Delivered by Ingenta to: Chinese University of Hong Kong IP: 37.18.42.72 On: Thu, 16 Jun 2016 08:52:10 Copyright: American Scientific Publishers Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Computational and Theoretical Nanoscience Vol. 8, 391–400, 2011 Localized State in Quantum Point Contacts: Possible Qubit Implementation? Lev Mourokh 1 ∗ , Pavel Ivanushkin 1 , and Jonathan Bird 2 1 Physics Department, Queens College of the City University of New York Flushing, NY 11367, USA 2 Department of Electrical Engineering, University at Buffalo, Buffalo, NY 14260, USA We discuss a possible implementation of a quantum bit as a localized state self-consistently formed in quantum point contacts (QPCs) near pinch-off conditions. Such formation has been connected to the so-called 0.7-anomaly, an additional feature observed in QPCs’ conductance below the first quantized step. We report experimental data showing a clear peak in the conductance of another (detector) QPC in close proximity to a QPC driven to pinch-off. This peak is visible for temperatures up to 35 K. We attribute this peak to a Fano resonance when the direct path for electrons from the source to the drain through the detector QPC coherently interfere with the path via a localized state in the pinched-off (swept) QPC. To support such a conclusion, we perform a theoretical analysis based on the equations of motion for electron operators, reproducing all essential features of exper- iment. Also, we discuss possible advantages of a qubit based on that localized state in comparison to the standard quantum dot case. Keywords: Quantum Point Contact, Fano Resonance, Quantum Computation. CONTENTS 1. Introduction ................................. 391 2. Quantum Point Contacts ......................... 392 3. Fano Resonance in Coupled Quantum Point Contacts: Experiment ................................. 393 4. Fano Resonance in Coupled Quantum Point Contacts: Theory .................................... 396 5. Advantages of the Quantum Point Contact-Based Qubit Implementation ............................... 399 Acknowledgment ............................. 399 References ................................. 399 1. INTRODUCTION The successful implementation of quantum computing has the potential to impact enormously in areas such as compu- tational science, quantum cryptography, and secure com- munications. The basic element of any quantum computer is the so-called qubit—a well established two-level sys- tem, which can be in a superposition of its two states for a time much longer than that needed for computational operations. While many approaches to quantum computa- tion have been explored to date, 1 one of the most promis- ing of these is based on the use of electron spin states in semiconductor quantum dots. 2 3 An important advantage of this approach is that it should benefit from compatibility ∗ Author to whom correspondence should be addressed. with current semiconductor microprocessing techniques. The spin of a single electron confined in a quantum dot provides a natural qubit which can be manipulated either electronically or optically. The decoherence time for spin degrees of freedom has furthermore been measured to be much longer than that of charge degrees of freedom, 4 5 and a universal set of quantum gates is also well established for spin-based qubits. 2 3 In recent years, considerable progress has been achieved in actual implementations of quantum-dot spin-based qubits. Single- and two-qubit operations have been demonstrated for GaAs/AlGaAs 6 7 and Si/Ge quantum dots. 8 Spin-charge conversion has furthermore been used to achieve successful spin readout. 9 In this approach, an external magnetic field is used to lift spin degeneracy, with the upper Zeeman state acquiring an energy larger than the Fermi energy in the reservoirs, thereby allowing it to be used as a route for electron escape from the dot. In the readout procedure, a current is driven through a quan- tum point contact (QPC) in close proximity to the quan- tum dot, and, if the dot contains an electron, the resulting electrostatic interaction slightly suppresses the QPC cur- rent. Consequently, the magnitude of this current reflects the population of the dot by a single electron, and it is this feature that allows for determination of the spin projection. While the above-mentioned progress in the implemen- tation of quantum computation is impressive, there are nonetheless several obstacles to further progress. One of the J. Comput. Theor. Nanosci. 2011, Vol. 8, No. 3 1546-1955/2011/8/391/010 doi:10.1166/jctn.2011.1703 391