Hawking effects on the entanglement near the Schwarzschild black hole

Hawking radiation effects on an entangled pair near the event horizon of a Schwarzschild black hole are investigated. The Hawking radiation was found to degrade both the quantum coherence of the entangled state and the mutual correlations of the entangled pair. When the black hole evaporated completely, the measure of entanglement vanished, but the classical correlation between the entangled pair still remained.Comment: Journal of Korean Physical Society, accepted for publicatio

T-count optimization of approximate quantum Fourier transform

The quantum Fourier transform (QFT) is a ubiquitous quantum operation that is used in numerous quantum computing applications. The major obstacle to constructing a QFT circuit is that numerous elementary gates are required. Among the elementary gates, T gates dominate the cost of fault-tolerant implementation. Currently, the smallest-known T-count required to construct an n-qubit QFT circuit approximated to error O(\varepsilon) is ~8nlog_2(n/\varepsilon). Moreover, the depth of T gates (T-depth) in the approximate QFT circuit is ~2nlog_2(n/\varepsilon). This approximate QFT circuit was constructed using Toffoli gates and quantum adders. In this study, we present a new n-qubit QFT circuit approximated to error O(\varepsilon). Our approximate QFT circuit shows a T-count of ~4nlog_2(n/\varepsilon) and a T-depth of ~nlog_2(n/\varepsilon). Toffoli gates, which account for half of the T-count in the approximate QFT circuit reported in the previous study, are unnecessary in our construction. Quantum adders, which dominate the leading order term of T-depth in our approximate QFT circuit, are arranged in parallel to reduce T-depth