 Developing experiments without loopholes to prove that the entanglement phenomenon is real has always been difficult. But there's nothing like actually using a phenomenon to remove all doubt. Quantum computing is doing just that for quantum linear superposition and entanglement. There is an amazing amount of work around the world going into the development of quantum computers and their subsystems. Here's just three of them. The superposition states, and quantum entanglement covered in the preceding segments represent the foundational physics for quantum computing. In order to illustrate how this is the case, we'll actually construct a two-electron quantum computer. The key difference between classical computers and quantum computers starts with their basic unit of information. For classical computers, it is the bit, with two values per bit, zero or one. Quantum computers use quantum bits, or qubits for short. And because of quantum linear superposition, a qubit has four values. For example, here's a state vector for the spin of an electron. Its position is determined by two angles that define its state. This state can be divided into two base states and two superposition states for a total of four, twice the number of possible values for a classical bit. What's more, because of quantum entanglement, every time we add a qubit, we double the number of classical bits the entangled whole can represent. Here's a table that compares classical computer bits to qubits. Three qubits are equivalent to eight bits, a full byte. The scaling grows into significant numbers as the number of qubits are increased. The real impact comes when we start talking about hundreds or even thousands of qubits. This exceptional scaling for the qubits has a significant impact on the time computer operations will take. For example, let's assume we have a computer with a clock speed of 3 GHz. It could perform 3 billion operations per second. Let's also assume one operation on one bit for qubit can be done in one clock cycle. This huge scaling potential is what's motivating the development of quantum computers. A bit has to be able to have its settings of zero or one, set, or changed, and have these settings persist over time. Its settings must also be detectable. In classical computers, bits are made of transistors. For a transistor, the absence of a voltage on its control line stops current from passing through making it off or equal to zero. An applied voltage will trigger a current making it on or equal to one. These values are easily set, changed, and read, and once set they persist for as long as needed. There are a number of ways to create quantum bits, atoms, photons, superconductors, etc. Silicon spin qubits are also promising. A number of companies are working on them. As of early 2022, Intel appears to have the lead with a 26 qubit product. The long term goal is to reach a million. To understand how quantum superposition and entanglement are used, we'll construct a quantum computer out of two electron spin qubits. We start with three layers of silicon. The yellow layer in the middle is made of stretched silicon. It is actually stretched. The distance between the atoms is increased, making it easier for electrons to move around. Electrons in this layer will not move up or down into the more compressed silicon without a push. On top, we construct an electrically controlled lattice of gates. Negatively biased electrostatic gates in gray and positively biased gates in brown are organized to create two energy wells capable of holding two electrons in place. These two wells are called quantum dots. On top of these two components, we add a micro-magnet to create a tapered magnetic field. This field couples electron spins to the electric field set up by the gates. With this configuration, we can introduce two electrons. The states of these electrons are controlled by microwave and voltage pulses applied to the gates by the quantum computing unit. For example, electron spin can be aligned with a magnetic field in the up or down direction. And the two electrons can also be put into an entangled state by managed exchange interactions across the Coulomb barrier between them. The Hadamard gate is one of the most important operations. It takes a single qubit in a base state as input and outputs a qubit in a superposition state with equal coefficients. The control NOT gate or C NOT gate is heavily used. It takes in two qubits and only flips the second qubit called the target from zero to one or one to zero if the first qubit called the control is a one. Otherwise it leaves the target unchanged. Having advantage of the fact that the up equals zero state has a slightly lower energy than the down equals one state, a series of microwave pulses will flip the target qubit only when the controlled qubit had enough energy to have measured it as in the one state. And it is done without reading the controlled qubit. Like changing a photon's polarity, this can be done for any number of entangled qubits without disturbing the entanglement state. This is the case for all quantum gates. Measurement is a special type of operation done on qubits at the end of a series of gate operations to get the final values. In a magnetic field, electrons have two discrete energy levels based on their spin. Detecting these energy levels tells us what the spin was. Compared to the gates, measurement is irreversible and hence is not actually a quantum gate. Its execution removes the qubit from its entangled superposition state into a zero or one. The results of a measurement are always stored in classical computer bits for analysis. Combinations of quantum gates are called quantum circuits. These combine to execute computer instructions. This is a two electron spin qubit quantum computer. Quantum dot states are extremely fragile. The slightest vibration or change in temperature can cause them to tumble out of superposition causing errors, lots of errors. That's why in order to best protect qubits from the outside world, they are housed in supercooled fridges and vacuum chambers. This makes them very expensive compared to classical computers. Because of this it is expected that quantum computers will only work on those problems that need a gigantic number of bits, jobs like factoring extremely large numbers. Schrodinger pointed out that superposition and entanglement are the two primary characteristics of the quantum world. And whenever particles find themselves close together, they will become entangled. Between unobservable quantum states.