Theoretical physicist David DiVincenzo is widely viewed as one of the pioneers of quantum computing. He authored a 1996 paper (PDF) outlining five criteria he predicted would make quantum computing a reality; it has become a de facto roadmap for most of the research in quantum computing since then. In 1998, with Daniel Loss, he proposed using electron spins for storing data as qubits in quantum dots, which might prove to be the best choice for creating a working quantum computer.
In 2010, DiVincenzo was invited by Germany's Alexander von Humboldt Foundation to become Director of the Institute of Theoretical Nanoelectronics at the Peter Grünberg Institute in Jülich, and a professor at the Institute for Quantum Information of RWTH Aachen University. Previously he was a research director at the IBM T.J. Watson Research Center in Yorktown Heights, N.Y.
IEEE Spectrum: You turned to investigating quantum computing while working as a theoretical physicist at IBM. What caught your interest?
DiVincenzo: I became interested in around 1993. It was not very much of a field at that time, but it was a field. There were two very eminent IBM scientists who were already involved for much longer: Rolf Landauer and Charles Bennett. Landauer is remembered for his contributions to the fundamental understanding of computing. Questions like what is the minimum amount of energy required to perform computational processes.
Landauer was quite important in the original discussions of quantum computing because he provided a skeptical point of view. He thought that the sensitivity to imperfections in matter would be devastating for quantum computing. But he was interested in the concept of error correction that arose at that time and that could be applied to quantum computing. And this really turned the story around. Bennett was famous for introducing the ideas of quantum physics into information science and cryptography. In 1993, he worked on what is now known as quantum teleportation.
I was fascinated by these developments, and at that time, IBM was flexible enough so that I could just jump in. I started contributing various ideas, and the following year, the Shor Factoring Algorithm was discovered. This made it clear that quantum computing could be done.
Spectrum: So, the research culture at IBM was definitely an important factor in your research career.
DiVincenzo: I would say that the research culture at IBM was always distinct. There was a whole evolution over the decades. For years it was thinking of itself in relation to Bell Labs: Are we as famous as Bell Labs? That’s the history of the 1970s. I joined the lab in the 1980s; I had many friends that were there from the beginning, and I think I had a feeling for what the culture was like. IBM tried to really build up its research in the 1960s. In that period, they were definitely looking at themselves hoping to be another Bell Labs, which was in its heyday. By the 1980s, I felt that at that point they really did not have to worry whether they [were turning out] science of a comparable quality as Bell Labs. The cultures were similar; they would take rather young scientists and immediately give them all the resources of an institute, basically, without any of the responsibilities. This is a fantastic model which has proven to be not so sustainable—at least not in the corporate world. [And beginning in the early 1990s, it wasn’t really sustainable within IBM.]
Spectrum: How did this change affect your work at IBM?
DiVinzenzo: IBM had a heavy financial crisis in 1993, its most severe one. It had a moment when it was really questioning its whole business model and whether it should be broken up into smaller companies. IBM undertook a whole sequence of different steps, such as getting out of personal computers, and each one made it appear that physics had become less relevant to IBM. The physics department got much smaller that year, and I remained in that smaller department. But we had a fantastic time after that in quantum computing.
Spectrum: So at least the research culture at IBM survived.
DiVincenzo: Here I would say something about the culture of IBM versus Bell. Bell evolved into a very competitive internal culture. People were really knocking against each other. Internal seminars were quite an ordeal because you were subjected to really heavy scrutiny. Internal dealings among scientists at IBM were much more congenial.
The rest of physics at IBM was suffering. IBM still has a physics department, but at this point almost every physicist is somehow linked to a product plan or customer plan. At IBM, right from the beginning, there was always hope that these physicists who were dreaming up interesting things could actually contribute. Research became more directed as time went on.
Spectrum: It is now five years since you joined two German universities. How would you compare the life of a researcher in Germany—a country traditionally known for its emphasis on academic freedom ever since the 19th century? Do researchers in Germany have the same freedom that researchers had at IBM during the 1960s?
DiVincenzo: No. But I think they are freer and have more flexibility than what you find in the U.S. academic culture. Of course, in the U.S., there is heavy attention given to third-party funding. In Germany, this is not completely true. If you have a chair, you actually do have fixed resources that go with the chair, which is not the case in the U.S. But it is typically not enough to do any major project, and it shrinks over time, so you should connect yourself with some third-party funding. However, there is here a pretty strong long-term consensus that we don't tinker with science funding too much.
Spectrum: Basically, future quantum computers might be based on qubits of two types: atoms or ions suspended by laser beams, and ions or electrons trapped in matter, such as in defects or in quantum dots. Which will prevail?
DiVincenzo: We're close enough to the quantum computer that we kind of can foresee its complexity in a classical sense—that is, how much instrumentation is required for this to work as a quantum computer. I think that systems that involve lasers add a really big jump in complexity because they would require a laser system for every qubit. Say we need a million qubits; we will have a system with a complexity well beyond anything that has ever been done in optical science.
Now, for example, with quantum dots there are no lasers in sight. Everything is done with electronics and at gigahertz frequencies. You will need controller devices containing transistors that work at 4 Kelvin. That is an interesting challenge. It turns out that some conventional transistors, right out of the foundry, do work at 4 Kelvin. This becomes the beginning of some real scientific research: How do you make a transistor that really functions in a good way at 4 Kelvin without dissipating too much energy? Energy dissipated in the instrumentation close to the quantum system at that temperature could be one of a whole set of challenges that will not be solved easily. My own personal view is that we're a decade or so away from some really new machines that at least will partially fulfill what we've been thinking about since 1993.