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Capabilities and Limitations of Quantum Computers

Umar Farooq

*Virtual University of Pakistan*

*Abstract* —Computers are evolving and growing for the last half a century. They are becoming smaller, faster and powerful but this growth has a limit. A new research has begun which will result in an entirely new kind of computer based on the Quantum physics rules. The quantum computer working on the Quantum physics rules will be able to break the security codes on which our modern computer infrastructure is running. The Quantum computers can solve many problems which are not solvable by the current computer systems. The large scale production of the quantum computers is near, which will result in many opportunities and every field of life will be affected.

*Keywords* — Quantum Computers, Qubit, Capabilities and limitations, Computation, Complexity

## I. INTRODUCTION

This is a long debate that what a quantum computer can do and what they can’t. Quantum computers can simulate the details of biological molecules, can work on intra galaxy research, and can do whatever an ordinary computer can’t. So this type of computer will become an important part of future progress in chemistry, physics, mathematics and engineering. Moreover the quantum computers will open new era of research, development and progress. Experts also see the quantum computers as threat because the computers of this magnitude will break all the cipher codes which are working in our ecommerce and computer systems.

Quantum computer works differently from the traditional computers. The traditional computers can compute only one transaction at a time, it is their speed by which they compute, they seem doing multiple transactions. On the other hand the quantum computers can perform multiple transactions at a time with the speeds more than traditional computers The fundamental building block of a quantum computer is Qubit as shown in the diagram.

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Fig1. The Bloch sphere is an illustration of a Qubit, the basic building block of quantum computers.[16]

Qubits are different from the bits and can hold much more information than the later. An ordinary bit can compute or store 0 or 1 but a Qubit can work in between the values of 0 and 1.

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Fig: 2 Quantum bit

Qubits are made up of two things first is the controlled particles and second one is the means of control.

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Fig: 3 Qubits[16]

The following table will show the comparison between the classical computing and quantum computing.

TABLE I COMPARISON OF TRADITIONAL AND QUANTUM COMPUTING

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## II. RELATED WORK

A Large 3-Dimensional cluster lattice is used as resource for quantum computer information processing and it is also used in mainframe computers. The idea can lead to a construction of 7.5 billion photonic chips Quantum Computer. [1]

The computer models which are motivated by physics have both physical as well as computational interpretation and it relates both of the systems.[2] Quantum mechanics represent most of our understanding related to microscopic physical phenomena and the operations of a computer should also be in form of quantum mechanics.[3]

Quantum computing is still evolving and no one knows when its requirements will be met, weather some tradeoffs will be made or research on entirely new path will begun.[4]

It is proposed that how explicit microscopically models of quantum computer register can be constructed, and sphere models bonded with the first and second kind can be used to represent quantum register.[17]

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Figure: 4 Shows a Quantum Charged Coupled Device (QCCD)[4]

Ion traps can be used as the building block of quantum computers. The basic implementation of the system is demonstrated but there are many problems to make the system scalable to large QBITS.[4]

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Figure: 5 Configuration of static and radio frequencies for QCCD.[4]

## III. HISTORY OF QUANTUM COMPUTING

The most advanced quantum computers till 2007 have yet managed more than 16 QBITS, which means that a useful quantum computer is far away. Quantum computers and the quantum theory have made some advancement in recent years.

In 1998 Los Alamos and MIT researchers tried to use a single Qubit in three nuclear spins in each alanine molecules in liquid form.

In 2000 March, Los Alamos National Laboratory scientists announced that they have succeeded to develop a 7-qubit quantum computer with a single drop of liquid.

In 2001 Scientists from IBM and Stanford University successfully experienced Shor's Algorithm on a quantum computer. This Algorithm is used for finding the prime factors of numbers. They used a 7-qubit computer to calculate the factors of 15. The computer correctly calculated that the prime factors were 3 and 5.

In 2005 Quantum Optics and Quantum Information Institute (University of Innsbruck) claimed that scientists are successful in creating the first Qubyte, or series of 8 Qubits, in which ion traps were used.

In 2006 Scientists in Waterloo and Massachusetts found methods to control on a 12-qubit system. Quantum computation related control becomes more complex with the increase of Qubits.

In 2007 Canadian start up company D-Wave created a 16- qubit quantum computer. The computer solved many pattern matching problems.

## IV.QUANTUM LANGUAGES

Quantum Computation involves some new concepts which are related to the movement of the particles like entanglement. So the Quantum programming language should be able to deal with these concepts.[6]

A quantum language QML is introduced which provides some practical semantics which could be understood by the quantum logic gates.[7]

The standard semantics are not compatible with the Quantum Mechanics due to its limitations but using semantic realism this difference can be removed. One can generate a Quantum Language which contains the Lindenbaum-Tarski algebra which is compatible to Quantum Language.[8]

Conventionally the quantum languages are defined at the low level and which discourages the normal programming practices so a new high language is described which has proper properties of a language.[9]

The Quantum Languages are continuously evolving and new dimensions are being added resulting in more structures and high level languages but still there is lot to do.

## V. HIGH PERFORMANCE QUANTUM COMPUTING

Various problems which need intensive computation will be solved by high performance Quantum computing. The High performance computing will get sustainable performance and will result in 1x 10[15] floating point operations per second.[18] The super computers having powers of petascale during the processing of some applications can reach to one quadrillion FLOPS.[18]

In the paper a parallelization solution is implemented, simulating the quantum computers related memory bounding problem, the technique is based on hybrid handling of MPI and OpenMP communication which used different threads.[19] An algorithm built for the purpose and which uses 1 to 2 QBIT gates works very well with IBM p690 having 1024 processors and 3TB of memory.[19]

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Fig. 6 Partitioning of 3d Global lattice for a HPQC mainframe.[20]

The global lattice shown above measures 4000 x 500,000 unit cells and it is prepared using approximately 7.5 x 10[9] photonic chips.[20]

[20] Have initiated the idea of a High Performance Quantum Computer, the quantum computer uses a 3 dimensional cluster lattice for processing multiple user related information.

## VI.CAPABILITIES AND LIMITATIONS

Quantum computers have lots of capabilities and they can solve problems with no time which a classical computer can take years to solve. Quantum computers can perform multiple transactions at one time as compared to the classical computers.

On the other hand the quantum computers are still evolving and they are not as much scalable yet. We should not expect magical things from quantum computers and we should not be too optimistic regarding quantum computers because they might let us down.[14] Noise distortion can lead to information corruption. Different implementations of Quantum Computers have been developed but they are of limited use and simply only for certain type of demonstration.

**[...]**

- Quote paper
- Umar Farooq (Author), 2014, Capabilities and Limitations of Quantum Computers, Munich, GRIN Verlag, https://www.grin.com/document/272738

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