# Quantum Computers

## FIELDS OF STUDY

Computer engineering

## ABSTRACT

Quantum computers are computing devices that can theoretically have computing power that is many orders of magnitude greater than that of conventional computers. The basic unit of data in a quantum computer is the quantum bit, or qubit, that is the quantum state of electrons in an atom. Qubits can theoretically exist in several superposed states simultaneously, enabling them to carry far more information than id available using conventional two-state bits. The mathematical basis of the proportionality of qubit states is similar to that of the input weights of neural networks. There has been some successful development of quantum computer technology, but a great deal of research and development remains to be done before quantum computers become viable as a mainstream technology, and there are arguments as to why this eventuality can never be achieved.

PRINICIPAL TERMS
• Constraints: limitations on values in computer programming that collectively identify the solutions to be produced by a programming problem.
• Control unit design: describes the part of the cpu that tells the computer how to perform the instructions sent to it by a program.
• Entanglement: the phenomenon in which two or more particles' quantum states remain linked even if the particles are later separated and become part of distinct systems.
• Logic implementation: the way in which a cpu is designed to use the open or closed state of combinations of circuits to represent information.
• Quantum bit (qubit): a basic unit of quantum computation that can exist in multiple states at the same time, and can therefore have multiple values simultaneously.
• Quantum logic gate: a device that alters the behavior or state of a small number of qubits for the purpose of computation.
• state: a technical term for all of the stored information, and the configuration thereof, that a program or circuit can access at a given time; a complete description of a physical system at a specific point in time, including such factors as energy, momentum, position, and spin.
• superposition: the principle that two or more waves, including waves describing quantum states, can be combined to give rise to a new wave state with unique properties. This allows a qubit to potentially be in two states at once.
A CONTROVERSIAL SUBJECT

Bits are controlled by the ‘clock speed’ of the particular computer, and is defined as being either the high state or the low state for the finite period of time determined by one clock cycle. A quantum bit (a qubit), however, is defined by the particular quantum state of the atom and can have several values at the same time, determined by the superposition or linear combination of several different states of the same atom. The qubit therefore exists as a quantum electronic state rather than as a macroscopic property. A transistor gate functions simply by switching the flow of electrons to change the state of a bit according to boolean relationships. A quantum gate, however, uses the quantum property of entanglement to change the state of a qubit. This is a very strange action that is not explained outside of quantum mathematics. Quantum entanglement occurs when two particles stay connected in a way such that any action on one particle equally affects the other particle, even when they are separated by great distances. Digital computer information is coded as strings of bits. In a quantum computer, the elements that carry the information are quantum states. This does not include just the ground and excited states, but also linear combinations and superpositions of states. This allows making use of quantum parallelism techniques that would be far more powerful than even the massively parallel techniques of digital computing. A fair question at this point would be ‘what are quantum states?’. In 1897, electrons and protons were positively identified as component particles of atoms, and though it was theorized at the same time, the existence of the neutron was not conclusively demonstrated until 1932. In the early 1900s, this model of atomic structure was refined using quantum mechanics to account for and describe the energies of electrons in atoms. Quantum mechanics describes the energy states that electrons in atoms are allowed to occupy. Each of these energy levels and the corresponding behavior and distribution of electrons is thus termed a ‘quantum state’. An electron can move from one quantum state to another by absorbing or emitting a ‘quantum’ of the appropriate energy. A quantum is the minimum ‘particle size’ of energy required for a specific change of quantum state, and each atom has numerous possible quantum states. By using quantum states instead of the binary states of transistors, the amount of information that can be carried through the system increases exponentially with each additional state.

QUANTUM COMPUTERS AND NEURAL NETWORKS TYPES OF QUANTUM COMPUTERS AND THE PROGRESS SO FAR

Research into quantum-scale phenomena is time-consuming and expensive. Nevertheless, many agencies in government and industry are actively pursuing research into the development of quantum computers, for several reasons. The most important reason, of course, is the vastly superior speed of computation that quantum computers should exhibit, enabling them to solve computational problems that cannot be solved by conventional computers. Quantum computers would be far more effective at dealing with and analyzing large quantities of data, as well as for service in the field of cryptanalysis and encryption. As might be expected, the different theoretical avenues by which quantum states can be described have given rise to several different approaches to defining qubits and achieving quantum computing devices. These include

• Use of superconducting josephson junctions
• Use of the internal electronic state of trapped ions
• Use of the internal electronic state of neutral atoms in an optical lattice
• Use of quantum dots and either the spin state or position of trapped electrons
• Use of nuclear spin state in nuclear magnetic resonance
• Use of electron spin
• Use of nuclear spin of phosphorus donors in silicon
• Use of bose-einstein condensates
• Use of light modes through various linear elements
• Use of carbon nanoparticles and fullerenes

Each of these methods has had some success, in accord with the principles of scientific investigation that seek to test s single hypothesis by the strict control of variables. None, however, have yet indicated a general or universal method of generating qubits for the creation of a quantum computer. In recent years, there has been some successful commercialization of quantum computer technology by the canadian company d-wave. The device does function using quantum properties, but has yet to achieve greater performance than is available using classical computers. Nasa revealed the \$15 million device publicly in december, 2015. More recently, in august, 2016, computer scientists at university of maryland constructed the first working quantum computer that is capable of being reprogrammed.

THE MAJOR PROBLEM FACING THE DEVELOPMENT OF QUANTUM COMPUTERS

That a quantum computer that functions at least partially as theorized has been built is perhaps verification of a paraphrased physical law that says ‘for every theory there is an equal and opposite set of empirical results’. On one hand, the strong investment into the development of quantum computers supports the view that quantum computer technology will eventually provide computing power that is “hundreds of orders of magnitude greater than that of conventional computers.” On the other hand, that such an increase has not been indicated by the performance of quantum technology so far supports the view that quantum computers will never be feasible. The principle argument for this latter view is based on the concept of ‘noise’. In conventional computers, that function on a macroscopic scale, electronic interactions from both internal and external sources produce a constant background of low energy random signals occasionally punctuated by individual events of higher energy. This is electrical ‘noise’. On the macroscopic scale, this interference can be readily filtered our or minimized such that the desired signals for data manipulation are essentially clean signals. On the quantum scale, however, there is no way to ‘filter out’ the quantum effects that arise from noise. The other major argument against the increased power of quantum computers is that even though the qubits of the process can be simultaneously in numerous states, their value can only be read as one of two states, making them no better than ordinary bits in terms of computing power. The reality is that, at the present time, neither argument for or against quantum computers has been demonstrated successfully, and a great deal of research is still required to either bring quantum computers out of the realm of theory and into the mainstream of technology or relegate them to the realm of failed scientific concepts.

—richard m. Renneboog m.sc.

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