Physics Seminar Series - Guest Speaker Dr. Lambert Giner-FR

Event date(s): September 25, 2025
Time(s): 01:15 PM - 02:15 PM
Category: Fredericton
Location: Fredericton

Tomography Without Tomography: Weak-Measurement State Reconstruction

At the heart of any quantum technology application (quantum computing, quantum communications, or quantum sensing) is the ability to characterize quantum states. The standard approach, quantum state tomography (QST), is indirect: one performs a set of predetermined projective measurements and then uses an algorithm to reconstruct the most likely state consistent with the data acquired. Quantum state tomography typically requires the number of measurements to scale quadratically with the space dimension and is poorly suited to real-time tracking because of the need to use an algorithmic reconstruction.

I will present an alternative, inspired by familiar indirect measurements in classical physics (e.g., inferring temperature from a sensor’s resistance via a calibration curve). Using a von Neumann–type interaction, the quantum state of interest (usually, one degree of freedom) is coupled to an ancillary “pointer” state (i.e another degree of freedom). Because the systems are coupled, when the system undergoes a weak measurement, it shifts the pointer by a quantity called the weak average, which is proportional to the wavefunction of the state of interest. With an appropriate calibration map (lookup table), one can determine the state directly from pointer readouts, without iterative algorithmic reconstruction.

I will show the first experimental results implementing this scheme: reconstructing polarization states of single photons by using photon time-of-arrival differences as the pointer. This weak-measurement-based protocol offers a compact route to state estimation with reduced post-processing and the potential for real-time monitoring.

Building: MacLaggan

Room Number: 105