You Can't Teleporter Anything He Doesn't Want to Hear

The early days of the experimental demonstrations of quantum teleportation, particularly the first three experiments by Zeilinger, DeMartini, and Kimble, were not without controversy. This I learned first hand in 1999 when I attended the Sixth International Conference on Squeezed States and Uncertainty Relations in Naples, Italy. As the three different experimental group’s publications had just appeared, the conference decided to have a panel discussion on these three different experiments. Curiously, the organizers asked me to chair the panel discussion. I was a bit puzzled by this request, and carefully stated that while I was honored by the invitation, I was hesitant to accept, since I had never carried out research (either theoretical or experimental) on quantum teleportation, and was certainly not an expert on the matter. The organizers looked about the room nervously and then in hushed tones took me aside and explained their rational — apparently there was a very contentious debate raging among the three experimental groups as to who had done the first “true” demonstration of quantum teleportation, and the organizers expected the panel discussion to be loud and chaotic and they feared the panelists might become unruly — quantum physicists run amok! Therefore, they explained to me nervously while averting their eyes from my gaze, that they needed a moderator who would be able to run the thing with a strong hand (and a loud voice) in order to keep order. “The organizers are unanimous in our decision that you, Dr. Dowling, are our only hope.” I had been in a moment demoted from world’s expert in quantum physics to the quantum mechanical equivalent of either Obi-Wan Kenobe or a barroom bouncer.

The panel was chaotic from the inception. As I expected, there was one panelist per experimental group, Austrian physicist Gregor Weihs from the Zeilinger group, DeMartini from his own group, and Australian physicist Samuel Braunstein from the Kimble group. (American physicist Marlan Scully once called me the “Bob Hope” of theoretical physics. If that is the case then my good friend and colleague Sam Braunstein is surely the “Woody Allen” of theoretical physics.) Since the debate to be among these three competing groups, I was again puzzled when the organizers, at the last minute, added two additional Italian physicists to the panel, other than DeMartini. I was even more puzzled that these two last minute additions seemed to have even less experience with quantum teleportation than me! I gentle inquired as to why they should be on the panel — my role as bouncer was clear — but their roles were not. Again more hushed tones and averted gazes and the organizers explained that they were both big-shot Italian professors who asked to be on the panel discussion on quantum teleportation, in spite of knowing absolutely nothing about quantum teleportation, but for the simple reason that they were big-shot Italian professors, and thought it would look prestigious to insert themselves onto the panel. I had lived in Italy for a year as a graduate student, and I knew a thing or two about Italian politics in the universities, and conceded to their admittance to the panel in the interest of keeping the peace. Peace, however, was not long kept.

The night before the panel discussion I was in a bit of a panic myself in my hotel room as I wondered how to organize things. I decided that in the hour-long time slot I would give each of the six participants, including myself, five minutes to speak or present a few slides, and then reserve the second half hour for questions from the audience. Now I knew there was this debate between the three experimental groups about whose teleportation experiment was the “best” experiment, where “best is very subjective, but I had not followed this debate at all, and did not really have time to read through all three of the experimental papers and try to figure it all out. Hence I had an experimentalist friend and colleague, German physicist Andreas Sizmann, give me a quick tutorial on the nature of the experimental debate, at the bar, and then went back and carefully reread the original theory proposal by Bennett and colleagues, and with the help of Sizmann constructed a few overhead transparencies on how I thought a quantum teleporter should work and how, if handed such a device, one might be able tell if it was working properly, and if handed three of them, how I might gauge which of the three was “best”. It was then, for the first time, I devised the story of the mythical National Institute of Quantum Standards and Technology (NIQuIST) and the equally mythical quantum teleporter-testing machine NIQuIST had constructed to test the three claimed experimental implementations. In other words I did not compare the three experiments at all, I figured the panelists could do that themselves, but instead I put up a series of tests that each teleporter should pass to get the NIQuIST seal of approval, or more accurately something like a Consumer Reports rating: Recommended, Best Buy, or Not Acceptable. The quantum teleporter-testing machine that I drew up by hand looked like that in figure.

In the figure we show a compactified version of the quantum teleporter, being slowly lowered into the NIQuIST teleportation-testing machine (brownish orange). As before we have Doug, now a NIQuIST employee (purple, left) who provides the teleporter with a photon whose quantum polarization state is unknown to Alice, Bob, or Charlie in the teleporter. Doug produces a large number of such unknown states using a machine called the “ensembler” that produces ensembles or collections of single photon polarization states, where the ensembler can be programmed to choose the states at random, or in a pre-selected sequence. There are an infinite number of states so ensembler should produce a large number of different states so the test of the teleporter will be statistically significant. That is if the sample of states is random enough and sample enough of the possible infinite space in a way that the teleporter operators cannot anticipate, the more likely that it will be a fair test and that the teleporter operators cannot somehow cheat by using some inside knowledge of the states being transmitted. 

The next part of the tester will be to see how good the teleporter is doing. If an unknown state Y is sent in by Doug to Alice, we want the state that emerges on Bob’s side of the teleporter to be as close to Y as possible. This is quality control and one measure of the quality of such a state transport is called the quantum fidelity. The fidelity is 100% if the outputted teleported state is identical to the inputted stated to be teleported. The fidelity is 0% if the outputted teleported state is as far from the inputted state as possible, which would be hard to arrange without trying. If the teleporter simply completely scrambles the input state then (on average) the fidelity of the output states, with respect to the input states, will be 50%. That is, if the teleporter sucks, on average the outputted state agrees with the inputted state only half the time. Hence anything better than 50% is considered good; I would call it the bronze standard of teleportation. The gold standard would be 82% fidelity, that is the minimum required for the teleporter to be used to teleport one half of an entangled photon pair and still violate Bell’s inequality. Anything less than 82% can be modeled with a classical local hidden variable and hence does not really test quantum mechanics. A new character in our pantheon, Ellen, on the right (purple), extracts the fidelity; she runs a machine called the “fidellerator” that measures the state of the teleported photon and compares it with the state that was actually teleported. Doug sends the complete information on the states he provided, and Ellen sends the complete information on the states she received, to François (bottom green), who runs a machine called the comparator (which I do not put in quotes because “comparator” is a real word), which compares what Doug sent to what Ellen got and then ranks the teleporter in the test.

 
The next part of the tester will be to see how good the teleporter is doing. If an unknown state  is sent in by Doug to Alice, we want the state that emerges on Bob’s side of the teleporter to be as close to that state as possible. This is quality control and one measure of the quality of such a state transport is called the quantum fidelity. The fidelity is 100% if the outputted teleported state is identical to the inputted stated to be teleported. The fidelity is 0% if the outputted teleported state is as far from the inputted state as possible, which would be hard to arrange without trying. If the teleporter simply completely scrambles the input state then (on average) the fidelity of the output states, with respect to the input states, will be 50%. That is, if the teleporter sucks, on average the outputted state agrees with the inputted state only half the time. Hence anything better than 50% is considered good; I would call it the bronze standard of teleportation. The gold standard would be 82% fidelity, that is the minimum required for the teleporter to be used to teleport one half of an entangled photon pair and still violate Bell’s inequality. Anything less than 82% can be modeled with a classical local hidden variable and hence does not really test quantum mechanics. A new character in our pantheon, Ellen, on the right (purple), extracts the fidelity; she runs a machine called the “fidellerator” that measures the state of the teleported photon and compares it with the state that was actually teleported.  Doug sends the complete information on the states he provided, and Ellen sends the complete information on the states she received, to François (bottom green), who runs a machine called the comparator (which I do not put in quotes because “comparator” is a real word), which compares what Doug sent to what Ellen got and then ranks the teleporter in the test. 

 
So I presented this slide and this spiel in about five minutes at the Naples panel discussion, along with an additional slide and discussion from my partner in crime and sidekick, Andreas Sizmann. Then I allowed each of the remaining five panelists to speak. As expected, the two-big shot Italian professors who were admitted to the panel for political reasons, gave presentations that had nothing to do with quantum teleportation, much to the consternation of the audience, consternation that was on display through audience member protestations consisting of gnarled grimaces (or audible snoring). Then in rapid fire came (in alphabetical order) the three true experimental teleporter panelists, Braunstein, DeMartini, and Wiehs. This took up about a half hour of time, as anticipated. As the other panelists spoke, Sam Braunstein scribbled away furiously on a little notepad and, periodically with great showmanship and dash, he would loudly tear off the note sheet with the scribbles on it, neatly fold it in half, and hand it to me, while he displayed a very serious face to the audience. On the notepad sheets, that I would carefully unfold like a poker hand of two cards (so that nobody else could see them), Braunstein had drawn cartoon caricatures of the various panelists (other then himself), with their name and such artistic embellishments as crossed eyes, drool, maniacal grins, or feathered and bloodied arrows sticking through their craniums from ear to ear. 

After he would hand these cartoons to me, he sitting and I standing, with great seriousness and I would peer intently at them. Astonished, I realized that he was simply trying to get me to laugh out loud during the panel discussion! Not to be fooled, I would inspect the crude cartoons carefully, out of eyesight from the other panelists and the audience, and then fold them neatly, tuck them into my shirt pocket, and announce, “A very good point, Prof. Braunstein, I will be sure to bring it up in the discussion session to follow!” All the panelists, the audience, and Braunstein would then nod in solemn approval of this curious ritual and then Braunstein would then return madly to his work of furiously scribbling out the next cartoon caricature on his note pad.

   
Each panelist had spoken their piece and then it was time for the grilling from the audience. I did my best to maintain order, timing the questions, cutting people off when sufficient, deafening others (with my ribald vocabulary of Italian curse words) when necessary, but all was lost when a young man (whose name I never got) with a Germanic accent (German, Swiss, or Austrian), grabbed the portable microphone and launched into a diatribe of his own. The young Teuton first assailed the structure and organization of the panel, “This panel discussion was badly organized. I have no idea who these first two Italian guys were or why they were even on the panel, as they had nothing to say about the topic of quantum teleportation!” The Italians looked at the German-accented Wünderkindt in horror as he dared speak the obvious. (I liked this Kindt already.) Then he continued, “Even the presentations from the panelists, who have done work on quantum teleportation, were mostly incomprehensible.” Now all the hackles were raised. “In fact,” continued this spawn of Odin, “The only person on this panel who made any sense was Dr. Jonathan Dowlings (sic)!” (I liked this Kindt even more.) I grinned ear from ear and smiled at all the organizers, but the were not smiling back. Then the blond-haired, German-accented, intemperate audience participant went in for the kill, “And you, DeMartini, your teleportation experiment meets none of Dowlings’ criteria for a success at all!” I stared at this guy in horror as a shudder when down my spine and spread rapidly through the floor and across the audience

That was it! DeMartini snatched up his transparencies and stormed off the podium, off the stage, down the steps, and out of the auditorium side door, never to be seen again (at least at that conference). I, jokingly, announced, “Did anybody see where DeMartini went? Can somebody check the toilet? No? Okay then I officially declare the panel discussion to be at an end!”

— The unedited story adapted from Schrödinger's Killer App: Race to Build the World's First Quantum Computer


 

Einstein's Least Famous Equation?

In 1980, my first year in graduate school, the English physicist, Paul Dirac, Nobel Prize 1933 awardee, came to the University of Colorado to give a popular talk at The Gamow Memorial Lecture. As a big fan of Dirac, I dragged all my non-physicist friends to the “popular” lecture early to get good seats in the middle and second row from the front. The place was packed with the mayor, the chancellor, the provost, the deans, all the physics professors, a blonde woman from the Sufi community dressed in a turban and a white cloak sporting ceremonial dagger in  her waistband, and so forth. (This is Boulder, Colorado, after all.) Dirac gave what I thought was a very interesting talk on the history of quantum theory, but with no slides, no notes, no audiovisual aides, and no nothing. He just stood at the podium and talked for an hour. He was 78 years old at the time and he spoke in a very soft high-pitched, English-accented, mouse-like voice. So soft it was that you could barely hear him at all and the technicians kept cranking up the amplifiers until it screeched periodically from the feedback. The talk put all the non-physicists in the audience immediately to sleep. Then Dirac got to the part where he discovered the Dirac equation predicting the existence of antimatter.

He clearly gets a bit excited and impossibly goes up an octave, whereupon the feedback kicks in waking everybody up, and Dirac says, “I was led to the idea of the discovery of antimatter by considering Einstein’s most famous equation, E = ….” All my buddies from the English department began to nudge me and the crowd visibly perked up. They had not understood a goddamn thing but for sure even the English majors knew what “…Einstein’s most famous equation, E = ???” was going to be. Dirac continues triumphantly onward to the hushed auditorium, E = the square root of p-squared times c-squared plus m-squared times c to the fourth!?"

(Einstein’s least famous equation?) The audience visibly collapsed upon themselves in utter disappointment—they understood nothing—and I in the tomb-like quiet that followed in the hallowed Rocky Mountain granite of the vast Macky auditorium— burst out laughing uncontrollably. (And I was the only one.) Dirac, normally an endearing bird-like little man, scowled, halted the talk, stepped out from behind the podium, and stared down at me in silence, vulture-like, for a full minute. The rest of the audience looked back and forth between Dirac and me as they coughed and inspected their watches. Then, without a word, after my torturous minute was up, he returned behind the podium and finished his talk as if nothing had happened at all.

From: Schrödinger's Killer App: Race to Build the World's First Quantum Computer (Page 42). Taylor & Francis Press.