Quantum Process Tomography of a Controlled-Phase Gate for Time-Bin Qubits

Abstract

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 2×2 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 used quantum process tomography to establish a process fidelity of 97.1%. Furthermore, we demonstrated the controlled-NOT gate operation with a process fidelity greater than 94%. 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 realization of distributed quantum computation based on time-bin qubits.

Approaching the theoretical limit in quantum gate decomposition

“…[45,46] reported on experimental demonstration of controlled-phase gates between transmon qubits, while another realization of controlled-phase gates between two photonic qubits encoded in photonic fields stored in cavities or between time-bin qubits were accomplished in Refs. [47] and [48], respectively. Usually, the CN OT gates in these platforms are generated as a composite gate operation formed from one two-qubit contolled gate and from suitable one-qubit rotations [46,48].…”

Complete frequency-bin Bell basis synthesizer

“…Generation and measurement of Bell states appear in a plethora of quantum communication protocols spanning dense coding [1], teleportation [2], entanglement-based cryptography [3][4][5], and entanglement swapping [6,7]. Production of a complete set of Bell states has been actively studied in various photonic degrees of freedom including polarization [1,8], orbital angular momentum [9], discrete timebins [10], pulsed time-frequency modes [11] and path encodings [12][13][14]. Recently, interest in frequency-bin encoding has grown due to particular advantages such as simple multiplexing capabilities and compatibility with both on-chip integration and optical fiber networks [15][16][17][18].…”

Realization of a Universal Quantum Gate Set for Itinerant Microwave Photons

“…Together with the non-linearity of photon detection, interference effects enable (probabilistic) Bell measurements [1,16] as used in quantum repeater protocols [17][18][19][20]. By additionally using ancilla measurements and classical feed forward, probabilistic photon-photon gates have been implemented in optical setups [21][22][23][24][25][26] and are now commonly used in linear optics quantum computing [26][27][28].…”

“…The total processes, however, are still limited by photon loss. In future, this may be overcome by the development of less lossy circulators [44,45] or by heralding of photons using time-bin [24,46], frequency-comb [23] or dual-rail encoding [21]. We expect the success rate of such heralding schemes to be an order of magnitude larger (close to η loss = 75 %) than in optical implementations of deterministic photon-photon gates [30,31].…”