Unitary Transforms and Quantum Gates

Huangjun Zhu, Haoyu Zhang

Efficient verification of the functioning of quantum devices is a key to the development of quantum technologies, but is a daunting task as the system size increases. Here we propose a simple and general framework for verifying unitary transformations that can be applied to both individual quantum gates and gate sets, including quantum circuits. This framework enables efficient verification of many important unitary transformations, including but not limited to all bipartite unitaries, Clifford unitaries, generalized controlled-$Z$ gates, generalized controlled-not gates, the controlled-swap gate, and permutation transformations. For all these unitaries, the sample complexity increases at most linearly with the system size and is often independent of the system size. Moreover, little overhead is incurred even if one can only prepare Pauli eigenstates and perform local measurements. Our approach is applicable in many scenarios in which randomized benchmarking (RB) does not apply and is thus instrumental to quantum computation and many other applications in quantum information processing.

Gianpiero Cattaneo, G. Conte, Roberto Leporini

Two possible quantum descriptions of the classical Not gate are investigated in the framework of the Hilbert space C2: the unitary and the anti-unitary operator realizations. The two cases are distinguished interpreting the unitary Not as a quantum realization of the classical gate which on a xed orthogonal pair of unit vectors, realizing once for all the classical bits 0 and 1, produces the required transformations 0 => 1 and 1 => 0 (i.e., logical quantum Not). The anti-unitary Not is a quantum realization of a gate which acts as a classical Not on any pair of mutually orthogonal vectors, each of which is a potential realization of the classical bits (i.e., universal quantum Not). Although the latter is not completely positive, one can give an approximated unitary realization of the gate by appending an ancilla. Finally, we consider the unitary and the anti-unitary operator realizations of two important genuine quantum gates that transform elements of the computational basis of C2 into its superpositions: the square root of the identity and the square root of the Not.

Xiaoguang Wang, YU Rong-jin, Jian-zheng Fu

By factorizing a general one-bit transformation matrix for one-bit gates in quantum computer, a general unitary transformation is constructed, which can serve as (controlled) m gate for the conditional quantum dynamics. When the quantized single-mode electromagnetic field containing n or no photons acts as the controlled bit, the quantum controlled-NOT gates and square root of the controlled-NOT gates in cavity-quantum electrodynamics are realized.