## Quantum and classical computers

### The design of a quantum algorithm needs to take into account what you can do such that measurement will correspond to what you want.

Updated on 19 Dec 2021, 10:23 pm

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The world of quantum physics is bizarre and most difficult to understand. Think of the formulation like ‘’Heisenberg’s Uncertainty Principle’’ where you cannot simultaneously measures the momentum and position of any particle or thought experiments like ‘’Schrodinger’s cat’’, where a cat is kept inside a sealed box with radioactive substance and the cat is simultaneously both alive and death or phenomena like quantum entanglement where two quantum particles can remain somehow entangled irrespective of the distance between them. All these properties/phenomena sound absurd but since quantum mechanics is a science of probabilities, it makes the absurd look possible. There are many ways to understand why quantum mechanics is hard to simulate. Perhaps the simplest is to see that quantum theory can be interpreted as saying that matter; at a quantum level is in a multitude of possible configurations (known as states).

Unlike classical probability theory, these many configurations of the quantum state, which can be potentially observed, may interfere with each other like waves in a tide pool. This interference prevents the use of statistical sampling to obtain the quantum state configurations. Rather we have to track every possible configurations, a quantum system could be in if we want to understand the quantum evolution.

Quantum mechanics was developed between 1900 and 1925 and it remains the cornerstone on which chemistry, condensed matter physics and technologies ranging from computer chip to LED lighting ultimately rest. The conventional method of computing is the most popular method for solving the desired problems with the estimated time complexities.

Algorithms of searching, sorting and many other are there to tackle daily life problems and are efficiently controlled over time and space with respect to different approaches. Certainly we use bits (either 0 or 1) for storing the information and with the help of these 2 bits; we calculate Giga to Tera to Petabytes of data and even more with quite unparalleled efficiency. Our CPU calculates at average 2.4 GHz apparently, it looks like that all combinations are calculated simultaneously but of course they are distinct from each other and CPU calculates one at a time. The fact is that our CPU calculates each combination one at a time. Here arises a big and advanced research question- can all of them be simultaneously calculated at once without having any multiprocessors? To answer this crazy question, Quantum Computing came into the picture.

Quantum Computers were proposed in the 1980s by Richard Feynman and Yuri Manin. The intuition behind quantum computing stemmed from what was often seen as one of the greatest embarrassments of Physics: remarkable scientific progress faced with the inability to model even simple systems. In 1976, Roman Stanislaw ingarden of Nicolas Copernicus University in Torun, Poland published one of the first attempts at creating a quantum information theory. In 1980, Paul Benioff of the Argonne National Laboratory published a paper describing a quantum mechanical model of Turing machine or classical computer, the first to demonstrate the possibility of quantum computing. In 1981, in a keynote speech titled ‘’ Simulating Physics with Computer ’’Richard Feynman of the California Institute of Technology’’, argued that a quantum computer had the potential to simulate physical phenomena that a classical computer could not simulate. That was the first time the term Quantum Computer was arguably introduced. Following that, in 1994, Peter Shor of Bell Laboratories, developed a quantum algorithm for factoring integers that has the potential to decrypt RSA-encrypted communications, a widely used method for securing data transmission.

Quantum computing is the branch of computer science that is based on the principle of the superposition of matter and quantum entanglement and uses a different computation method from the classical one. Quantum computers look very different from anything we’ve associated with computers for decades. The technology is centred on the use of quantum bits or qubits, units of information, which represent zeros or ones, qubits, can represent both at the same time. At a very basic level, a high voltage on a transistor or a state represented a 1 and a low voltage represented by a 0. And that is how bits are constructed. Quantum Bits or Qubits on the other hand are constructed by the superposition or entanglement of quantum particles and hence making quantum computing fundamentally different from classical computing.

Qubits store 2 raised to power N numbers, which means that if a Qubit is added (N becomes N +1), the storage doubles, thereby making the growth exponentially. This translates into significant computing power thereby making Quantum computing much more powerful than classical computing. There are logic gates, as there are in a classical computer but they have a fundamental different. Quantum logic must be non-dissipative. Therefore you cannot have the classical three-port logic gates such as AND, OR etc. Instead you need a non-dissipative gate-type such as the four-port controlled NOT (CNOT) gate. It only takes a minimal set of quantum logic gates to build a universal quantum computer which is similar to classical logic needed for a universal Turing machine. Just as a quantum computer can store multiple numbers at once, so it can process them simultaneously. Instead of working in serial, it can work in parallel.

Only when you try to find out what state it’s actually in at any given moment does it collapse into one of its possible states and that gives you the answer to your problem. A quantum logic gate acts on a quantum state to transform it according to some conditions. The condition could be a local setting or it could depend on the state of another qubit. Such a conditional logic is very important because quantum information can interact and consequently affects the evolution of the entire quantum system.

There is couple of reasons for this. Unlike classical computer, quantum computers are not easy to construct. Building a quantum computer simply means building complex networks of logic elements. Quantum mechanics is deterministic if we exclude measurement. That means only future quantum state can be predicted based on the present state. The design of a quantum algorithm needs to take into account what you can do such that measurement will correspond to what you want. This is the domain of quantum algorithm design. Computer scientists believe the technology needed to create a practical quantum computer is years away. Quantum computers must have at least several dozen qubits to be able to solve real –world problems.

Sanjenbam Jugeshwor Singh

Faculty, JCRE Global College, Imphal, Manipur. The writer can be reached at sjugeshwor7@gmail.com