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IEEE/LEOS Summer Topical Meetings

Experimental Quantum Process Tomography of Controlled-phase Gate for Time-bin Qubits

Hsin-Pin Lo, Takuya Ikuta, Nobuyuki Matsuda, Toshimori Honjo, William J. Munro, Hiroki Takesue

Published Jul 1, 2019• 0 citations
Publisher

Abstract

We demonstrate a controlled-phase gate for time-bin qubits using a 2 × 2 electro-optic switch based on lithium niobate waveguides. We reconstructed the process matrix using quantum process tomography with 16 distinct inputs from which we determined a process fidelity of 82%.
DOI
10.1109/phosst.2019.8794864

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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.
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We experimentally demonstrate quantum process tomography of controlled-Z and controlled-NOT gates using capacitively coupled superconducting phase qubits. These gates are realized by using the ∣2⟩|2⟩∣2⟩ state of the phase qubit. We obtain a process fidelity of 0.70 for the controlled phase and 0.56 for the controlled-NOT gate, with the loss of fidelity mostly due to single-qubit decoherence. The controlled-Z gate is also used to demonstrate a two-qubit Deutsch-Jozsa algorithm with a single function query.