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Quantum-limited measurements: One physicist’s crooked path from quantum optics to quantum information I.Introduction II.Squeezed states and optical interferometry III.Ramsey interferometry and cat states IV.Quantum information perspective V.Beyond the Heisenberg limit Carlton M. Caves Center for Quantum Information and Control, University of New Mexico School of Mathematics and Physics, University of Queensland http://info.phys.unm.edu/~caves Collaborators: E. Bagan, S. Boixo, A. Datta, S. Flammia, M. J. Davis, JM Geremia, G. J. Milburn, A Shaji, A. Tacla, M. J. Woolley. Center for Quantum Information and Control

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I. Introduction View from Cape Hauy Tasman Peninsula Tasmania

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A new way of thinking Quantum information science Computer science Computational complexity depends on physical law. Old physics Quantum mechanics as nag. The uncertainty principle restricts what can be done. New physics Quantum mechanics as liberator. What can be accomplished with quantum systems that can’t be done in a classical world? Explore what can be done with quantum systems, instead of being satisfied with what Nature hands us. Quantum engineering

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Metrology Taking the measure of things The heart of physics Old physics Quantum mechanics as nag. The uncertainty principle restricts what can be done. New physics Quantum mechanics as liberator. Explore what can be done with quantum systems, instead of being satisfied with what Nature hands us. Quantum engineering Old conflict in new guise

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II. Squeezed states and optical interferometry Oljeto Wash Southern Utah

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(Absurdly) high-precision interferometry Laser Interferometer Gravitational Observatory (LIGO) Hanford, Washington Livingston, Louisiana 4 km The LIGO Collaboration, Rep. Prog. Phys. 72, 076901 (2009).

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Laser Interferometer Gravitational Observatory (LIGO) Hanford, Washington Livingston, Louisiana 4 km Initial LIGO High-power, Fabry- Perot-cavity (multipass), power- recycled interferometers (Absurdly) high-precision interferometry

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Laser Interferometer Gravitational Observatory (LIGO) Hanford, Washington Livingston, Louisiana 4 km Advanced LIGO High-power, Fabry- Perot-cavity (multipass), power- and signal-recycled, squeezed-light interferometers (Absurdly) high-precision interferometry

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Mach-Zender interferometer C. M. Caves, PRD 23, 1693 (1981).

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Squeezed states of light

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G. Breitenbach, S. Schiller, and J. Mlynek, Nature 387, 471 (1997). Groups at ANU, Hannover, and Tokyo continue to push for greater squeezing at audio frequencies for use in Advanced LIGO, VIRGO, and GEO. Squeezed states of light Squeezing by a factor of about 3.5

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Quantum limits on interferometric phase measurements Quantum Noise Limit (Shot-Noise Limit) Heisenberg Limit As much power in the squeezed light as in the main beam

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III. Ramsey interferometry and cat states Truchas from East Pecos Baldy Sangre de Cristo Range Northern New Mexico

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Ramsey interferometry N independent “atoms” Frequency measurement Time measurement Clock synchronization

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Cat-state Ramsey interferometry J. J. Bollinger, W. M. Itano, D. J. Wineland, and D. J. Heinzen, Phys. Rev. A 54, R4649 (1996). Fringe pattern with period 2π/N N cat-state atoms

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Optical interferometryRamsey interferometry Quantum Noise Limit (Shot-Noise Limit) Heisenberg Limit Something’s going on here.

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Optical interferometryRamsey interferometry Entanglement? Between arms Between atoms (wave entanglement) (particle entanglement) Between photons Between arms (particle entanglement) (wave entanglement)

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IV. Quantum information perspective Cable Beach Western Australia

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Heisenberg limit Quantum information version of interferometry Quantum noise limit cat state N = 3 Fringe pattern with period 2π/N Quantum circuits

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Cat-state interferometer Single- parameter estimation State preparation Measurement

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Heisenberg limit S. L. Braunstein, C. M. Caves, and G. J. Milburn, Ann. Phys. 247, 135 (1996). V. Giovannetti, S. Lloyd, and L. Maccone, PRL 96, 041401 (2006). Generalized uncertainty principle Cramér-Rao bound Separable inputs

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Achieving the Heisenberg limit cat state

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Is it entanglement? It’s the entanglement, stupid. But what about? We need a generalized notion of entanglement /resources that includes information about the physical situation, particularly the relevant Hamiltonian.

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V. Beyond the Heisenberg limit Echidna Gorge Bungle Bungle Range Western Australia

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Beyond the Heisenberg limit The purpose of theorems in physics is to lay out the assumptions clearly so one can discover which assumptions have to be violated.

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Improving the scaling with N S. Boixo, S. T. Flammia, C. M. Caves, and JM Geremia, PRL 98, 090401 (2007). Metrologically relevant k-body coupling Cat state does the job. Nonlinear Ramsey interferometry

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Improving the scaling with N without entanglement S. Boixo, A. Datta, S. T. Flammia, A. Shaji, E. Bagan, and C. M. Caves, PRA 77, 012317 (2008). Product input Product measurement

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Improving the scaling with N without entanglement. Two-body couplings S. Boixo, A. Datta, S. T. Flammia, A. Shaji, E. Bagan, and C. M. Caves, PRA 77, 012317 (2008); M. J. Woolley, G. J. Milburn, and C. M. Caves, arXiv:0804.4540 [quant-ph].

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Improving the scaling with N without entanglement. Two-body couplings S. Boixo, A. Datta, M. J. Davis, S. T. Flammia, A. Shaji, and C. M. Caves, PRL 101, 040403 (2008). Super-Heisenberg scaling from nonlinear dynamics, without any particle entanglement

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Bungle Bungle Range Western Australia

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Pecos Wilderness Sangre de Cristo Range Northern New Mexico Appendix. Two-component BECs

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Two-component BECs S. Boixo, A. Datta, M. J. Davis, S. T. Flammia, A. Shaji, and C. M. Caves, PRL 101, 040403 (2008).

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Two-component BECs J. E. Williams, PhD dissertation, University of Colorado, 1999.

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Let’s start over. Two-component BECs Renormalization of scattering strength

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Two-component BECs Integrated vs. position-dependent phaseRenormalization of scattering strength

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? Perhaps ? With hard, low-dimensional trap Two-component BECs for quantum metrology Losses ? Counting errors ? Measuring a metrologically relevant parameter ? Experiment in H. Rubinsztein-Dunlop’s group at University of Queensland S. Boixo, A. Datta, M. J. Davis, A. Shaji, A. B. Tacla, and C. M. Caves, “Quantum-limited meterology and Bose-Einstein condensates,” PRA 80, 032103 (2009).

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San Juan River canyons Southern Utah Appendix. Quantum and classical resources

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Making quantum limits relevant The serial resource, T, and the parallel resource, N, are equivalent and interchangeable, mathematically. The serial resource, T, and the parallel resource, N, are not equivalent and not interchangeable, physically. Information science perspective Platform independence Physics perspective Distinctions between different physical systems

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Making quantum limits relevant The serial resource, T, and the parallel resource, N, are equivalent and interchangeable, mathematically. The serial resource, T, and the parallel resource, N, are not equivalent and not interchangeable, physically. Information science perspective Platform independence Physics perspective Distinctions between different physical systems

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Making quantum limits relevant. One metrology story A. Shaji and C. M. Caves, PRA 76, 032111 (2007).

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Using quantum circuit diagrams Cat-state interferometer C. M. Caves and A. Shaji, “Quantum-circuit guide to optical and atomic interferometry,'' Opt. Comm., to be published, arXiv:0909.0803 [quant-ph].

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