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Quantum Computing, Communication, and Simulation

Unidirectional bosonic quantum walk and entanglement redistribution from high-dimensional quantum states with tunable entropies

Raktim Haldar, Robert Johanning, Philip Rübeling, Anahita Khodadad Kashi, Thomas Bækkegaard, Surajit Bose, Nikolaj Thomas Zinner, Michael Kues

Published Mar 8, 2023• 0 citations
Publisher

Abstract

We generate high-dimensional bi-photon quantum frequency combs with tunable entropies exploiting the second-order nonlinearity of a periodically-poled lithium niobate waveguide through a novel pumping and filtering scheme. Using these quantum states with varying degrees of entanglement, we demonstrate unidirectional bosonic quantum walks, asymmetric energy transfer, and entanglement transport. Our non-maximally entangled quantum states can serve as excellent testbeds for several computational protocols. Moreover, we achieve the steering of the directionality in a scalable format, which enables a new control mechanism for quantum walks as well as novel modification means of joint probability distributions.

DOI
10.1117/12.2650007

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We generate high-dimensional bi-photon quantum frequency combs with tunable entropies exploiting the second-order nonlinearity of a periodically-poled lithium niobate waveguide through a novel pumping and filtering scheme. Using these quantum states with varying degrees of entanglement, we demonstrate unidirectional bosonic quantum walks, asymmetric energy transfer, and entanglement transport. Our non-maximally entangled quantum states can serve as excellent testbeds for several computational protocols. Moreover, we achieve the steering of the directionality in a scalable format, which enables a new control mechanism for quantum walks as well as novel modification means of joint probability distributions.
Quantum walks are central to a wide range of applications such as quantum search, quantum information processing, and entanglement transport. Gaining control over the duration and the direction of quantum walks (QWs) is crucial to implementing dedicated processing. However, in current systems, it is cumbersome to achieve in a scalable format. High-dimensional quantum states, encoded in the photons' frequency degree of freedom in on-chip devices are great assets for the scalable generation and reliable manipulation of large-scale complex quantum systems. These states, viz. quantum frequency combs (QFCs) accommodating huge information in a single spatial mode, are intrinsically noise tolerant, and suitable for transmission through optical fibers, thereby promising to revolutionize quantum technologies. Existing literature aimed to generate maximally entangled QFCs excited from continuous-wave lasers either from nonlinear microcavities or from waveguides with the help of filter arrays. QWs with flexible depth/duration have been lately demonstrated from such QFCs. Here, instead of maximally-entangled QFCs, we generate high-dimensional quantum photonic states with tunable entropies from periodically poled lithium niobate waveguides by exploiting a novel pulsed excitation and filtering scheme. We confirm the generation of QFCs with normalized entropies from ∼0.35\sim 0.35∼0.35 to 111 by performing quantum tomography with high fidelities. These states can be an excellent testbed for several quantum computation and communication protocols in nonideal scenarios and enable artificial neural networks to classify unknown quantum states. Further, we experimentally demonstrate the steering and coherent control of the directionality of QWs initiated from such QFCs with tunable entropies. Our findings offer a new control mechanism for QWs as well as novel modification means for joint probability distributions.
Quantum superpositions enable quantum walks (QWs) to potentially speedup certain computational tasks such as database searches, tests of graph isomorphism, ranking nodes in a network, quantum many-body simulations, etc. [1] . To implement QWs, photonic platforms have the advantages of being robust at room temperature and immune to decoherence [2] . However, photonic circuits either require huge overhead or necessitate modifying the physical layout to attain the tunability of the QW. Recently, QWs exhibiting enhanced ballistic transport (bosonic) or strong energy confinement (fermionic) have been demonstrated [3] using high-dimensional bi-photon quantum frequency combs (QFCs) [4] , which do not require any change of the device arrangement.
Control over the duration of a quantum walk is critical to unlocking its full potential for quantum search and the simulation of many-body physics. Here we report quantum walks of biphoton frequency combs where the duration of the walk, or circuit depth, is tunable over a continuous range without any change to the physical footprint of the system-a feature absent from previous photonic implementations. In our platform, entangled photon pairs hop between discrete frequency modes with the coupling between these modes mediated by electro-optic modulation of the waveguide refractive index. Through control of the phase across different modes, we demonstrate a rich variety of behavior: from walks exhibiting enhanced ballistic transport or strong energy confinement, to subspaces featuring scattering centers or local traps. We also explore the role of entanglement dimensionality in the creation of energy bound states, which illustrates the potential for these walks to quantify high-dimensional entanglement.
Entanglement and encoding in discrete frequency bins – essentially a quantum analogue of wavelength-division multiplexing – represents a relatively new degree of freedom for quantum information with photons. In this talk I discuss biphoton frequency combs, generated either by spontaneous four-wave mixing (SFWM) from on-chip microring resonators or by spectral filtering of spontaneous parametric down conversion (SPDC) in second order nonlinear crystals. Potential advantages include generation of high dimensional units of quantum information (qudits), which can carry multiple qubits per photon, robust transmission over fiber, and frequency parallelism and routing. Since the initial experiments 2-3 years ago, frequency bin quantum photonics has been advancing rapidly [1, 2]. In this talk I will give special attention to high dimensional entanglement. One of the interesting possibilities is to perform mixing of multiple frequency bins in a single operation, going well beyond nearest neighbor “interactions.” In this vein I will comment on two recent experiments in our lab, each involving more than a dozen frequency bins. One experiment focuses on quantum walks of frequency entangled photon pairs, in which the input state can be coherently steered toward either correlated or anticorrelated quantum walk behavior [3]. In the second case, we show that high-dimensional frequency bin entanglement enables measurement of signal-idler delay at the few picosecond level, ~30× faster than the single photon detectors employed [4].