Superposition: The core idea in quantum computing

- We look in the ocean and we see waves, or a pond, and you drop a rock in a pond and you see waves ripple out. And if you drop a rock in a different point of the pond, at the same time, then you see waves ripple, and then they come together and the waves form superpositions. You'll have crests and troughs that add and subtract from each other in some coherent way that can be destructive or constructive. - So what would happen if we could have a computational model that can manage a one, zero, or both all at the same time? This is what a qubit is. The ability for a qubit to have multiple states simultaneously is called superposition. It's the physics of quantum mechanics that enables particles and waves to be in multiple states until measured, at which point they collapse to either a one or zero. Qubits are created by changing the state of certain atoms and other quantum-scale particles. Electrons, the nucleus, and even photons can be used. To change an electron into a qubit, we can use laser beams, electromagnetic fields, radio waves, and other techniques. Let me use an example to further explain this idea of multiple states until measured. For this, we'll use one of our coins. I'm going to go ahead and spin this on the table. Is it currently heads or tails? As the coin is spinning, it's neither. It's only when it comes to a rest that we know whether it will be heads or tails. So this binary device, a coin, with only two sides when spinning, it technically in multiple states. If it were a qubit, it will be in a state called superposition. What does this mean in practice? - So we have superposition, we have entanglement, and there's another really key element for quantum algorithms, and that's interference. And so, when we think about interference, how does that work, what does that mean? If you imagine a tide pool, and you create a ripple. I touch the corner of the pool and then I touch the corner on the other side of the pool, and this creates waves. So in quantum mechanics, waves play a very large role. This creates waves in that pool. And as those waves ripple across, they interact, they interfere. When those waves interfere, you can see that the height, the amplitude of those waves, some of them, the amplitude grows, they get taller. Some of them, in fact, disappear. Some of those waves disappear, they cancel each other out. This is actually exactly what we use in the design of quantum algorithms, the notion of some of these waves causing amplitudes to increase and magnify, and some of the waves to disappear. Now, the key is encoding your solution into those waves. All of the possible solutions become, in some sense, one of these waves. And so, the wrong solutions, you want to interfere and cancel each other out. And the correct solutions, you want to interfere and magnify, amplify. And that's exactly how we design quantum algorithms. - A classical computing model will try each possible solution in sequence until it reaches the correct answer. A quantum computer would guess many answers in parallel, thus finding the solution in no time at all. Qubits are at the center of what makes quantum computing work. Today, we are able to tie a few qubits together to create quantum gates, their equivalent of logic gates. You'll recall these gates are what take an input, test it, and then create a new output. In classical computing, these are called Boolean operators, like an and, and an or. Quantum gates are conceptually similar. Finally, quantum computing, while is already achieving remarkable results, has some big challenges to overcome. Among them is managing qubits. Both vibration and changes in temperature quickly make them unstable, this can hinder computation or cause errors in the output. We need to solve and address these current limitations if quantum computing is to become reliable and used with confidence.