Faults and Tests in Quantum Circuits

Quantum computing is a recently developed approach to information processing, which is based on quantum mechanics rather than classical physics. Information is represented by quantum bits (qubits) that correspond to microscopic states such as photon polarization. Up to 2n n-bit words can be stored simultaneously in n qubits, implying a type of massive parallelism. Powerful forms of quantum interaction such as interference and entanglement exist which have no counterparts in classical computer science. Some important and hitherto intractable problems such as prime factorization of large numbers can be solved efficiently using quantum methods. In practice, however, quantum computing devices and circuits are extremely difficult to design and build, since they are nanoscale in size and operate at very low energy levels. Consequently, they have many more failure modes than classical (non-quantum) circuits. For example, quantum signal states are inherently unstable and tend to decay rapidly due to interaction with the environment (decoherence). Quantum gate operations are defined by continuous parameters that allow small errors to arise and propagate to other gates. Furthermore, state measurement is probabilistic and the measurement process itself affects the state being measured. This talk will review the history and development of quantum circuits, with emphasis on their failure modes and testing requirements. It will be seen that quantum circuits are highly testable for classical faults. However, they are also subject to various complex, nonclassical failure modes, which are still not well understood. Some methods for error correction and recovery that have been developed specifically for quantum circuits will also be reviewed.