# Quantum Computing Since their invention, computers have been getting smaller and more powerful. (We’ve all heard about Moore’s Law which is the observation that the number of transistors in a dense integrated circuit doubles about every two years) But soon we may reach its physical limits as the size of transistors reaches close to a few atoms.  A transistor is the simplest form of a data processor in a computer. It is basically a switch which either does or does not allow a flow of current. This on/off mechanism can be used to represent bits – 1/0 respectively and a combination of several bits can represent more complex information. A transistor working as a switch (https://www.build-electronic-circuits.com/how-transistors-work/)

#### Need for Quantum Computers:

As transistors reduce to the size of a few atoms, we begin to face some issues. Other than the increasing difficulty in producing such small structures, it is the fact that the laws of physics do not work as we expect at such a minuscule scale. Particles of these sizes tend to follow the weird rules of quantum mechanics and transistor would fail its most basic task of acting as a switch to control the flow of current as some the electrons may just jump to the other side using a process called quantum tunnelling. To solve these issues, we are trying to instead use these unusual quantum properties in our favour by building quantum computers. How Quantum Tunnelling works (https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Quantum_Mechanics/02._Fundamental_Concepts_of_Quantum_Mechanics/Tunneling)
• The properties we could take advantage of are:
• Superposition
• Quantum Entanglement

#### The Qubit:

In normal computers, bits are used as the smallest unit of information whereas, in quantum computers, qubits are used. A qubit can also be set to one of any two states. The states of a qubit could be the spin of an electron in a magnetic field or the horizontal or vertical polarisation of a photon Bit vs Qubit (https://blogs.umass.edu/Techbytes/2017/06/28/quantum-computers-how-google-nasa-are-pushing-artificial-intelligence-to-its-limit/)

Unlike a regular bit which can either be in one of the two states (0 or 1), in the quantum world, a qubit can be in any proportion of both states at once because of quantum superposition. But as soon as you test/observe it, the wave function collapses and it has to choose one state. Thus as long as the qubit is unobserved, the qubit is in superposition with probabilities for either state and we can not predict which state it is in. Superposition of qubits is what gives quantum computers their inherent parallelism.

#### The advantage:

Quantum superposition is a huge advantage for us as it can be clearly seen in the following example. Consider 4 classical bit, they can be in either 1 of 16 possible combinations but 4 qubits in superposition can be in all 16 combinations at once. This number grows exponentially as the number of qubit increases. Note: A quantum computer stores the same 2^n states that the classical computer stores. The difference is that the quantum computer stores a linear superposition of those states, where the classical computer can only store one of those states at a time.

#### Quantum Gate:

Qubit manipulation is very hard and we can’t use normal logic gates, thus quantum gates are used. A quantum gate manipulates superpositions, rotates probabilities and produces another superposition as its output. For example, in a photon its spin in a magnetic field where polarization represents the value and we can test the value of a photon by passing it through a polarizing filter, and it will collapse to be either vertically or horizontally polarized (0 or 1). #### The Process:

Consider a quantum computer has a set of qubits encoded with some information( ie – we currently know their quantum states). The quantum computer passes applies the qubits to quantum gates, thus entangling and manipulating them to be in a superposition of all possible states. It then measures/observes the output by collapsing the superposition to the actual sequence of 0/1s

One thing to notice is that although possible combinations existed at once before observation, ultimately we will only get 1 of these possible results and it may only probably be the one we require. So this requires us to try many times till we get the desired result. This may seem counter-intuitive as I have built up quantumm computer to be the future but we can’t even be sure of what result we are going to get. In spite of this, if we manipulate the results effectively, this method will still be much more powerful than regular computing.

#### Applications:

Quantum computers can have many potential applications in various fields such as:

• Cryptography – It could because of its extra computational power, simply brute-force passwords in no time. It would allow the completion of various cryptographic tasks that are considered classically impossible. For example, it would be impossible to copy data encoded in a quantum state as the very act of reading data encoded in a quantum state, changes the state. This could be used to detect eavesdropping in quantum key distribution.
• Chemical Simulations – can untangle the complexity of molecular and chemical interactions leading to the discovery of new medicines and materials.
• Artificial Intelligence – Making AI and ML much more powerful when data sets are very large, such as in searching images or video.