Cold Trapped Ions as Quantum Information Processors

In this tutorial we review physical implementation of quantum computing using a system of cold trapped ions. We discuss systematically all the aspects for making the implementation possible. Firstly, we go through the loading and confining of atomic ions in the linear Paul trap, then we describe the collective vibrational motion of trapped ions. Further, we discuss interactions of the ions with a laser beam. We treat the interactions in the travelling-wave and standing-wave configuration for dipole and quadrupole transitions. We review different types of laser cooling techniques associated with trapped ions. We address Doppler cooling, sideband cooling in and beyond the Lamb-Dicke limit, sympathetic cooling and laser cooling using electromagnetically induced transparency. After that we discuss the problem of state detection using the electron shelving method. Then quantum gates are described. We introduce single-qubit rotations, two-qubit controlled-NOT and multi-qubit controlled-NOT gates. We also comment on more advanced multi-qubit logic gates. We describe how quantum logic networks may be used for the synthesis of arbitrary pure quantum states. Finally, we discuss the speed of quantum gates and we also give some numerical estimations for them. A discussion of dynamics on off-resonant transitions associated with a qualitative estimation of the weak coupling regime and of the Lamb-Dicke regime is included in Appendix.Comment: 44 revtex pages, 23 figures, to appear in Journal of Modern Optic

Quantum Computation with Quantum Dots and Terahertz Cavity Quantum Electrodynamics

A quantum computer is proposed in which information is stored in the two lowest electronic states of doped quantum dots (QDs). Many QDs are located in a microcavity. A pair of gates controls the energy levels in each QD. A Controlled Not (CNOT) operation involving any pair of QDs can be effected by a sequence of gate-voltage pulses which tune the QD energy levels into resonance with frequencies of the cavity or a laser. The duration of a CNOT operation is estimated to be much shorter than the time for an electron to decohere by emitting an acoustic phonon.Comment: Revtex 6 pages, 3 postscript figures, minor typos correcte

Vibrational coherent quantum computation

Published versio

Practicality of Quantum Random Access Memory

Quantum computers are expected to revolutionize the world of computing, but major challenges remain to be addressed before this potential can be realized. One such challenge is the so-called data-input bottleneck: Even though quantum computers can quickly solve certain problems by rapidly analyzing large data sets, it can be difficult to load this data into a quantum computer in the first place. In order to quickly load large data sets into quantum states, a highly-specialized device called a Quantum Random Access Memory (QRAM) is required. Building a large-scale QRAM is a daunting engineering challenge, however, and concerns about QRAM’s practicality cast doubt on many potential quantum computing applications. In this thesis, I consider the practical challenges associated with constructing a large-scale QRAM and describe how several of these challenges can be addressed. I first show that QRAM is surprisingly resilient to decoherence, such that data can be reliably loaded even in the presence of realistic noise. Then, I propose a hardware-efficient error suppression scheme that can further improve QRAM’s reliability without incurring substantial additional overhead, in contrast to conventional quantum error-correction approaches. Finally, I propose experimental implementations of QRAM for hybrid quantum acoustic systems. The proposed architectures are naturally hardware-efficient and scalable, thanks to the compactness and high coherence of acoustic modes. Taken together, the results in this thesis both pave the way for small-scale, near-term experimental demonstrations of QRAM and improve the reliability and scalability of QRAM in the long term

Basics of quantum computation

Quantum computers require quantum logic, something fundamentally different to classical Boolean logic. This difference leads to a greater efficiency of quantum computation over its classical counter-part. In this review we explain the basic principles of quantum computation, including the construction of basic gates, and networks. We illustrate the power of quantum algorithms using the simple problem of Deutsch, and explain, again in very simple terms, the well known algorithm of Shor for factorisation of large numbers into primes. We then describe physical implementations of quantum computers, focusing on one in particular, the linear ion-trap realization. We explain that the main obstacle to building an actual quantum computer is the problem of decoherence, which we show may be circumvented using the methods of quantum error correction.Comment: 28 pages including 17 figures, invited basic review article for Progress in Quantum Electronic