PIRSA:12110068

# What's an unknown POVM, and what does a two qubit state look like?

### APA

Pusey, M. (2012). What's an unknown POVM, and what does a two qubit state look like?. Perimeter Institute. https://pirsa.org/12110068

### MLA

Pusey, Matthew. What's an unknown POVM, and what does a two qubit state look like?. Perimeter Institute, Nov. 06, 2012, https://pirsa.org/12110068

### BibTex

@misc{ pirsa_PIRSA:12110068, doi = {10.48660/12110068}, url = {https://pirsa.org/12110068}, author = {Pusey, Matthew}, keywords = {}, language = {en}, title = {What{\textquoteright}s an unknown POVM, and what does a two qubit state look like?}, publisher = {Perimeter Institute}, year = {2012}, month = {nov}, note = {PIRSA:12110068 see, \url{https://pirsa.org}} }

Matthew Pusey University of York

**Collection**

**Talk Type**Scientific Series

## Abstract

If probabilities represent knowledge, what is an "unknownprobability"? De Finetti's theorem licenses the view that it is simply a

convenient metaphor for a certain class of knowledge about a series of

events. There are quantum versions for "unknown states" and "unknown

channels". I will explain how "unknown measurements" can be rehabilitated

too.

I will then move to a totally different topic. The Bloch sphere is handy

for representing qubit states, but the equivalent for two qubits is

15-dimensional! I will advocate instead drawing the set of states that Bob

can steer Alice to, the "steering ellipsoid". I will show how entanglement

and discord look from this perspective, and outline a geometric

classification of separable two qubit states.