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IEEE Journal of Selected Topics in Quantum Electronics

Experimental Characterization of a Telecommunications-Band Quantum Controlled- not Gate

Monika Patel, Joseph B. Altepeter, Matthew Hall, Milja Medić, Prem Kumar

Published Jan 1, 2009• 3 citations
Publisher

Abstract

The quantum controlled-not gate is an example of the maximally entangling gate, which is a broad class of operations that are necessary for scalable linear optics quantum computation. Here, we characterize a telecommunications-wavelength (1550 nm) quantum controlled- not gate, and for the first time, experimentally bound its process fidelity by measuring its operation in two complementary polarization bases. The gate's final process fidelityFis given by 91% ¿F¿ 95%.

DOI
10.1109/jstqe.2009.2025704

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The quantum controlled-not gate is an example of the maximally entangling gate, which is a broad class of operations that are necessary for scalable linear optics quantum computation. Here, we characterize a telecommunications-wavelength (1550 nm) quantum controlled- not gate, and for the first time, experimentally bound its process fidelity by measuring its operation in two complementary polarization bases. The gate's final process fidelityFis given by 91% ¿F¿ 95%.
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We demonstrate complete characterization of a two-qubit entangling process--a linear optics controlled-NOT gate operating with coincident detection--by quantum process tomography. We use a maximum-likelihood estimation to convert the experimental data into a physical process matrix. The process matrix allows an accurate prediction of the operation of the gate for arbitrary input states and a calculation of gate performance measures such as the average gate fidelity, average purity, and entangling capability of our gate, which are 0.90, 0.83, and 0.73, respectively.
Abstract The realization of strong photon–photon interactions has presented an enduring challenge across photonics, particularly in quantum computing, where two-photon gates form essential components for scalable quantum information processing (QIP). While linear-optic schemes have enabled probabilistic entangling gates in spatio-polarization encoding, solutions for many other useful degrees of freedom remain missing. In particular, no two-photon gate for the important platform of frequency encoding has been experimentally demonstrated, due in large part to the additional challenges imparted by the mismatched wavelengths of the interacting photons. In this article, we design and implement an entangling gate for frequency-bin qubits, a coincidence-basis controlled-NOT (CNOT), using line-by-line pulse shaping and electro-optic modulation. We extract a quantum unitary fidelity of 0.91 ± 0.01 via a parameter inference approach based on Bayesian machine learning, which enables accurate gate reconstruction from measurements in the two-photon computational basis alone. Our CNOT imparts a single-photon frequency shift controlled by the frequency of another photon—an important capability in itself—and should enable new directions in fiber-compatible QIP.
The promise of tremendous computational power, coupled with the development of robust error-correcting schemes, has fuelled extensive efforts to build a quantum computer. The requirements for realizing such a device are confounding: scalable quantum bits (two-level quantum systems, or qubits) that can be well isolated from the environment, but also initialized, measured and made to undergo controllable interactions to implement a universal set of quantum logic gates. The usual set consists of single qubit rotations and a controlled-NOT (CNOT) gate, which flips the state of a target qubit conditional on the control qubit being in the state 1. Here we report an unambiguous experimental demonstration and comprehensive characterization of quantum CNOT operation in an optical system. We produce all four entangled Bell states as a function of only the input qubits' logical values, for a single operating condition of the gate. The gate is probabilistic (the qubits are destroyed upon failure), but with the addition of linear optical quantum non-demolition measurements, it is equivalent to the CNOT gate required for scalable all-optical quantum computation.