How can “objective classical reality” -- states we can find out -- arise from the fragile quantum states that are perturbed by measurements? (“Quantum Darwinism”) Cannot justify “real” collapse -- inconsistent with 1&2. But can justify “most of what’s needed”.

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**States that can survive “being found out” intact must be orthogonal.**

Proof: measurement is an information transfer from a quantum system S to a quantum apparatus A. So, for any two possible repeatable (predictable) (Axiom 3) outcome states of the same measurement it must be true that: NOTE: IN CONTRAST WITH DECOHERENCE, PROOF DOESN’T RELY ON BORN’S RULE! Cannot “find out” quantum states as a result! Connection with cloning! By unitarity (Axiom 2) scalar product of the total (S+A) state before and after must be the same. So: But So either (measurement was not successful) or QED!!!!

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**Consequences and extensions**

Derivation of the key to Collapse Postulate from Axioms 1-3: explains why in general one cannot “find out” preexisting states. Implies that observables are Hermitean (given an extra assumption that eigenvalues are real). Proof similar to “no cloning theorem” -- information about preexisting states cannot be “found out” -- passed on (Cloning -- making a “perfect copy”.) Proof can be extended to the case when the apparatus is initially in a mixed state. Axiom 3 -- predictability -- is the key to the proof! Information transfer need not be due to a deliberate measurement: any information transfer that does not perturb outcome states will have to abide by this rule: Pointer states, predictability sieve, and DECOHERENCE. ? Axiom 5 not used!!! “preferred states” emerge -- how does this compare with decoherence?

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Plan Derive controversial axioms 4&5 from the noncontroversial 0-3. Understand emergence of “objective classical reality” -- how real states that can be found out by us arise from quantum substrate. Why the measurement outcomes are limited to an orthogonal subset of all the possible states in the Hilbert states? Why does “Born’s rule” yield probabilities? How can “objective classical reality” -- states we can find out -- arise from the fragile quantum states that are perturbed by measurements? (“Quantum Darwinism”) WE HAVE “EVENTS”!

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We have a PHYSICAL argument why probabilities of the outcome states have same Schmidt coeff’s have the same probabilities. This is an OBJECTIVE property of a global entangled pure state!!!!! ENVARIANCE does not work classically!!!

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**ENVARIANCE (Entanglement-Assisted Invariance)**

DEFINITION: Consider a composite quantum object consisting of system S and environment E. When the combined state is transformed by: but can be “untransformed” by acting solely on E, that is, if there exists: Symmtery!!! (“ASSISTED SYMMETRY”) then is ENVARIANT with respect to Envariance is a property of and the joint state of two systems, S & E .

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**ENTANGLED STATE AS AN EXAMPLE OF ENVARIANCE:**

Schmidt decomposition: Above Schmidt states are orthonormal and complex. Lemma 1: Unitary transformations with Schmidt eigenstates: Local phase of Schmidt coeff. has no physical significance! Explain (“domesticate”) Schmidt basis / decomposition. leave envariant. Proof: LOCALLY, SCHMIDT PHASES DO NOT MATTER: DECOHERENCE!!!

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**PHASE ENVARIANCE THEOREM**

Fact 1: Unitary transformations must act on the system to alter its state (if they act only somewhere else, system is not effected). Fact 2: The state of the system is all that is necessary/available to predict measurement outcomes (including their probabilities). Fact 3: A state of the composite system is all that is needed/available to determine the state of the system. Moreover, “entanglement happens”: THEOREM 1: State (and probabilities) of S alone can depend only on the absolute values of Schmidt coefficients , and not on their phases. Phases lose local physical significance …. Decoherence!!! Proof: Phases of can be changed by acting on S alone. But the state of the whole can be restored by acting only on E. So change of phases of Schmidt coefficients could not have affected S! QED. By phase envariance, { } must provide a complete local description of the system alone. Same info as reduced density matrix!!!

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**Envariance of entangled states: the case of equal coefficients**

In this case ANY orthonormal basis is Schmidt. In particular, in the Hilbert subspace spanned by any two { } one can define a Hadamard basis; This can be used to generate ‘new kind’of envariant transformations: A SWAP: Can be ‘undone’ by the COUNTERSWAP: LEMMA 3: Swaps of states are envariant when Schmidt coefficients have same absolute value.

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**Probability of envariantly swappable states**

By the Phase Envariance Theorem the set of pairs provides a complete description of S. But all are equal. With additional assumption about probabilities, can prove THEOREM 2: Probabilities of envariantly swappable states are equal. “Pedantic assumption”; when states get swapped, so do probabilitites; When the state of the system does not change under any unitary in a part of its Hilbert space, probabilities of any set of basis states are equal. (c) Because there is one-to-one correlation between Therefore, by normalization:

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**Probabilities from envariance**

Symmetries can reflect ignorance Probabilities from envariance (Environment-assisted iNVARIANCE) follows! Note: Swaps do change unentangled states ! Phases matter! IS ORTHOGONAL TO

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**Special case with unequal coefficients**

Consider system S with two states The environment E has three states and An auxilliary environment E’ interacts with E so that: States have equal coefficients. Therefore, Each of them has probability of 1/3. Consequently: p(0) = p(0,0)+p(0,1) = 2/3, and p(2) = 1/3. ….. BORN’s RULE!!! no need to assume additivity! (p(0)=1-p(2))!

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**Probabilities from Envariance**

The case of commensurate probabilities: Attach the auxiliary “counter” environment C: THEOREM 3: The case with commensurate probabilities can be reduced to the case with equal probabilities. BORN’s RULE follows: General case -- by continuity. QED.

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**ENVARIANCE* -- SUMMARY**

New symmetry - ENVARIANCE - of joint states of quantum systems. It is related to causality. In quantum physics perfect knowledge of the whole may imply complete ignorance of a part. 3. BORN’s RULE follows as a consequence of envariance. 4. Relative frequency interpretation of probabilities naturally follows. 5. Envariance supplies a new foundation for environment - induced superselection, decoherence, quantum statistical physics, etc., by justifying the form and interpretation of reduced density matrices. From now on, we can use the usual tools of decoherence (reduced density matrices, trace, etc.). *WHZ, PRL 90, ; RMP 75, 715 (2003); PRA 71, (2005).

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Plan Derive controversial axioms 4&5 from the noncontroversial 0-3. Understand emergence of “objective classical reality” -- how real states that can be found out by us arise from quantum substrate. Why the measurement outcomes are limited to an orthogonal subset of all the possible states in the Hilbert states? Why does “Born’s rule” yield probabilities? How can “objective classical reality” -- states we can find out -- arise from the fragile quantum states that are perturbed by measurements? (“Quantum Darwinism”)

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Quantum Darwinism The imprint left by the system S in the environment E is the cause of decoherence The focus of decoherence is the information that is left in S in spite of E. (“reduced density matrix” of S) Quantum Darwinism is focused on the information about S that can be found out indirectly from E !!! How many copies of the information about S can be extracted from E? (Redundancy) What is this information about? (i.e, what observable of S gets redundantly imprinted in E?) Why does this matter? Robin Blume-Kohout (Cal Tech - IQI) Harold Ollivier (Perimeter Institute) David Poulin (Cal Tech - IQI)

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