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Physics

Shaping the Future of Quantum Computation

Peter van Loock

Published May 27, 2014• 0 citations
Publisher

Abstract

We present a detailed analysis of the multimode quantum state embedded in an optical frequency comb generated by a synchronously pumped optical parametric oscillator (SPOPO) [J.Roslund, R. Medeiros de Araújo, S. Jiang, C. Fabre, and N. Treps, Nat. Photon. 8, 109 (2014)].The full covariance matrix of the state is obtained with homodyne detection where the local oscillator is spectrally controlled with pulse-shaping techniques.The resulting matrix reveals genuine multipartite entanglement.Additionally, the beam is comprised of several independent eigenmodes that correspond to specific pulse shapes.The experimental data is confirmed with numerical simulations.Finally, the potential to create continuous-variable cluster states from the quantum comb is analyzed.Multiple cluster states are shown to be simultaneously embedded in the SPOPO state, and these states can be revealed by a suitable basis change applied to the measured covariance matrix.

DOI
10.1103/physics.7.56

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We present a detailed analysis of the multimode quantum state embedded in an optical frequency comb generated by a synchronously pumped optical parametric oscillator (SPOPO) [J.Roslund, R. Medeiros de Araújo, S. Jiang, C. Fabre, and N. Treps, Nat. Photon. 8, 109 (2014)].The full covariance matrix of the state is obtained with homodyne detection where the local oscillator is spectrally controlled with pulse-shaping techniques.The resulting matrix reveals genuine multipartite entanglement.Additionally, the beam is comprised of several independent eigenmodes that correspond to specific pulse shapes.The experimental data is confirmed with numerical simulations.Finally, the potential to create continuous-variable cluster states from the quantum comb is analyzed.Multiple cluster states are shown to be simultaneously embedded in the SPOPO state, and these states can be revealed by a suitable basis change applied to the measured covariance matrix.
Multimode nonclassical states of light are an essential resource in quantum computation with continuous variables [1]. They can be generated either by mixing different squeezed light sources using linear optical operations [2], or directly in a multimode optical device [3, 4]. Synchronously Pumped Optical Parametric Oscillators (SPOPO) — which correspond to an OPO pumped by a train of ultrashort pulses that are synchronized with the cavity round-trip time — have been theoretically shown to be promising devices for the generation of multimode nonclassical frequency combs, since a SPOPO is a sole optical cavity acting as an assembly of independent squeezers in a particular basis called the supermodes basis [5]. The supermodes are optical frequency combs with specific spectrum and phase profiles that are determined by the length of the intracavity nonlinear crystal and the pulse shape of the pump.
Multimode nonclassical states of light are an essential resource in quantum computation with continuous variables, for example, in cluster state computation. We report in this Letter the first experimental evidence of a multimode nonclassical frequency comb in a femtosecond synchronously pumped optical parametric oscillator. In addition to a global reduction of its quantum intensity fluctuations, the system features quantum correlations between different parts of its frequency spectrum. This allows us to show that the frequency comb is composed of several uncorrelated eigenmodes having specific spectral shapes, two of them at least being squeezed, and to characterize their spectral shapes.
Cluster states, in which entanglement is distributed amongst numerous parties, have attracted interest due to their role as a resource for measurement-based quantum computing. The traditional means for fabricating these highly-entangled states has been to sequentially combine the squeezed outputs from individual optical parametric oscillators (OPO) with a network of phase-shifters and beamsplitters. While successful, this approach demands an optical footprint commensurate with the dimensionality of the produced state. Within the continuous-variable regime, a promising alternative is to begin with a source that is intrinsically multimode. Toward that end, the frequency comb underlying femtosecond optical pulses contains upwards of ~ 105individual frequencies, which makes it an attractive starting point for the production of multipartite entanglement. This work demonstrates the ability of OPOs synchronously pumped by a femtosecond pulse train to create scalable, adaptable multipartite photonic states in a single beam while exploiting femtosecond pulse shaping to perform a complete state analysis.
Entanglement among multiple modes is a necessary resource in the field of quantum information technology. In this context, frequency comb sources of femtosecond pulsed light represent a useful resource, with approximately 10^5 individual frequency modes across the pulse bandwidth and it has been recently shown that a synchronously pumped optical parametric oscillator (SPOPO) can generate highly multimode nonclassical states of light. A high degree of control over the mode structure of the output can thus be achieved by shaping the spectral mode of the pump beam using a pulse shaper. The SPOPO setup is also a promising platform for the simulation of quantum optical networks. Energy exchanges have been characterized for several models of networks via the measurement of the spectral density function and the observation of quantum non-Markovian behavior.
Continuous-variable (CV) multipartite entangled states, particularly Greenberger–Horne–Zeilinger (GHZ) states, are an indispensable resource for quantum networks and quantum information processing. Conventional approaches to generating CV GHZ states rely on combining multiple independently prepared squeezed lights through complex interferometric networks, which impose significant challenges in scalability and stability. Here, we experimentally demonstrate a compact and resource-efficient scheme for generating a tripartite CV GHZ state from a single quantum frequency comb produced by a synchronously pumped optical parametric oscillator. By employing multipixel homodyne detection, we simultaneously measure quadrature fluctuations across multiple frequency bands and reconstruct the full multimode covariance matrix in a single measurement. Through eigenmode decomposition, we extract a set of independent squeezed supermodes, which are subsequently transformed via a tailored linear unitary operation implemented in post-processing. Using this approach, we deterministically construct a tripartite GHZ state from three supermodes and verify its entanglement via the van Loock–Furusawa criteria, with the corresponding correlation variances measured to be 0.57, 0.75, and 0.57, all below the shot noise limit. This method effectively realizes a scalable entanglement generation scheme within a single optical beam, eliminating the need for complex interferometric networks. Our results provide a scalable and versatile platform for multimode quantum information processing.