# Causal Constraints on Possible Measurements

### APA

Borsten, L. (2012). Causal Constraints on Possible Measurements. Perimeter Institute. https://pirsa.org/12060078

### MLA

Borsten, Leron. Causal Constraints on Possible Measurements. Perimeter Institute, Jun. 28, 2012, https://pirsa.org/12060078

### BibTex

@misc{ pirsa_PIRSA:12060078, doi = {10.48660/12060078}, url = {https://pirsa.org/12060078}, author = {Borsten, Leron}, keywords = {Quantum Fields and Strings}, language = {en}, title = {Causal Constraints on Possible Measurements}, publisher = {Perimeter Institute}, year = {2012}, month = {jun}, note = {PIRSA:12060078 see, \url{https://pirsa.org}} }

Heriot-Watt University

**Collection**

Talk Type

**Subject**

Abstract

A
crucial question in any approach to quantum information processing
is: first, how are classical bits
encoded
physically in the quantum system, second, how are they then manipulated and,
third, how are they finally read out?
These
questions are particularly challenging when investigating quantum
information processing in a relativistic spacetime. An obvious
framework for such an investigation is relativistic quantum field
theory. Here, progress is hampered by the lack of a universally
applicable rule for calculating the probabilities of the outcomes of ideal
measurements on a relativistic quantum field in a collection of spacetime
regions. Indeed,
a straightforward relativistic generalisation of the non-relativistic formula
for these probabilities leads to superluminal signalling.

Motivated by these considerations we ask what interventions/ideal measurements can we in principle make, taking causality as our guiding criterion. In the course of this analysis we reconsider various aspects of ideal measurements in QFT, detector models and the probability rules themselves. In particular, it is shown that an ideal measurement of a one–particle wave packet state of a relativistic quantum field in Minkowski spacetime enables superluminal signalling. The result holds for a measurement that takes place over an intervention region in spacetime whose extent in time in some frame is longer than the light crossing time of the packet in that frame.

Motivated by these considerations we ask what interventions/ideal measurements can we in principle make, taking causality as our guiding criterion. In the course of this analysis we reconsider various aspects of ideal measurements in QFT, detector models and the probability rules themselves. In particular, it is shown that an ideal measurement of a one–particle wave packet state of a relativistic quantum field in Minkowski spacetime enables superluminal signalling. The result holds for a measurement that takes place over an intervention region in spacetime whose extent in time in some frame is longer than the light crossing time of the packet in that frame.