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PS-09 Quantum Measurement Theory on a Half Line Yutaka Shikano Department of Physics, Tokyo Institute of Technology Collaborator: Akio Hosoya Y. Shikano and A. Hosoya, in preparation Outline and Aim What is Quantum Information? What is Measurement? (e.g. Measuring Process) What is Covariant Measurement? Comments on the Momentum on a Half Line Optimal Covariant Measurement Model Why need we consider Quantum Measurement? Why need we consider the Half Line system? (Details: Y. Shikano and A. Hosoya, in preparation) 8/7/2007 The 52nd Condensed Matter Physics Summer School at Wakayama 2 What is Quantum Information? Operational Processes in the Quantum System Similar to Information Process Preparation Initial Conditions My Research Field Output Data Object Solve the Schroedinger Equations. 8/7/2007 Measurement What information do we obtain from this result? The 52nd Condensed Matter Physics Summer School at Wakayama 3 Aim of Quantum Information Solve the Schroedinger Equation. Understand how to obtain Information from Quantum System. How much information can we get? What method can we obtain information optimally? Question: Is the essence of quantum mechanics the operational concept? 8/7/2007 The 52nd Condensed Matter Physics Summer School at Wakayama 4 Axioms of Quantum Mechanics 1. Definition of state, state space, observable Observable is defined as the self-adjoint operator since the operator has real spectrums. 2. Time evolution of state (Schroedinger Equation) 3. Born’s probablistic formula 4. Definition of the combined system 8/7/2007 The 52nd Condensed Matter Physics Summer School at Wakayama 5 What is Measurement? Measured System Probe System t=0 3 t = ⊿t t= ⊿t+⊿T time 8/7/2007 1 We can evaluate the “measurement” value t = 0 on the measured system from the measurement value t = ⊿t. This process is called magnification or observation and is different from measurement. The 52nd Condensed Matter Physics Summer School at Wakayama Interaction between the measured system and probe system. 2 We obtain the measurement value on the probe system. We obtain the macroscopic value. (e.g. Photomultiplier) 6 To describe the Measuring Process We have to know the follows to describe the measuring process physically. 1. Hamiltonian on the combined system between the measured system and probe system. 2. Evolution operator on the combined system from the Hamiltonian 3. Measuring time of the measuring process. 4. Measurement value of the probe system. 8/7/2007 The 52nd Condensed Matter Physics Summer School at Wakayama 7 What is Covariant Measurement? For any bases “0” on the space, Measured System 3 0 Probe System 1 Shifted 3 0 measurement value measurement value 0 3 This condition is satisfied by the ideal measuring device. Remark The measure on the measured system, that is POVM, is constrained. 2 Shifted as same!! 8/7/2007 The 52nd Condensed Matter Physics Summer School at Wakayama 8 Formulation of the Covariant Measurement Definition transformation of the momentum. Property of the covariant measurement. Using the Born formula. (Axiom 3) (Holevo 1978,1979,1982) 8/7/2007 The 52nd Condensed Matter Physics Summer School at Wakayama 9 Comments on the Momentum on a Half Line NOT SAME 8/7/2007 The 52nd Condensed Matter Physics Summer School at Wakayama 10 Lesson from this example. is symmetric but not self-adjoint operator. This means that the momentum on a half line is NOT observable. Lesson: When we consider the infinite dimensional Hilbert space, e.g. momentum and position in quantum mechanics, we have to check the domain of the operator. How to classify the operator. Deficiency Theorem 8/7/2007 (Weyl 1910, von Neumann 1929, Bonneau et al. 2001) The 52nd Condensed Matter Physics Summer School at Wakayama 11 Prescription: Naimark Extension Naimark Extension Theorem: When we extend the domain of any symmetric operators, the symmetric operators become the self-adjoint operator on the extended domain. 3 position 0 1 Copy 0 8/7/2007 Combined 2 Inversion position position 0 3 position The 52nd Condensed Matter Physics Summer School at Wakayama 0 12 Optimal Covariant Measurement Model Aim: We find the Hamiltonian to satisfy the optimal covariant POVM to minimize the variance between the measurement value on the probe system at t = ⊿t and the evaluated “measurement” value on the measured system at t = 0. Optimal Covariant POVM 8/7/2007 The 52nd Condensed Matter Physics Summer School at Wakayama 13 Model Hamiltonian Assume that the measured system alone is coupled to the bulk system at zero temperature. Evolution operator 8/7/2007 The 52nd Condensed Matter Physics Summer School at Wakayama 14 Remarks Following iεprescription, we can obtain the optimal covariant POVM. Bulk System t=0 time 8/7/2007 Probe System Instantaneous interaction T=0 Energy Dissipates! t=∞ Measured System T=0 Precise evaluation from the momentum conservation. Controllable Ground State Assumption: Ground Energy = 0 The 52nd Condensed Matter Physics Summer School at Wakayama Measurable 15 Concluding Remarks We have explained the overview of quantum information and the measurement theory of quantum system. We have shown the strange example of the half line system. We have obtained the optimal covariant measurement model. Thank you for your attention although my poster presentation may be out of place in this session. 8/7/2007 The 52nd Condensed Matter Physics Summer School at Wakayama 16 References Y. Shikano and A. Hosoya, in preparation J. von Neumann, Mathematische Grundlagen der Quantmechanik (Springer, Berlin, 1932), [ Mathematical Foundations of Quantum Mechanics (Princeton University Press, Princeton, 1955) ] A. S. Holevo, Rep. Math. Phys. 13, 379-399 (1978) A. S. Holevo, Rep. Math. Phys. 16, 385-400 (1979) A. S. Holevo, Probabilistic and Statistical Aspects of Quantum Theory (North-Holland, Amsterdam, 1982) H. Weyl, Math. Ann. 68, 220-269 (1910) J. von Neumann, Math. Ann. 102, 49-131 (1929) G. Bonneau et al. Am. J. Phys. 69, 322-331 (2001) 8/7/2007 The 52nd Condensed Matter Physics Summer School at Wakayama 17