Due to a Lack of Entanglement, NTT’s Coherent Ising Machine May Not Qualify as a Quantum Computer

Three weeks ago I wrote about a new prototype quantum computer hosted by Nippon Telegraph and Telephone (NTT) in Japan.  This prototype was exciting because it was freely available, operating at room temperature, and using very little power to do so.

In response to my post, I received this message from Dr. Hidetoshi Nishimori, Tokyo Institute of Technology:

I read your new post on the Japanese effort with the Coherent Ising Machine (CIM) from NTT, etc. The machine is a fine technological achievement toward a new, useful optimizer. Nevertheless, there has been a big debate in Japan since last week if this CIM can be called a quantum computer. The computational unit, a laser pulse, operates as a qubit (in a superposition of 0 and 1). However, the CIM never entangles qubits, because a classical circuit FPGA processes the most important part of the computation (the introduction of interactions between qubits). This is, I believe, disqualifying for the CIM to satisfy the MINIMUM requirements to be called a “quantum computer.”

In other words, the core part of the computation is classically processed.  Hence, not a quantum computer.

We Can’t Even Agree on What a Quantum Computer Is Now?

I kid, but the NTT team describes their machine as a quantum computer, as does all the coverage I’ve read.  This reminds me of a comment Helmut Katzgraber  made about quantum annealers not being true quantum computers.  In that case, Professor Katzgraber was questioning whether D-Wave’s annealer satisfies the dictionary definition, because it doesn’t take instructions from a variable program.  In this case, Dr. Nishimori’s point gets to the heart of what quantum computing is all about: taking advantage of quantum mechanical phenomena.

This doesn’t diminish the team’s achievement, but it does put a spot light on why standard terminology matters. Dr. Nishimori is an established contributor to the field with a ton of credibility.  He’s concerned that the announcement is inadvertently propagating confusion on an international stage.  This is exactly why the IEEE’s Quantum Computing Working Group’s effort to establish standard quantum computing definitions is crucial, and why I’m so excited to be a part of the effort.

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