Note/Quantum Computing
A quantum Arithmetic Logic Unit (ALU) utilizes qubits that exist in a state of superposition, allowing it to process multiple inputs at once. It then uses quantum interference to cancel out wrong answers and amplify the correct one, ultimately collapsing all possibilities into a single, highly accurate result upon measurement.
A qubit (quantum bit) is the fundamental unit of information in quantum computing, equivalent to the classical bit but governed by quantum mechanics. A classical bit can exist in either a 0 position or a 1 position. However, qubits can also occupy a third state known as a superposition (of 0 and 1). A superposition represents 0, 1 and all the positions in between taken at once for a total of three separate positions. In other words, unlike classical bits that can only be a 0 or a 1, qubits can exist in both states simultaneously. While a qubit can be in a superposition (i.e., encode three separate positions), the term qubit in quantum computing still refers to either the material or process used to represent a 0 or 1, or the measurement of that bit, meaning whether it is a 0 or a 1.
As a helpful analogy for understanding bits versus qubits, imagine you are standing in the center of a complicated maze. To escape the maze, a traditional computer must “brute force” the problem, trying every possible combination of paths to find the exit. This kind of computer would use bits to explore new paths and remember which ones are dead ends. Comparatively, a quantum computer might, figuratively speaking, at once derive a bird’s-eye view of the maze, “testing multiple paths simultaneously” and revealing the correct solution. However, qubits do not “test multiple paths” at once. Instead, quantum computers measure the probability amplitudes of qubits to determine an outcome. As these amplitudes function like waves, they also overlap and interfere with each other. When asynchronous waves overlap, it effectively eliminates possible solutions to complex problems and the realized coherent wave or waves present the solution.
Another way to think about it is that a “classical” computer operating on bits uses a single state (e.g., encoded as a byte, i.e., 8 classical bits) as its input to produce a single state as its output; whereas a quantum computer operating on qubits uses multiple states (i.e., qubits in superposition) as its input and computes a single (deterministically measurable) state as its output. While the previous analogy referred to “testing multiple paths simultaneously” this is not to be confused with computing all possible solutions given an input state space in parallel and then selecting “the right answer”; complex mathematical concepts underpinning quatum mechanics effectively eliminate possible solutions to complex problems in a quantum computer. In other words, a quantum ALU can effectively reduce qubits in superposition by eliminating possible solutions to yield “the right answer”.