,
20 tweets,
4 min read
Read on Twitter
1/ What is a qubit? This thread is a prerequisite to understanding my thread on Quantum Computing (which will come in the near future).
2/ As you read this thread, it might all seem unnecessarily arcane, and it might be difficult to see the connection between these bizarre beasts and Quantum Supremacy
3/ Before you understand qubit, you need to understand a "bit". As you know, computers understand only 1s and 0s. And these 1s and 0s are stored inside the computer in little locations called bits. A bit then is a location/thing/unit which can take on one of two values: 0 or 1.
4/ A "bit" is a conceptual/theoretical thing. In reality, there are different ways in which the concept of a bit can be implemented.
5/ For example, In a CPU high voltage across a transistor is 1 and low voltage is 0. On a hard disk, a location magnetized in one direction is 1 and the other direction is 0. In a fibre optic cable, light with high brightness is 1 and low brightness is 0.
6/ In summary, a bit is a conceptual entity that can take one of two (and only two) different values. One of the values is designated as 1, and the other 0. And in reality, bits can be implemented in many different ways.
7/ A qubit is like a bit, in that it has two different values, with the difference that it can have both values at the same time. If you've never studied quantum physics, this is where you do a double-take. It's non-intuitive, but this is how the (sub-)atomic universe works.
8/ Remember Schrödinger's Cat? Dead and alive at the same time as long as the box is closed, but as soon as you open the box, then it will (randomly) be either just dead, or just alive: en.wikipedia.org/wiki/Schr%C3%B…
A qubit is a real-life Schrödinger's Cat
A qubit is a real-life Schrödinger's Cat
9/ In fact, a qubit that has both 0 and 1 values at the same time is said to be in "cat state" (by actual serious scientists).
10/ So, a qubit is a weird bit that can have both values, 0 and 1, at the same time. And like Schrödinger's Cat you never see ("observe") its two values. When you try to measure the value, it randomly "collapses" to one of the values, and you see only that value
11/ How is this useful? For that you'll need to wait for my Quantum Computing thread. But in the mean time, let me tell you about a few more properties of a qubit.
12/ Actually, a qubit having two different values is not completely random. For any qubit, there is a specific probability of having the value 1 vs having the value 0.
13/ Specifically, there is a probability p associated with a qubit such that when you measure the value of the qubit, the probability of seeing 1 is p, and the probability of seeing 0 is (1-p).
14/ This is important, so let me repeat it in different words. Assume that we know a process by which a qubit can be put in a cat state X, where the probability of seeing value 1 when the qubit value is measured is 25%. What does this mean?
15/ If we run this same process 100 times on 100 different qubits, and measure the output value, 25 times we'll see the value 1 and 75 times we'll see the value 0. You probably can't imagine how, but this is extremely powerful and will play a key part in quantum supremacy.
16/ A few more little tidbits. It is possible that a qubit is in a state where the probability p is 1, in which case there is a 100% guarantee that we'll see the value 1 when the qubit value is measured. Or p could be 0, in which case we'll see 0 output 100% of the time.
17/ Also, most importantly, we have methods by which we can manipulate the value of p. These methods are called "operators" or "gates". So, we can start with p=1, then apply a gate and p becomes 0 (called a NOT gate). Apply another gate and p becomes 0.5 (called a Hadamard gate)
18/ Just as in bits, qubits can also be physically realized in many different ways? Usually it is based on quantum effects at the (sub-)atomic level. e.g. an electron orbiting around one of two atoms near each other. 1 means electron around the left atom, 0 means right atom
19/ Or photon polarization: 1 = horizontally polarized photon, 0 = vertically polarized. Or atomic spin: 1 = up, 0 = down. Or superconducting circuit: 1 = clockwise current, 0 = anticlockwise. (It's OK if you don't understand most of these. Even I don't understand half of them.)
20/ All of this might seem like a lot of annoying physics with not much of a payoff in the end. Be patient. The good stuff is coming in future threads.
