Quantum limits on estimating a waveform I.What’s the problem? II.Quantum Cramér-Rao bound (QCRB)...
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Transcript of Quantum limits on estimating a waveform I.What’s the problem? II.Quantum Cramér-Rao bound (QCRB)...
Quantum limits on estimating a waveform
I. What’s the problem? II. Quantum Cramér-Rao bound (QCRB) for classical force on a
linear system
Carlton M. CavesCenter for Quantum Information and Control, University of New Mexico
http://info.phys.unm.edu/~caves
M. Tsang and C. M. Caves, “Coherent quantum-noise cancellation for optomechanical sensors,” PRL 105,123601 (2010).
M. Tsang, H. M. Wiseman, and C. M. Caves, “Fundamental quantum limit to waveform estimation,” arXiv:1006.5407 [quant-ph].
Collaborators: M. Tsang, UNM postdoc
H. M. Wiseman, Griffith University
measurement noise
Back-action force
SQL
Langevin force
When can the Langevin force be neglected?
Narrowband, on-resonance detection
Wideband detection
The right story. But it’s still wrong.
SQL for force detection
Use the tools of quantum information theory to formulate
a general framework for addressing quantum limits on
waveform estimation.
Noise-power spectral densitiesZero-mean, time-stationary random process u(t)
Noise-power spectral density of u
QCRB: spectral uncertainty principle
Prior-information term
At frequencies where there is little prior information,
Quantum-limited noise
No hint of SQL, but can the bound be achieved?
1. Back-action evasion. Monitor a quantum nondemolition (QND) observable.
2. Quantum noise cancellation (QNC). Add an auxiliary negative-mass oscillator on which the back-action force pulls instead of pushes.
Achieving the force-estimation QCRB Oscillator and negative-mass oscillator paired sidebands
paired collective spins
Collective and relative coördinates
Quantum noise cancellation (QNC)
W. Wasilewski , K. Jensen, H. Krauter, J. J. Renema, M. V. Balbas, and E. S. Polzik, PRL 104, 133601 (2010).
Cable BeachWestern Australia
QCRB for waveform estimation Classical waveform Prior information
Measurements
Estimator
Bias
Handling the measurements
Hamiltonian evolution of pure states, but all ancillae subject to measurements
QCRB for waveform estimation
Gives a classical Fisher information for the prior information.
Apply the Schwarz inequality!
Gives a quantum Fisher information involving the generators.
QCRB for waveform estimation
Two-point correlation function of generator h(t)
QCRB for force on linear system
Force f(t) coupled h=qto position
Gaussian prior Classical prior Fisher information is the inverse of the two-time correlation matrix of f(t).
Time-stationary Two-time correlation matrices are processes diagonal in the frequency domain.
Diagonal elements are spectral densities.
QCRB becomes a spectral uncertainty principle.
Optomechanical force detector
Optomechanical force detector
(a) Flowchart of signal and noise.
(b) Simplified flowchart.
QNC I
(a) QNC and frequency-dependent input squeezing.
(b) QNC by output optics (variational measurement) and squeezing. W. G. Unruh, in Quantum Optics, Experimental Gravitation,
and Measurement Theory, edited by P. Meystre and M. O. Scully (Plenum, New York, 1983), p. 647.
M. T. Jaekel and S. Reynaud, Europhys. Lett. 13, 301 (1990).
H. J. Kimble, Y. Levin, A. B. Matsko, K. S. Thorne, S. P. Vyatchanin, PRD 65, 022002 (2002).
S. P. Vyatchanin and A. B. Matsko, JETP 77, 218 (1993).
QNC II
(a) QNC by introduction of anti-noise path.
(b) Simplified flowchart.(c) Detailed flowchart of
ponderomotive coupling and intra-cavity matched squeezing.
(d) Implementation of matched squeezing scheme.
QNC III
Input (a) and output (b) matched squeezing schemes and associated flowcharts (c) and (d).