Novel Applications of ESR/EPR: Quantum Computing/Quantum Information Processing

Abstract

A qubit (quantum bit) is a quantum mechanical two-state system. The qubit can afford to give an arbitrary superposition of quantum states, and it has substantially higher complication than a classical bit. A canonical example of the qubit is the matter spin with spin-1/2. Thus, electrons with the spin quantum number 1/2, as physical qubits, have naturally been anticipated for implementing quantum computing and information processing (QC/QIP). Recently, electron spin-qubit systems in organic molecular frames such as extremely stable radicals have emerged as a hybrid spin-qubit system along with a nuclear spin-1/2 qubit, termed molecular electron-bus qubits. Indeed, molecular electron spins are the latest arrival as qubits. Among promising candidates for QC/QIP from the materials science side, the reasons why electron spin-qubits such as synthetic electron spin systems, i.e., unpaired electron spins in molecular frames, have potentialities for serving for QC/QIP are briefly described in this chapter. Issues relevant to synthetic multi-electron qubit systems are important for the implementation of qubit scalability, but are not included. Compared with NMR-based QC/QIP, pulse ESR-based QC/QIP is totally immature, simply because of the intrinsic technical restrictions and decoherence inherent in electron spins in ensemble physical systems. In terms of the linkage between QC/QIP and pulsed electron magnetic resonance as enabling ensemble-spin manipulation technology, there are many important and emerging issues. The linkage between QC/QIP and chemistry or materials science is also important, providing insights into the quest for practically scalable spin qubits. In this chapter, we only emphasize that current pulsed electron magnetic resonance enables us to manipulate an electron spin and nuclear spin qubits in an equivalent manner. Super dense coding (SDC) experiments by the use of pulse ENDOR are introduced to understand QC ENDOR and how it differs from QC NMR based on modern nuclear spin technology. Direct observation of the spinor inherent in an electron spin, detected for the first time, will be shown in connection with the entanglement of an electron-nuclear hybrid system (the simplest electron-bus system).