## ABSTRACT

In the last chapters, we led up to the idea of building a quantum computer using entangled particles as the underlying building blocks. If the goal is to run Grover’s search algorithm, then a quantum search engine probably already exists and can be purchased from D-Wave for $10 million. If, on the other hand, the goal is to have a truly universal quantum computer capable of running Shor’s factoring algorithm, in order to crack a 1024-bit public key, a quantum computer with all the bells and whistles and running full error correction would

be a tera-qubyte machine, that is, 1012 qubytes or approximately 1013 qubits. e current record in an ion-trap quantum computer is around 13 qubits not 1013 qubits-we are 12 orders of magnitude away from a machine that would be of use for factoring such large numbers. Nevertheless, work continues apace in the study and development of ever-larger machines. A recent Intelligence Advanced Research Projects Activity program, in which I participate, proposes to estimate the resources, both quantum and classical, that a future quantum computer would need to solve a pantheon of dierent algorithms, not just factoring. To quote the head of the program, American scientist Mark Heiligman, of the Oce of the Director of National Intelligence, “National security decisions will now be made on the basis that there is no objective physical reality.”2 But as Nobel Laureate William Phillips is fond of saying, “e chances of building a quantum computer are 50-50: and by that I mean a 50% chance in 50 years.” What are we then to do in the meantime?