Synopsis: Predicting the Quantum Past

Predictions for a quantum measurement are improved by probing the system after the measurement and evolving a model backward in time.
Synopsis figure
K. Murch/Washington University, St. Louis

Hindsight in quantum physics isn’t exactly 20/20, but observing an object at a later time can allow a better guess about its earlier quantum state. In a new experiment, researchers used a weak probe to continuously monitor a single qubit over several microseconds, and with that data they tried to predict the qubit state at some intermediate time. Using only the “before” data, the prediction was right in just 50% of the trials. But adding “after” data boosted the success rate to 90%, suggesting that the quantum state reveals more of itself when past and future measurements are combined.

Quantum physics has always been a bit of a guessing game. In the classic double-slit experiment, for example, a precise measurement of the initial (or final) velocity will not tell you for sure through which slit the particle will go (or has gone). Physicists have, however, developed a way to track particles—and other quantum objects—with so-called weak measurements that can provide imprecise information at several points along the path. The question is, does this limited—but extended—information help you make a better guess?

Kater Murch from Washington University in St. Louis, Missouri, and his colleagues played this guessing game with a superconducting qubit in a microwave cavity. The qubit constantly evolves as a superposition of two different energy states, and the team can monitor this seemingly random behavior with weak measurements using microwave photons. Halfway through each experimental run, the team temporarily concealed the microwave data and then tried to predict these “hidden results” by extrapolating the measurements from before and after. The predictions were markedly different and more confident when future measurements were included. These findings give new understanding of weak probes and how they might be used to make precision measurements of, for example, gravitational waves.

This research is published in Physical Review Letters.

–Michael Schirber


Announcements

More Announcements »

Subject Areas

Quantum Physics

Previous Synopsis

Biological Physics

Magnetic Cells

Read More »

Next Synopsis

Related Articles

Synopsis: Twisting in Thin Air
Quantum Physics

Synopsis: Twisting in Thin Air

Researchers characterize the rotational jiggling of an optically levitated nanoparticle, showing how this motion could be cooled to its quantum ground state. Read More »

Viewpoint: Mapping Out the State of a Quantum System
Optics

Viewpoint: Mapping Out the State of a Quantum System

Researchers have developed a new technique to measure the density matrix—a more general way of characterizing the state of a quantum system than that provided by the wave function. Read More »

Synopsis: A Tight Squeeze
Quantum Physics

Synopsis: A Tight Squeeze

Researchers have created quantum states of light whose noise level has been “squeezed” to a record low. Read More »

More Articles