Photonic quantum information processing: generalized concurrence of multimode photonic states

Changhoon Baek, Danbi Kim, Juntaek Oh +4 more

High-dimensional photonic operations are an important resource for quantum information processing, yet achieving scalability and long-distance operation remains a formidable challenge. Here, we demonstrate reconfigurable high-dimensional spatial-mode state synthesis using spatial-mode-encoded qu d its in a 20-m-long multimode fiber. By exploiting the ultra-long coherence of heralded single photons from a warm 87 Rb atomic ensemble, we overcome the modal dispersion limits that restricted previous short-distance demonstrations. Using digital optical phase conjugation, we deterministically synthesize spatial-mode states structured by target unitary operations, including generalized Pauli operators and quantum Fourier transforms, through the complex fiber medium. We achieve process fidelities exceeding 92.5(4)% for dimensions up to d = 7. These results establish long multimode fibers as a reconfigurable platform for high-dimensional quantum state synthesis and transport in fiber-based quantum information processing.

Teodor Strömberg

Sebastian Malewicz

Rosa Tualle-Brouri, Alexei Ourjoumtsev, Aurélien Dantan +3 more

We present a general model to account for the multimode nature of the quantum electromagnetic field in projective photon-counting measurements. We focus on photon-subtraction experiments, where non-Gaussian states are produced conditionally. These are useful states for continuous-variable quantum-information processing. We present a general method called mode reduction that reduces the multimode model to an effective two-mode problem. We apply this method to a multimode model describing broadband parametric down-conversion, thereby improving the analysis of existing experimental results. The main improvement is that spatial and frequency filters before the photon detector are taken into account explicitly. We find excellent agreement with previously published experimental results, using fewer free parameters than before, and discuss the implications of our analysis for the optimized production of states with negative Wigner functions.

Seungbeom Chin, Joonsuk Huh

Abstract A fundamental question in linear optical quantum computing is to understand the origin of the quantum supremacy in the physical system. It is found that the multimode linear optical transition amplitudes are calculated through the permanents of transition operator matrices, which is a hard problem for classical simulations (boson sampling problem). We can understand this problem by considering a quantum measure that directly determines the runtime for computing the transition amplitudes. In this paper, we suggest a quantum measure named “Fock state concurrence sum” C S , which is the summation over all the members of “the generalized Fock state concurrence” (a measure analogous to the generalized concurrences of entanglement and coherence). By introducing generalized algorithms for computing the transition amplitudes of the Fock state boson sampling with an arbitrary number of photons per mode, we show that the minimal classical runtime for all the known algorithms directly depends on C S . Therefore, we can state that the Fock state concurrence sum C S behaves as a collective measure that controls the computational complexity of Fock state BS. We expect that our observation on the role of the Fock state concurrence in the generalized algorithm for permanents would provide a unified viewpoint to interpret the quantum computing power of linear optics.

Pramod Kumar, Alan L. Migdall, Seungmin Nam +2 more