As some of you might already know, on October 1st I finally started to focus on my PhD in (roughly speaking) Quantum Computing by taking a sabbatical leave from TTTech in order to do research at Frank Verstraete’s Quantum Information Theory group at University of Vienna. For the next 6 months I will finally have time to sit down and think through more deeply a couple of ideas I have in this area and for crystallizing them into papers. I will have the same opportunity another time in a year from now in order to continue where I left off. I’m hoping to create the essential content of my PhD thesis within this sabbatical, even though it seems unrealistic to expect completion of the thesis entirely within that time frame. Nevertheless, now I have time to work full-time on what has been an enjoyable hobby and spare-time activity so far.

Many of my computer scientist friends may wonder how it is possible to skip over to an entirely different field, doing a PhD in physics without having a physics undergraduate degree. Whether this is really going to work out I don’t even know myself for sure, but I can tell you the story as it has developed so far: Back in the undergraduate days, probably in 2002, I got interested in quantum computing for the first time. At this time Prof. Jozef Gruska was teaching a one-semester guest class on Quantum Computing at TU Vienna, where I was studying Computer Science. I was curious about that class, yet frightened to take it by the mere thought of a computer scientist having to master quantum mechanics in a semester just in order to complete that class. So I was too scared to actually take that class. But what I did do was buying Gruska’s 1999 book on Quantum Computing, which was the first ever text book in this area covering all the basics in one place. So I had this book sitting on my book shelf for years and every now and then I came back to it for curiosity. But most of the time I was simply overwhelmed by the strange physicist notation of and and tensor products () when jumping right to the gems of the area: Shor’s and Grover’s algorithms. But once I figured out, that Quantum Computing is essentially Linear Algebra and the bra and ket symbols are merely short-hand notation for row and column vectors representing states, while state transitions are performed just by matrix multiplications, most mysteries resolved in delight. I thought “Hey, that’s just Linear Algebra! Well, I can do that!” and after mastering the basics, a quite lively and active research area was open and accessible to me. What I enjoyed most was that another Computer Science area I have alway been fond of was central to this area: Complexity Theory ( and related questions). Naturally, one of the most intriguing question of this area was: How powerful are quantum computers? Can a quantum computer perform certain tasks significantly more efficient than any classical computer? (for the experts: ?) Furthermore, with the language of Quantum Complexity Theory, it was now possible to formally ask the question how hard it is to compute the ground state of quantum systems using a quantum computer, which essentially boils down to a quantum version of the question, namely ? Given Quantum Mechanics as humanity’s best description of the physical world, what are the hardest problems that can be possibly solved within *Nature* and what kind of problems will for ever stay out of reach?

Gruska’s book is a good introduction to the field, but the presentation is still quite biased towards physicists rather than computer scientists. The current standard text book in this area is Nielsen and Chuang, which is almost “physics-free” and probably even better accessible to computer scientists than Gruska. All of this I found extremely exciting, so I started to devote much of my spare time to learn everything about this area and at some point in time I started to ask myself, whether I shouldn’t channel this efforts into something with fruitful outcome, like a PhD. During that time I started reading very many papers in this area, some of them (co-)authored by Frank Verstraete, who is a well-known researcher in this field. At one point in time, I observed that Frank is now with University of Vienna, having been appointed as full professor in 2007. So I figured out it might be a good idea to get in contact with him to discuss some of my ideas, which I did in early 2009. Since that loose collaboration seemed to work out excitingly for both of us, Frank accepted me as a PhD student and so I inscribed at University of Vienna for the PhD programme in Physics in summer semester 2009. Of course, with a Computer Science undergraduate degree this was not entirely straightforward, but in the end I got accepted conditioned on taking an extra Quantum Optics lab course including the associated theory course in addition to the classes I need to take for the PhD programme itself. So I took the theory class by Markus Arndt and Phillip Walter, which I very much enjoyed and which I did on vacation time of my regular job at TTTech. But the lab itself is blocked into one entire month of full-time lab work. How could I align that with my full-time job at TTTech? And more generally, how could I align doing a PhD with my job at all? So I was looking for funding opportunities, being aware of the fact that any option would mean a severe pay cut. Nevertheless, I was willing to accept that, for the enjoyment and intellectual excitement to be expected. Finally, the most straightforward funding opportunity to support a PhD without quitting my job turned out to be the sabbatical programme (Bildungskarenz) offered by the government of Austria: If your employer accepts, you are free to take off 1 year within a time-frame of 4 years for (provable) personal education while being redeemed some part of your last net salary. As this was acceptable to me, I agreed with TTTech to split the sabbatical into 2 times 6 months starting October 1st. Splitting it in a more fine-grained way (e.g. part-time job/part-time PhD) would have been the ideal solution, but unfortunately this was not compatible with the legal requirements of the sabbatical programme offered by the government.

Now I’m here at the university, working with Frank and his small and brilliant team on exciting Quantum Algorithms! I hope you enjoyed reading my story so far! Be sure, I will keep you informed.