Collaborative Research: RUI: Weak and Protective Measurements in the Time Domain

This project will realize new ways of measuring a quantum system. In physics, the concept of a measurement is extremely important. We can only learn about the world around us by performing measurements, e.g., when you look at something you are “measuring" it with your eye.

In classical physics, measurements can be performed without disturbance—one gains information about an object without changing its state. (The trajectory of a baseball is unchanged as you look at it). In quantum mechanics, however, it was long believed that a measurement would nearly always change the state of the system, the wave function would “collapse," and there was no way around that fact.

Recently that belief has been challenged, as physicists have come to realize that it is possible to perform so-called weak or protective measurements in which one gains information about the object while only minimally disturbing its state. Such types of measurements have been transforming the landscape of quantum measurements. They also offer technologically important applications in quantum information processing.

In this project, the group will implement new experimental techniques for performing weak and protective measurements. These techniques promise to provide significant improvements in measurement performance and flexibility over existing experiments. In parallel, the group will advance our understanding of weak and protective measurements through theoretical investigations, whose predictions will in turn be tested by the experiments.

Much of the work will be performed by a diverse group of undergraduate students, providing an excellent opportunity for them to develop research skills and further their scientific careers.

This project will experimentally realize weak and protective measurements with single photons and complement these experiments with theoretical studies. The experimental approach is characterized by using the temporal differential group delay (DGD) between polarization components as a measuring “pointer." The group will construct a source of narrow-band, pure-state, single photons in the telecommunications C-band (~1550 nm) and use it to first implement a weak measurement of polarization by directly measuring photon arrival times after the DGD.

This basic setup will then serve as the central building block of a novel experimental realization of a protective measurement in the time domain by utilizing DGD in a loop configuration to measure expectation values of photon polarization. The optical loop will enable the team to pass the photons repeatedly through the same measurement and state-protection stage, thus implementing the protective measurement.

In the theoretical portion of the project, the group will study several important properties of protective measurements and their performance, including: the effect of diminished state purity on the measurement, photon survival probabilities, and the uncertainty in the final measurement. These theoretical activities will generate models and predictions that can be tested by experiment and that will enhance our understanding of protective measurements.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.