Contribution of Quantum Computing in Advance Computing with Practical Challenges

— Quantum computation is a nanotechnology that is getting quite a buzz in the field of technology over past few years because of in polynomial time its ability to solve exponentially hard problems. Alternatively, an important feature is having an ultra-low power dissipation and high frequency. CMOS technologies have some drawbacks in Nano-scales. There are 7 billion transistors in NVidia graphics chip so it is simply understood that it is about to achieve subatomic level. This paper explains the concept of quantum computing and quantum universal gates are also discussed in order to provide a good foundation for this emerging technology. This paper also covers the review of the current experimental status of quantum computers and various challenges faced by this technology are also discussed .


INTRODUCTION
Ever since, the Quantum mechanics is discovered, scientists have now found an alternative mechanics to develop computer system. Quantum mechanical phenomena behave differently than the classical physics. In 1982, beginning of the story of quantum computing the physicist Richard Feynman considered simulation of quantum-mechanical objects by other quantum systems. Though the unusual strength of quantum computation was not really measured until the 1985 when David Deutsch of the University of Oxford published a critical theoretical paper in which he described a universal quantum computer. After the Deutsch paper, the search was to find something interesting for quantum computers to do. Richard Feynman was the first one to ask what effect this has on computation [1]. In traditional computer it is possible to implement each operation by logic gates. For example universal gate is operationally complete by itself. In classical computing NAND gate is used by combining and recombining such set of operations, it is possible to implement each operation functionally complete. Quantum gate acts on the n-qubit as 2 n x 2 n unitary matrix. When the input is fetched through gate, the vector of the system state is multiplied by the matrix of the gate and producing new vector of the state. Quantum gate operations are reversible because the gate is unitary matrix and one to one mapping between inputs and outputs. In this formal review, the basic quantum gates will be explained and why they possess universal property. In 1994 Peter Shor from AT&T's Bell Laboratories in New Jersey proposed the first quantum algorithm. This algorithm can perform efficient factorization. This was known as a `killer application' at that time and it was something that only a quantum computer can perform. Difficulty of factorization is used in security of many widespread methods of encryption; for example RSA. Quantum computing implementation faces problems related to de-coherence, error correction, and implement ability of gates.

A. QUANTUM COMPUTING OVER CLASSICAL COMPUTING
Classical computation theory became prominent after Church and Turing made their study into the characteristics of computability in the year 1936 [2]. Logic gates and logic circuits took an important part in the theory of computation.
In the passage of time the execution, sophistication and optimal structure of classical logic circuits have been developed [3]. The classical world-view works fine at the everyday level and much of modern engineering relies on this but there are things at the macroscopic level that cannot be comprehend. Using classical physics, these include the color of a heated object, the existence of solid objects. So where does classical physics come unstuck?
The classical computation was implemented using variables 0 and 1 Boolean logic [4].The physical analysis of 0 and 1 is the voltage On/Off. But when it comes to quantum computation, quantum mechanics provides a new protocol that works beyond than the classical paradigm of Boolean logic [5][6]. The basic variable in quantum computing is a quantum bit or qubit which is represented as a vector in a two dimensional complex Hilbert space.

B. QUANTUM COMPUTATION-UNITARY OPERATIONS
Quantum computation is a reversible process, logically and physically. All reversible Boolean functions are some particular cases of unitary transformations. For example, if a two-bit function mapped |01〉to |11〉, the inverse would map |11〉back to |01〉, which is what we would get if we took the transpose of the corresponding unitary matrix. Therefore, loosely speaking, any problem that can be simulated classically can also be simulated quantum mechanically. However, the new features of the quantum computersuperposition, interference between qubits, entanglement, and measurementallow quantum computers to solve some certain problems, like the factoring and database search problems, exponentially faster than can be done on any classical computer. According to the axioms of quantum mechanics, the evolution of a closed quantum system is described by a unitary transformation. If a system is in state |ᴪ| at time t 1 is related with its state |ᴪ'| at time t 2 by a unitary operator U.
In the study of quantum computation and quantum information, Pauli matrices are four extremely useful unitary matrices. The matrices, and their notations, are shown in Figure 1 In classical computer there are number of multiple input gates except NOT gate. These are AND, OR, NAND, NOR etc. There we have also multiple input gates in Quantum computer. In 1955, a number of researchers declared that the two-qubit gates are basic gates [11][12] Controlled NOT-Gate (CNOT) In CNOT Gate we have two input qubit as Control qubit and Target qubit.
In CNOT Gate if control qubit is set to 0 then target qubit remains unchanged. The target qubit is inverted only if the control qubit is set to 1.

D. UNIVERSAL GATE
In classical computer NAND and NOR gates are universal gate. Using these universal gates we can derive any other logic gate. Similarly we can derive classical NAND gate using Quantum gate with reversible property.

Controlled-Controlled Not-Gate (CCNOT) As Classical NAND Gate-
In CCNOT-GATE there are two control bits and a target bit.
When both the control bits input is |1 then the target bit is inverted.
Control bit | A Control bit | B Target bit | C Figure. 3 Table 3-TRUTH TABLE OF CCNOT-GATE   Above truth table have the input as |ABC where input A and B is the control bits and input C is the target bit. The input bits B and C from Row 5-8 and there corresponding output bit C have the similar bits as the NAND in classical computer and the other bits provide reversibility.

III. IMPLIMENTATON OF QUANTUM COMPUTER
The implementation of quantum hardware is remarkable, obtained from atomic physics, quantum optics and electron magnetic resonance spectroscopy, in superconducting device physics, in electron physics, and in microscopic and quantum dot research.

 Ions Implanted in Silicon
The Kane [13] experiment of phosphorus in silicon builds upon modern semiconductor fabrication, design upon known physical properties. The nuclear spin of a phosphorus atom integrated with an electron embedded in silicon under a high magnetic field and low temperature is used to design Quantum bit [14].  Solid-State Devices The design of solid-state devices involves subjecting a substrate to a sequence of physiochemical process. Solid-state device are electronic device in which electricity flows through solid semiconductor crystals (silicon, gallium arsenide, and germanium. Solid-state technology offers an appealing option for manufacturing thousands of qubits as small objects.

IV. D-WAVE QUANTUM COMPUTING HARDWARE FOR THE NEXT GENERATION
D-Wave One (128 bits) is the first-generation version of quantum computer. That world's first commercial quantum computer-Wave successively shape the hardware design of D-Wave Two (512 bits), a second-generation machine that has already been leased by customers such as Google, NASA. The technologies for hardware design used by D-Wave's quantum computing architecture explore the future approach for the researcher to build more powerful quantum computers.
A. Hardware in D-Wave's quantum computer D-Wave's quantum computing hardware based on metal loops of niobium that have tiny electrical currents running through them.

B. D-Wave's Quantum Machine
In metal loops of niobium current creates a tiny magnetic field pointing up and down by running counterclockwise and clockwise through the loop. The niobium loops become superconductors when chilled to frigid temperatures of 20 mill kelvin (-273 degrees C). Magnetic field pointing up and down property is used as qubit state 0 and 1. The low temperature makes this possible the currents and magnetic fields can enter the strange quantum state known as "superposition" that allows them to represent both 1 and 0 states simultaneously. That allows D-Wave to use these "superconducting qubits" as the building blocks for making a quantum computing machine.
C. Information Processing Quantum Annealing Each metal loops of niobium also contains a number of Josephson junction's two layers of superconductor which is separated by a thin insulating layer that act as a mechanism of switches for routing magnetic pulses to the correct locations. D-Wave has worked on a specialized technique known as "quantum annealing" a method of tackling optimization problems instead of building quantum computers using the traditional logic-gate model of computing. D-Wave starts a group of qubits in their lowest energy state and then moderately turns on interactions between the qubits, which encodes a quantum algorithm. When the qubits settle back down in their new lowest-energy state, D-Wave can read out the qubits to get the results .the result is used for solving optimization problems means finding the lowest minimum.

V. CHELLENGES IN IMPLEMENTATION OF QUANTUM COMPUTER
Quantum computing faces problems related to decoherence, state preparation, error correction, and implement ability of gates. The requirements for physical implementation of a quantum computer include long decoherence time, a stable memory, feasible state preparation for the initial state, a universal set of gate operations and capability for single quantum measurements [15].
A. Decoherence Quantum computers are extremely sensitive to interaction with the surroundings since any measurement leads to a collapse of the state function an unwanted interaction of quantum information with extraneous entities, is a hazard of quantum computing in that it destroys the information the system is processing. This phenomenon is called decoherence. It is really difficult to separate a quantum system, especially an engineered one for a computation, without it getting entangled with the environment [16]. It is harder to maintain the coherence with larger number of qubits. Decoherence cannot be reversed by redundancy coding [17] even it raise the quantum computer's vulnerability against decoherence by adding extra physical quantum bits to achieve redundancy.

B. Errors And Their Correction
Computation invariably involves errors, which are internal or externally induced. The classical computers has the capability to perform well because the non-linearly of the computation process makes it possible to eliminate small errors by redundant error correction bit. In quantum error correcting algorithms [18] consider only bit flips, phase flips or both between the relative phases but these are not all the errors that might encounter in a quantum computation. Unlike the situation in classical computing, small errors cannot be eliminated in quantum computing [19] as it is a linear process, and it rules out operations comparable to clamping and hard-limiting. The no-cloning theorem prevents us from copying an unknown state and doing additional testing on it. In initialization phase of qubit have random unitary transformation errors can occur [20]. The characteristic of errors is also need to be considered they might be component proliferation, nonlocal effects and amplitude errors. We cannot group the errors in a systematic way because in quantum register the errors can be due to a variety of reasons. In quantum arena information error is nonlocal compared to the local in classical arena and hence the concept of controlling errors using higher dimensional code word space is not realistic.

VI. CONCLUSION
Quantum computation is a nanotechnology that has a great future. New features of the quantum computer -superposition, entanglement, and measurement, allow quantum computers to solve certain problems, like the factoring and database search problems, exponentially faster than can be done on any classical computer. This paper provides the concept of quantum computing, quantum gates have been discussed with graphical and mathematical representation. This paper also provides quantum computer implementation techniques designed by D-Wave (first commercial Quantum computer) briefly. The challenges with decoherence and error correction in Quantum computer also discussed. Hence using quantum computer can overcome the irreversibility nature of classical computation and avoid information loss. This technology opened a number of great opportunities for researcher with some challenges to change the whole scenario of computation.