Noncommuting Mixed States Cannot Be Broadcast

Howard Barnum, Carlton M. Caves, Christopher A. Fuchs, Richard Jozsa, Benjamin Schumacher
1996 Physical Review Letters  
We show that, given a general mixed state for a quantum system, there are no physical means for broadcasting that state onto two separate quantum systems, even when the state need only be reproduced marginally on the separate systems. This result generalizes and extends the standard no-cloning theorem for pure states.
doi:10.1103/physrevlett.76.2818 pmid:10060796 fatcat:juxhx2xh7va6noh55hqzy3ff4e