Scientists such as myself were delighted to learn that the National Security Agency (NSA) of the United States plans to build a quantum computer. The news first appeared in the Washington Post, you can read it here. Previous attempts at snooping on civilians have tended to use physics mainly in the form of electronics, a subject that was exciting many decades ago but is no longer considered to be a part of physics at all. Quantum computing, by contrast, is a major buzzword these days. With the NSA's move in this direction, everyone living outside the US (the "snooped-upon") has a chance to be involved with the deepest questions in science, or at least to be the victim of people engaged in studying these questions.
For the benefit of readers who do not understand how quantum mechanical spying will enrich their lives, let me imagine a system where it is possible to be in one of two states: "with us" and "against us". The classical dynamics of this two-state system was famously analysed by one George W. Bush, who correctly observed that it was possible to be in only one of these states. But in quantum mechanics things are different: one can be in a quantum superposition of the two. A simple example would be a person who is "with us" with an amplitude of one over the square root of 2, and also "against us" with the same amplitude. Importantly, the phase of "against us" can be arbitrary relative to "with us", leading to the possibility of "quantum interference".
Imagine a person in the quantum state just described. As long as the NSA does not spy on her, she will simply remain in that state (for which reason it's called a "stationary state"). But suppose they measure whether she is "with us", as the NSA will surely want to do. This leads to a disastrous phenomenon called "collapse of the wave function". The poor soul will instantly find herself to be either "with us", or "against us", and the probability of collapsing into each of these states will be exactly a half. Moreover, and I can hardly stress this enough, all subsequent measurements will return the same state as the first one. We physicists like to say that the person went into an eigenstate.
It is not clear, at the time of writing, whether collapsing a person's wave function and forcing them into an eigenstate is as serious a violation of human rights as collapsing their humanity and forcing them into Guantanamo prison. As usual, the NSA is way ahead of the United Nations on this matter. Even physicians can't be certain: is forcing you into an eigenstate as painful as forcing water up your nose? Today this is a known unknown, but once the NSA unveils its powerful quantum computer we will be sure. Or perhaps we will only know with a definite probability?
As a human-rights supporter, I look forward with interest to the first quantum trial in a court of law. The dialogue might run like this:
Judge: Is the defendant with us or against us?
NSA: Yes, your honour.
Judge: You mean she is the sum of both?
NSA: Not necessarily, your honour. She could be the difference of both. Or a complex combination.
Judge: You mean you couldn't detect the phase?
NSA (looks at shoes): No, your honour. Our quantum computer programmer isn't good with complex numbers.
Judge: Case dismissed!