November 24, 2016
Published by Stephan Sponar
*Weak Measurements*

What can be said about the *value* of an observable during the time interval between two measurements ? A possible answer to this question has been given by Aharonov, Albert and Vaidman (AAV) in 1988. In their seminal paper “*a new kind of value for a quantum variable*” the so-called *weak value* is introduced, again demonstrating that quantum mechanics predicts striking and astonishing counter-intuitive phenomena. The weak value is obtain via a procedure, referred to as **weak measurement**, where the probed quantum system is left minimally disturbed and pursues its evolution from an *initial* *state* towards its *final* *state*. The weak value of a variable may differ significantly from the eigenvalues of an associated operator in that sense, that weak values may lie far *outside* the eigenvalue range of the operator.

September 8, 2017
Published by Stephan Sponar
The **pigeonhole effect** principle tells us that if you put *n* items are put into m containers, with *n > m*, then at least one container must contain more than one item. However in quantum mechanics this basic property does **not** hold. This violation of the classical effect is referred to as *quantum pigeonhole effect* and is demonstrated in our neutron interferometric experiment.

January 7, 2017
Published by Stephan Sponar
We developed a measurement scheme, used in a matter-wave interferometric experiment, in which the neutron path system’s quantum state was characterized via **direct measurements** using both **strong** and **weak** interactions. Experimental evidence is given that strong interactions outperform weak ones for tomographic accuracy. Our results are not limited to neutron interferometry, but can be used in a wide range of quantum systems.

December 21, 2014
Published by Stephan Sponar
The **Cheshire Cat** featured in **Lewis Caroll’s** novel **“Alice in Wonderland” ** is a remarkable creature: it disappears, leaving its grin behind. Can an object be separated from its properties? It is possible in the quantum world. In our experiment, neutrons travel along a different path than one of their properties – their magnetic moment. This is the “*Quantum Cheshire Cat*”.

February 25, 2013
Published by Stephan Sponar
A straightforward implementation of **weak measurements** for massive particles, as originally proposed by **Yakir Aharonov**, is not feasible, due to the tiny coherence volume of particles used in matter-wave optics. We have overcome this hurdle by developing a method to weakly measure a massive particle’s spin component. This is achieved in a neutron optical approach by utilizing neutron interferometry, where the neutron’s spin is coupled weakly to its spatial degree of freedom.