Quantum computers information processing

It was reahzed quite some decades ago that the amount of information accumulated by chemists can, in the long run, be made accessible to the scientific community only in electronic form in other words, it has to be stored in databases. This new field, which deals with the storage, the manipulation, and the processing of chemical information, was emerging without a proper name. In most cases, the scientists active in the field said they were working in "Chemical Information . However, as this term did not make a distinction between librarianship and the development of computer methods, some scientists said they were working in "Computer Chemistry to stress the importance they attributed to the use of the computer for processing chemical information. However, the latter term could easily be confused with Computational Chemistry, which is perceived by others to be more limited to theoretical quantum mechanical calculations. 

Two properties, in particular, make Feynman s approach superior to Benioff s (1) it is time independent, and (2) interactions between all logical variables are strictly local. It is also interesting to note that in Feynman s approach, quantum uncertainty (in the computation) resides not in the correctness of the final answer, but, effectively, in the time it takes for the computation to be completed. Peres [peres85] points out that quantum computers may be susceptible to a new kind of error since, in order to actually obtain the result of a computation, there must at some point be a macroscopic measurement of the quantum mechanical system to convert the data stored in the wave function into useful information, any imperfection in the measurement process would lead to an imperfect data readout. Peres overcomes this difficulty by constructing an error-correcting variant of Feynman s model. He also estimates the minimum amount of entropy that must be dissipated at a given noise level and tolerated error rate. 

Here we will focus on electron spin qubits and thus we will not be discussing NMR quantum computing, where molecules played a key role in the early successes of quantum information processing. 

Entanglement is the main resource of quantum information processing, without which quantum computation will not be faster than its classical counterpart [8] and quantum communication protocols will not work [113-115]. Moreover, as shown... 

The first volume contained nine state-of-the-art chapters on fundamental aspects, on formalism, and on a variety of applications. The various discussions employ both stationary and time-dependent frameworks, with Hermitian and non-Hermitian Hamiltonian constructions. A variety of formal and computational results address themes from quantum and statistical mechanics to the detailed analysis of time evolution of material or photon wave packets, from the difficult problem of combining advanced many-electron methods with properties of field-free and field-induced resonances to the dynamics of molecular processes and coherence effects in strong electromagnetic fields and strong laser pulses, from portrayals of novel phase space approaches of quantum reactive scattering to aspects of recent developments related to quantum information processing. 

Part I Role of entanglement in quantum computing and information processing... 

ROLE OF ENTANGLEMENT IN QUANTUM COMPUTING AND INFORMATION PROCESSING... 

Group theory aspects of information processing in quantum ensembles is discussed in the first paper of this chapter. It presents the basic concepts of the information processing and teleportation, based on the group theory approach. It also addresses the question whether the information processing can be carried out when the ensemble of qubits is not in a pure quantum state, being subject to thermalization prior to the quantum computation. 

In this article I will discuss entanglement and its role in quantum information processing - especially, but not exclusively in the context of quantum computation. 

In the context of quantum information, the effects of interactions with the environment, known as "quantum errors", may render information storage and processing unreliable [Preskill 1998 (c) Nielsen 2000]. Since Shor s demonstration that error-correcting schemes exist in quantum computation [Shor 1995], a general framework of quantum error-correction has been built upon the formalism of quantum operations. The idea of quantum error-cor-... 

Approximate versions of the translational EPR state, wherein the -function correlations are replaced by finite-width (Gaussian) distributions, have been shown to characterize the quadratures of the two optical-field outputs of parametric down-conversion, or of a fiber interferometer with Kerr nonlinearity. Such states allow for various schemes of continuous-variable quantum information processing such as quantum teleportation [Braunstein 1998 (b) Furu-sawa 1998] or quantum cryptography [Silberhorn 2002], A similar state has also been predicted and realized using collective spins of large atomic samples [Polzik 1999 Julsgaard 2001]. It has been shown that if suitable interaction schemes can be realized, continuous-variable quantum states of the original EPR type could even serve for quantum computation. 

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