RQC Seminar

127th RQC Seminar

• Speaker

  Dr. Erkka Haapasalo
  ( National University of Singapore )

• Date

  14:00-15:00, July 19, 2024 (Friday)

• Venue

  Hybrid(ZOOM・ Wako Main Research 4F 445-447 Seminar Room C01）

• Title

  Strategies for joint quantum measurements: retrieving and broadcasting

• Inquiries

  rqc_info[at]ml.riken.jp

Abstract
We need to carry out joint measurements in order to obtain varied data of a quantum system. For example, approximate joint measurements of position and momentum can be used for quantum state reconstruction. However, incompatibility – the impossibility of accurately measuring mutually excluding quantities simultaneously – sets limitations on carrying out joint measurements. We investigate methods of realizing joint measurements in different scenarios: (1) sequential measurements where we retrieve the second part of the joint measurement after the measurement of the first part and (2) parallel schemes where we first approximately broadcast the quantum system and realize the two parts of the joint measurements locally on the compound system exiting the broadcaster. The retrieving strategy (1) provides a generalization of the Leggett-Garg conditions for macrorealistic behaviour, leading to a strictly broader class of hidden-variable theories than those having a macrorealistic description. In qubit systems the optimal retrieving protocols further relate to the fundamental precision limit of quantum theory given by Busch-Lahti-Werner error-disturbance uncertainty relations. This is also demonstrated in an experimental photonic setting. The broadcasting strategy (2) is of more limited applicability given no-go results for certain continuous measurements. The method however remains a valid way of generating joint measurements for discrete measurements in finite dimensional systems as demonstrated by examples treating joint noisy basis measurements.