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In my previous question I asked who invented a quantum computer using qubits.

As a follow-up to this question I want to ask who built the first quantum computer using at least two qubits.

During my research I have discovered that in 1998, Jonathan A. Jones and Michele Mosca developed a quantum computer using two qubits specifically to solve Deutsch's problem. Have there been other working quantum computers before to solve other problems or general attempts not specifically bound to one problem?

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  • $\begingroup$ @Blue, is it better now? $\endgroup$
    – luap42
    Mar 24 '18 at 20:17
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    $\begingroup$ Great question. I am unaware of any demonstrable quantum computer before this, but have access to a ton of old research materials and will do some digging this weekend to see what I can find. $\endgroup$
    – whurley
    Mar 24 '18 at 20:27
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What is a qubit? And what is a quantum computer? Any claim about about which is first will depend on our definitions.

One suggestion might be the 1981 experiment by Aspect, Grangier and Roger to demonstrate a violation of Bell’s inequality.

My arguments for this are:

  • It uses a physical degree of freedom (photon polarization) which has since been considered for qubits.
  • It performs a task (Bell’s inequality violation) that has since been used in quantum information theoretic tasks (like cryptography).

So though the authors would have had no concept of their setup being a two qubit quantum computer at the time, I’d say that it was.

For some other early two qubit systems, see references 7, 8 and 9 in this paper (which has arguably the first 3 qubit system).

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It's difficult to define the point where an experimental setup is a quantum computer. But the crucial feature of a quantum computer is that it's able to perform a quantum computation. The first experimental realization of an algorithm was indeed Jones' and Mosca's implementation of the Deutsch algorithm in 1998 using an NMR setup.

Of course previous experiments showed components that could be used in a quantum computer.

However, it is quite reasonable to demand that a quantum computer is able to perform arbitrary arithmetics, whether programmable or by minor adjustments to the setup. By this definition we don't have a quantum computer, yet. This is related to the DiVincenzo Criteria for Quantum Computers.

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An honorable mention might go to Bennett, Brassard, and friends' prototype machine for implementing the BB84 protocol, built in the late '80s and described in Scientific American in 1992.

In his article on the history of quantum cryptography, Brassard states:

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, precisely on the tenth anniversary of our meeting at the San Juan beach!

This (presumably) used a plurality of qubits, way more than $2$ as in the question, but all in a product state so not really a quantum computer.

Nonetheless Brassard comments that the act of building the prototype did cause physicists to start paying attention to quantum cryptography, and, presumably by extension, to quantum computing. I'm trying to find a publicly available image of the machine - I remember seeing it in my (hardcopy) Scientific American from when it was published. Such a lovely piece of history.

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