What is actually teleported?

IBM Journal of Research and Development, Jan 2004 by Peres, A

There are no "unknown quantum states." The phrase is self-contradictory. Moreover, Alice and Dob are only inanimate objects: They know nothing. What is teleported instantaneously from one system (Alice) to another system (Bob) is the applicability of the preparer's knowledge of the state of a particular qubit in the systems. The operation necessitates the use of dual classical and quantum channels. Other examples of dual transmission, including "unspeakable information," are presented and discussed. This paper also includes a narrative of my recollections of how quantum teleportation was conceived.

Landmarks

It was a great pleasure to participate in the May 2003 symposium at the IBM Thomas J. Watson Research Center honoring the sixtieth birthday of Charles Bennett. I learned of Charlie's work when I became interested in quantum information upon visiting John Wheeler at the University of Texas at Austin in 1979. However, I actually met Charlie in the summer of 1986, when I spent two months at MIT. We both lived in the house of Tom Toffoli, who was also our host at MIT. Tom had bought a dilapidated house on Howard Street and was busy making it livable. His family occupied the third floor; I was on the second floor in a tiny apartment that was perfect for me. Charlie, Theo, and her children were in a larger apartment, also on the second floor. The ground floor had not yet been rebuilt and looked like a construction site.

The symposium also marked the tenth anniversary of the discovery of quantum teleportation, described in the paper that I had the honor of coauthoring with Charles Bennett, Gilles Brassard, Claude Crepeau, Richard Jozsa, and William Wootters [1]. I shall discuss only the title of the paper, "Teleporting an Unknown Quantum State via Dual Classical and Einstein-Podolsky-Rosen Channels," and relate what I recall of the conception of the work. I apologize if my recollections arc imperfect or if I have unwittingly distorted any aspect of the story.

In October 1992, Bill Wootters (whom I knew from The University of Texas at Austin, where he had been a student) sent me an e-mail note indicating that he and others at the University of Montreal had found an interesting problem, and he asked for my advice. When things became clearer and we thought of writing a paper with six coauthors, we started arguing on every nuance of the text. all this had to be done by e-mail, because we were then scattered in five different places in four countries and eight time zones. Some of us worked while others were sleeping. Charlie quipped, "the sun never sets on our collaboration," and thereby started an argument about which king had said that. First we thought it might have been Charles Quint, but after some research work we learned that it was Philip II of Spain.

There were some memorable moments while the text was finalized. One Friday afternoon, Claude Crepeau sent me an e-mail note from Paris: What happens if Alice's particle, whose state has to be teleported, is itself entangled with another one, far away? Will Bob's particle become entangled with this other particle without having ever interacted with it? I was puzzled, but it was time to start the traditional Shabbat dinner with my family. As we were eating, I suddenly jumped from my seat, ran to the computer and wrote to Claude, "mais oui." Hc had reinvented entanglement swapping [2]!

Charlie did most of the editing. When everything looked fine, I sent him an e-mail note with a subject designation of imprimatur (the seal of approval of the Great Inquisitor). Charlie submitted the paper to Physical Review Letters (PRL), and wrote to us, alea jacta est, as if we had crossed the Rubicon. Contrary to expectations, our opus was not rejected by the referees. Later we learned that one of them was David Mermin, who gave a very strong recommendation that it be published. It is only more recently that David has deconstructed telcportation, and also dense coding [3].

Not only are the contents of the teleportation paper interesting, but also what is not in it. There are no acknowledgments for support by the National Science Foundation (NSF), the National Aeronautics and Space Administration (NASA), the Defense Advanced Research Projects Agency (DARPA), the Naval Research Laboratory (NRL), or other research support agencies. We never submitted a research proposal: It would have been rejected anyway. There was no time for that.1 Next, let's start to analyze the title of the paper.

Teleporting

I don't watch television and was suspicious of the term teleportation. In my dictionary [4] I found the definition, "theoretical transportation of matter through space by converting it into energy and then reconverting it at the terminal point." I protested that this was not at all what we had in mind, but Charlie reassured me, saying that we would cite Roger Penrose's 1989 book, The Emperor's New Mind [5]. I threatened that if we cited it, I would not be a coauthor. A few days later, Charlie wrote to me that he wanted to use weak measurements and cite a paper by Aharonov and Vaidman. This time, I didn't fall into the trap. Actually, we could have cited another paper by those authors [6], which introduced the use of dense coding. However, the presentation of this technique was so bizarre that the paper was not noticed then by the small quantum information community.