A controlled-NOT gate for frequency-bin qubits

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) [1]. While linear-optic schemes have enabled probabilistic entangling gates in spatio-polarization encoding [2,3], solutions for many other useful degrees of freedom remain missing. In particular, no two-photon gate for the important platform of frequency encoding [4][5][6][7] 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 the first 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 gate fidelity of 0.91 ± 0.01 via a novel 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.arXiv:1809.05072v1 [quant-ph]

Robust Architecture for Programmable Universal Unitaries

“…Similar ideas of configuring the transformation were reported in [21][22][23][24]. The universality theorem was proven for a multiport circuit of similar topology [25], however the proof relies on the ability to construct specific multiport mixing gates to fit the overall transformation to the desired unitary matrix, which is not the case considered in this work.…”

High-dimensional optical quantum logic in large operational spaces

“…This operation, needed for a two-qubit gate, is probabilistic with standard linear optics and photon counting. 3 Quantum gates have been demonstrated in a number of different photonic degrees of freedom, such as polarization, 4 orbital angular momentum, 5 time, 6 and frequency, 7,8 and to sidestep the challenges of probabilistic multiphoton interactions, encoding qubits in different degrees of freedom (DoFs) in a single photon has been demonstrated, where each DoF carries one qubit and, now, operations between different qubits can be made deterministic. 9,10 This scheme allows encoding more quantum information in single photons, and can find use in stand-alone processing tasks or be subsequently incorporated into larger systems built on true photon-photon interactions, thus offering a potentially more efficient method for photonic quantum information processing.…”

Arbitrary linear transformations for photons in the frequency synthetic dimension

“…Future work could leverage synthetic frequency dimensions for complicated quantum information protocols beyond single-qudit unitary transformations, such as realizing probabilistic entangling gates for linear optical quantum computing (LOQC) 17 , 36 . In particular, spectral LOQC using EOMs and pulse shapers has been shown to be universal for quantum computation 17 .…”

“…Previous works have considered implementing photonic linear transformations using different frequency channels in parallel but without frequency conversions among them 6 , 7 , 9 , 11 by demultiplexing the different frequencies into separate spatial channels. Additionally, optimized fast modulation has been used for tailoring single photon spectra from two-level quantum emitters 35 , or for quantum frequency conversion 15 and linear optical quantum computation 17 , 36 , where the modulator is used as a generalized beam splitter in synthetic frequency dimensions. However, the design of an entire scattering matrix that implements an arbitrary N × N linear transformation in synthetic space, which is essential for many applications in quantum information processing and neural networks, has not yet been shown.…”