Isolating a Single Qubit in a Multi Qubit System

Quantum computing, a whole new paradigm to computing, is based on using Quantum Mechanics for computation. The basic element in a quantum computer is a “qubit” which is a two-state physical system. Unlike a classical bit which can exist in one of two states, 0 or 1, a qubit can not only exist in its two basis states, but also in any linear combination of these states. A quantum computer comprises several qubits interacting with each other. The states of qubits are manipulated by performing singleand multi-qubit gate operations on them wherein one or more parameters of the system are varied via classical control lines. Since every control line opens the quantum system to a noisy environment, in any design of a quantum computer, it is desirable that the number of control lines be minimal. Just like the NAND gate is a universal gate in classical computing, there are a set of gates universal to quantum computing. It has been shown that single qubit gates together with a two-qubit Controlled-NOT (CNOT) gate is universal for quantum computing, that is, any operation within a quantum computer can be decomposed into a series of single qubit and CNOT gates [1]. However, in a quantum computer having nearest-neighbor architecture, each qubit is coupled to several qubits adjacent to it. Therefore, to perform a gate operation on a single qubit, it needs to be isolated from the qubits adjacent to it by switching off the coupling. This greatly increases the complexity of the control circuitry. We show here how this problem can be overcome by designing a method whereby a single qubit can be isolated in a multi-qubit system without having to switch off the coupling. 2. Isolating a Qubit Without Shutting off Coupling