Quantum information processing

Three immediate issues face any chemist attempting to design molecules for QIP understanding QIP at an appropriate level, designing 

The second algorithm is the Shor algorithm, ° which demonstrates that a quantum computer could factorise very large numbers into primes much more quickly than a classical computer. This immediately becomes important because modern cryptography is dependent on the fact that classical computers are very good at multiplying many primes together to 

More recent proposals have involved MNMs which have S = 1/2 ground states, with the two microstates l- -l/2 and l-l/2 acting as the 0 and 1 that will undergo quantum superposition. Most work has been published on CryNi rings, ° with other work on V15 poly-oxometallates (POMs) ° and on a mixed valence polyoxometallate [PM012O40 this last case contains two S = 1/2 centres on 

This leads to a splitting of some of the EPR transitions in the dimer, which is clear evidence for the superposition. As a physical demonstration of the principle this is extremely beautiful work however, switching the superposition on and off requires recrystallisation of the Mn4 cages to achieve a different crystal packing. 

More recently, Lehmann et proposed a scheme where electrical 

---

Abstreiter, G., Finley, J. J. and Zrenner, A. (2005) Recent advances in exciton-based quantum information processing in quantum dot nanostructures. New J. Phys., 7, 184-1-184-27. 

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. 

To illustrate an application of nonlinear quantum dynamics, we now consider real-time control of quantum dynamical systems. Feedback control is essential for the operation of complex engineered systems, such as aircraft and industrial plants. As active manipulation and engineering of quantum systems becomes routine, quantum feedback control is expected to play a key role in applications such as precision measurement and quantum information processing. The primary difference between the quantum and classical situations, aside from dynamical differences, is the active nature of quantum measurements. As an example, in classical theory the more information one extracts from a system, the better one is potentially able to control it, but, due to backaction, this no longer holds true quantum mechanically. 

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... 

Efficient and selective excitation of electronic target states in atoms and molecules lies at the heart of photochemical applications (see corresponding references in Section 6.1) as well as quantum information processing [102, 103]. Here we demonstrate the potential of SPODS, introduced in the previous sections, for ultrafast electronic switching in a multistate model system. In the previous... 

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. 

Monroe, C. Quantum information processing with atoms and photons. Nature (London) 416,238-246... 

Role of quantum statistics. When considering complex systems by methods of statistical physics, one operates with their time-dependent distributions. In fermionic systems (see Yu. Ozhigov), statistical requirements imply that we must replace the independent-particle description by a quasiparticle formalism for quantum information processing. Effects of statistical fluctuations on coherent scattering processes (see M. Blaauboer et al.) suggest the need for furher exploration of the role of statistics on the dynamics of entangled systems. 

Abstract I briefly review the status of entanglement in quantum information processing. 

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

I hope I have given you some glimpse into one of the more subtle areas in quantum information processing. Both in terms of some of the progress that we have made in recent years towards understanding when and how entanglement really matters, and in terms of the still open questions. 

---