Shival is right, but not because of "data moving at the sped of light". Currently when computers communicate across the internet some of the data already moves at the speed of light (along fibre optic cables, or through the air as radio and micro waves)(tehcnicall the sped of light divided by the refractive index of the mdeium, but this is pretty close to the speed of light in free space).
The reason that quantum computers have such potential for breaking and making encryptions is because of something totally different, entanglement.
A quantum computer works on the principle of perfomring computations with qubits (quantum bits) instead of normal bits. A normal bit can be a 1 or a 0 at any time a quantum bit can be both at once. Also when you have a collection of normal bits they are ech a 1 or a 0, when you have a collection of qubits they are all in a shared superpositon of many states, taking all the possible values of your collection at once. This means quantum computers can run some very unusual algoritms which let normally slow "trial and error" based "brute force" tasks (like factorising a number into two prime factors) be done much more rapidly, this ability to skip the trial and error bit and just get straight to an answer is what means quantum computers could break any encryption, because the current best encryption systems use assymetric ciphers where there is a public key (related to a single huge number) and a private key (related to the two prime factors of that number) and quantum computers are very good at factorising.
The reason quantum computers can also MAKE (as well as break) ciphers is because they can use entangled channels to communiate between a sender and receiver, any eavesdropper attempting to snoop on the channel would break the entanglement and therefore be unable to extract any useful information, the breaking of entanglement would also signal to the receiving party that their communications were being spied upon. In theory it would be against the laws of physics for any method of successfully intercepting the quantm channel to work, although the message could still be snooped on by anyone watching the wires between alice's computer and her end of the quantum link, or anyone watching the wires between bob's end of the quantum link and his computer.
In the past the american government was so scared of encryption that they tried to treat it's export like the export of armaments, so how scared many governments (and also citizens with private data to hide, that is anyone who has ever logged into an https website) would be of a working quantum computer is easy to imagine.
Edited by rp88, 19 May 2015 - 11:16 AM.
Back on this site, for a while anyway, been so busy the last year.
My systems:2 laptops, intel i3 processors, windows 8.1 installed on the hard-drive and linux mint 17.3 MATE installed to USB