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Demonstration of controlled-phase gate for time-bin qubits

Hsin-Pin Lo, Takuya Ikuta, Nobuyuki Matsuda, Toshimori Honjo, Hiroki Takesue

Published Jan 1, 2018• 0 citations
Publisher

Abstract

We demonstrate a controlled-phase (C-Phase) gate for the time-bin qubits using a 2×2 optical switch as a time-dependent beam splitter. We confirmed that independent time-bin qubits were entangled through the C-Phase gate.
DOI
10.1364/cleopr.2018.w2g.5

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Quantum logic gates are important for quantum computation and quantum information processing in numerous physical systems. Although time-bin qubits are suitable for quantum communication over optical fiber, many essential quantum logic gates for them have not yet been realized. Here, we demonstrated a controlled-phase (C-Phase) gate for time-bin qubits that uses a 2 × 2 optical switch based on an electro-optic modulator. A Hong–Ou–Mandel interference measurement showed that the switch could work as a time-dependent beam splitter with a variable splitting ratio. We confirmed that two independent time-bin qubits were entangled as a result of C-Phase gate operation with the switch.
Time-bin qubits, where information is encoded in a single photon at different times, have been widely used in optical-fiber- and waveguide-based quantum communications. With the recent developments in distributed quantum computation, it is logical to ask whether time-bin encoded qubits may be useful in that context. We have recently realized a time-bin qubit controlled-phase (C-phase) gate using a $2\ifmmode\times\else\texttimes\fi{}2$ optical switch based on a lithium-niobate waveguide, with which we demonstrated the generation of an entangled state. However, the experiment was performed with only a pair of input states and thus the functionality of the C-phase gate was not fully verified. In this research, we use quantum process tomography to establish a process fidelity of $97.1\mathrm{%}$. Furthermore, we demonstrate the controlled-not gate operation with a process fidelity greater than $94\mathrm{%}$. This study confirms that typical two-qubit logic gates used in quantum computational circuits can be implemented with time-bin qubits and thus it is a significant step forward for the realization of distributed quantum computation based on time-bin qubits.
A time-bin qubit is a promising candidate qubit for advanced quantum information processing systems operating over optical fibers or integrated quantum photonic circuits. However, the single- and two-qubit operations of time-bin qubits have been largely unexplored compared with those of polarization qubits. Here, I present a simple scheme for implementing a basic two-qubit operation, namely an ``entangling'' operation of two independent time-bin qubits, based on Hong-Ou-Mandel quantum interference in a high-speed $2\ifmmode\times\else\texttimes\fi{}2$ optical switch. With the proposed scheme, the formation of a time-bin entanglement was confirmed experimentally.