Unraveling the Quantum Ensemble

Monday, April 13, 2015 - 4:15pm
Room #1 LeConte Hall
Irfan Siddiqi
University of California, Berkeley
Public Lecture: 

In the quantum world, an object can simultaneously exist in multiple states – the “dead” and “alive” character of Schrödinger’s proverbial feline being a quintessential example.  It is the act of measurement which drives such an exotic superposition to a more familiar classical outcome, “dead” or “alive” for the cat, thus bridging the gap between quantum mechanics and our concept of reality. However, the precise nature of this so-called wavefunction collapse remains a topic of debate at the intersection of physics, mathematics, and philosophy.  In particular, quantum mechanics does not in general predict the specific result of a single experiment, but rather defines the probability distribution associated with different measurement outcomes given an ensemble of identical systems or repetitive sequences. Applying continuous weak measurement techniques in conjunction with Bayesian statistics to superconducting quantum circuits, we reconstruct the real-time collapse of the wavefunction describing a two-state system at the level of the individual constituent quantum trajectories that form an ensemble measurement. With this dense dataset, a variety of statistical metrics can be extracted, including the most probable path—analogous to the geodesic in space-time—between two points in Hilbert space.  Further, we have applied these weak measurement protocols to stabilize coherent oscillations using quantum feedback, and to generate entanglement between remote objects. Future extensions to many-body quantum systems may promise a route to efficiently probe systems with exponentially increasing complexity, generating natural parallels with questions in high energy physics.