Our current computers perform calculations and process information using methods which date back to Turing and von Neumann. In this model, all information is reducible to bits, which can take the values of either 0 or 1. All processing can be performed via simple logic gates (AND, OR, NOT, XOR, XNOR, NAND, NOR) acting on one or two bits at a time. At any point in its computation, a classical computer’s state is entirely determined by the states of all its bits, so that a computer with n bits can exist in one of 2^n possible states, ranging from 00…0 to 11…1
The power of the quantum computer is in its much richer range of states. A quantum computer also has bits — but instead of 0 and 1, its quantum bits, or qubits, can represent a 0, 1, or linear combination of both, which is a property known as superposition. A quantum computer takes advantage of a special kind of superposition that allows for exponentially many logical states at once, all the states from (00…0 to 11…1). This is a powerful feat, and no classical computer can achieve it.
The quantum superpositions most useful for quantum computation, are entangled. Entangled states are states of the whole computer that do not correspond to any assignment of digital or analog states of the individual qubits. A quantum computer is therefore significantly more powerful than any classical computer.
The IBM Quantum Composer provides a hands-on opportunity to experiment with operations on a real quantum computing processor. A field guide contains a series of topics to allow experiments to be constructed, run in simulation, and executed on real quantum processors available through IBM Cloud®.
The true challenge of quantum physics is in ideas that are counterintuitive to day-to-day experiences in the physical world, which of course are constrained by classical physics. To comprehend the quantum world, a new intuition for a set of simple but different (and often surprising) laws needs to be built.
There is no single simple physical principle from which these conclusions follow. The best that can be done is to distil quantum mechanics down to a few mathematical laws, from which all the observed behaviour of quantum particles (and qubits in a quantum computer) can be deduced and predicted.