To be unconditionally secure, both Alice and Bob (the two most maligned and praised people in physics literature, at least since quantum physics widely adopted an information theoretic approach to QM and QFT) must be able to exchange information such that if Alice commits to some (scalar multiple or weighted superposition) of bit values used to encode some information, when/if she later "reveals" to Bob what is needed to decode the information Bob can be sure that Alice cannot have changed the actions needed to encrypt the information using quantum computing and Bob cannot decrypt it without the "key" provided by Alice. Instead of using number theory and the computational power needed to factor prime numbers, information in quantum computing is encoded in a prepared system that must be measured (interfered with) in order to be decoded, and must be both prepared and measured in particular ways (and preparation of some specified kind requires measurement in some particular way).
In general (and simplistically) the same reasons for looking to quantum computing as the next generation of encryption technology is the reason it doesn't work (unconditionally, that is). The uncertainty principle and measurement problem (and in general the statistical nature of quantum mechanics) require that in order for information to be encoded using qubits in such a way that decoherence occurs only by measurement and reliably. It's very easy to prepare a system that can't be "cracked", but this is a bit like writing down information on a piece of paper and then burning it. Sure, nobody is going to "crack" such security, but neither can anybody transmit information on paper by burning it before anybody can see it. The main problem, then, is for Alice to have some way of committing to a preparation that Bob can trust, as it is easy for a dishonest Alice to commit to some protocol but not do so (different systems can yield identical measured states given some transformation by Alice). However, once Bob is assured that Alice cannot commit to some encoding protocol that she does not follow, how is Alice to know that Bob cannot decode the information before Alice wishes? Currently, quantum mechanics appears to render impossible secure bit commitment if Alice and Bob do not trust one another (unconditional security). However, there are recent experimental realizations of unconditionally secure bit commitment given constraints imposed by relativity, as well as a number off theoretical proposals. The problem is that such constraints (no-signaling) are, at present, part of the theoretical aligning of quantum physics with relativity, but this melding is largely mathematical and remains perhaps the foremost problems of modern physics.
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