Circular CNOT circuits: Definition, analysis and application to fault-tolerant quantum circuits

Abstract

The work proposes an extension of the quantum circuit formalism where qubits (wires) are circular instead of linear. The left-toright interpretation of a quantum circuit is replaced by a circular representation which allows to select the starting point and the direction in which gates are executed. The representation supports all the circuits obtained after computing cyclic permutations of an initial quantum gate list. Two circuits, where one has a gate list which is a cyclic permutation of the other, will implement different functions. The main question appears in the context of scalable quantum computing, where multiple subcircuits are used for the construction of a larger fault-tolerant one: can the same circular representation be used by multiple subcircuits? The circular circuits defined and analysed in this work consist only of CNOT gates. These are sufficient for constructing computationally universal, fault-tolerant circuits formed entirely of qubit initialisation, CNOT gates and qubit measurements. The main result of modelling circular CNOT circuits is that a derived Boolean representation allows to define a set of equations for X and Z stabiliser transformations. Through a well defined set of steps it is possible to reduce the initial equations to a set of stabiliser transformations given a series of cuts through the circular circuit.