David Deutsch emailed me with nice comments and suggestions about my just posted paper. I told him the funny story behind the paper.
A few years ago I was watching a NOVA program about time travel and closed timelike curves. On watching the program I thought: “hmm, I wonder if quantum circuits always give you self-consistent evolutions?” After thinking about the problem for a few days, I was able to hack out a proof. Only then did I decide to go look in the literature to see what others had done. There I discovered Deutsch’s 1991 paper and found the result I had hacked out. The scary thing was how close our two proof were (I seem to recall even the symbols we used were identitical!) Getting deja vu when writing a paper on time travel is really spooky.
O brave new quantum world!