According to Wikipedia's timelines, quantum computing may have had some inceptions as early as the late 60's, when Stephen Wiesner invented conjugate coding.

Around the early 80's, Wiesner's ideas inspired Charles Bennett and Gilles Brassard to hypothesize a cryptographic scheme - now called BB84 - to use quantum mechanics to enable information-theoretically secure cryptographic communication.

I sense that in the mid 80's, quantum key distribution was at best somewhat well-received, but more likely mostly ignored. Nonetheless researchers really began to take notice and appreciation when Bennett, et al. built a working prototype machine that transmitted and received a plurality of qubits, encoded in the polarity of photons, over a short distance to implement the BB84 scheme:

First BB84 Machine

I recall seeing the prototype in my copy of Scientific American in 1992. From Brassard's remembrances:

Essentially without any special budget allocated to the project, we were able, in late October 1989, to establish history’s first secret quantum transmission, over a staggering distance of 32.5 centimetres...!

Clearly this prototype wasn't practical or useful, but I'd posit that the theoretical and experimental researchers in the then nascent field of quantum computing/quantum information science were inspired not just by the theoretical work on BB84 but by images and descriptions of the above actual, physical tabletop device, as bulky and noisy as it was.

Based on the SciAm article the device was built at IBM Yorktown Heights (where Bennett had worked since '74).

As to my question, could the prototype still exist anywhere? Or put in a basement in IBM's campus and forgotten about, or was it more than likely scavenged for parts?

Such a prototype might be worth saving, at the very least as a bit of a relic to the field of quantum information...

The SciAm article was in '92. Around then, Vazirani gave a famous lecture at Bell Labs on the Bernstein-Vazirani algorithm, with Shor (and maybe Simon?) in the audience. We also have the quantum teleportation protocol from '93, the Elitzur-Vaidman bomb tester from then as well...


1 Answer 1


(Self-answer, making CW)

In Bennett's 2021 talk about the early history of quantum information at about the 8 minute mark, he has a quick slide picturing the quantum cryptographic device. Indeed, a black-and-white version of the same photo is part of the Journal of Cryptology article mentioned above.

The later SciAm picture looks professionally photographed, while the earlier image below looks like the device is labelled for a presentation or educational purposes. I'm not positive that the machine pictured above in the SciAm article is necessarily the same as the one pictured below in Bennett et al.'s report; one could be a replica or a Ship of Theseus from the original.

Labelled BB84 machine

But, Brassard's article states that the first demonstration of the device was in October of 1989, while the Cryptology article indicates that a further run was done in February of 1991, and the SciAm article came out in October of 1992, so the machine was around at least for a couple of years.

Regardless, my heart sings a little bit of joy in thinking that the same device that first demonstrated quantum cryptography may still be extant.

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    $\begingroup$ interesting. I always thought the “prototype” had been built in Montreal. I remember Brassard mentioning the racket made by the device. $\endgroup$ Mar 26 at 2:07
  • $\begingroup$ That might indeed be the case! But, John Smolin was apparently instrumental in building it- he and Bennett are out of NY. $\endgroup$
    – Mark S
    Mar 26 at 3:47
  • $\begingroup$ You could be right of course. Brassard would likely not have had access to this equipment as UofM does not have a big optics lab with Pockels cells handy, which is why I always found “my version” strange. $\endgroup$ Mar 26 at 3:50
  • $\begingroup$ But you are right that Brassard indicated that the power supply for the Pockels cells were loud, and indeed gave different sounds depending on the polarization of the qubits. $\endgroup$
    – Mark S
    Mar 26 at 3:51

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