BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Talks.cam//talks.cam.ac.uk//
X-WR-CALNAME:Talks.cam
BEGIN:VEVENT
SUMMARY:Optimal estimation of quantum Markov chains using coherent absorbe
 rs and displaced-null measurements - Madalin Guta (University of Nottingha
 m)
DTSTART:20241204T090000Z
DTEND:20241204T100000Z
UID:TALK218623@talks.cam.ac.uk
DESCRIPTION:In this presentation I will discuss the problem of estimating 
 dynamical parameters of a quantum Markov chain. The key tool will be the u
 se of a coherent quantum absorber which transforms the problem into a simp
 ler one pertaining to a system with a pure stationary state at a reference
  parameter value. Motivated by the proposal in [1] I will consider countin
 g output measurements and show how the statistics of the counts can be use
 d to compute a simple\, asymptotically optimal estimator of the unknown pa
 rameter. For this\, I will introduce translationally invariant modes (TIMs
 ) of the output and show that these modes are Gaussian in the limit of lar
 ge times and capture the entire quantum Fisher information of the output. 
 Moreover\, the counting measurement provides an effective joint measuremen
 t of the TIMs number operators. The unknown parameter is estimated using a
  two stage estimation procedure. A rough estimator is first computed using
  a simple measurement\, and is used to set the absorber parameter. Due to 
 non-identifiability issues of the counting measurement the reference param
 eter needs to be shifted away from the initial rough estimator\, as shown 
 in the displaced-null measurements theory [2]. Finally\, an optimal estima
 tor is computed in terms of the total number of excitations of the TIMs\, 
 avoiding the need for expensive estimation procedures. Details can be foun
 d in [3].\n&nbsp\;&nbsp\;\n&nbsp\;[1] D. Yang\, S. F. Huelga\, and M. B. P
 lenio PRX Quantum 13\, 031012 (2023)&nbsp\;\n&nbsp\;[2] F. Girotti\, A. Go
 dley and M. Guta\, arXiv:J. Phys. A 57 245304 (2024)\n&nbsp\;[3] F. Girott
 i\, A. Godley and M. Guta\, arXiv: 2408.00626
LOCATION:Seminar Room 1\, Newton Institute
END:VEVENT
END:VCALENDAR