Directions in Quantum Optics

H. J. Carmichael, Roy J. Glauber, Marlan O. Scully

Ilhwan Kim, Donghwa Lee, Seongjin Hong +4 more

Linear optical multiports are widely used in photonic quantum information processing. Naturally, these devices are directionally-biased since photons always propagate from the input ports toward the output ports. Recently, the concept of directionally-unbiased linear optical multiports was proposed. These directionally-unbiased multiports allow photons to propagate along a reverse direction, which can greatly reduce the number of required linear optical elements for complicated linear optical quantum networks. Here, we report an experimental demonstration of a 3 × 3 directionally-unbiased linear optical fiber multiport using an optical tritter and mirrors. Compared to the previous demonstration using bulk optical elements which works only with light sources with a long coherence length, our experimental directionally-unbiased 3 × 3 optical multiport does not require a long coherence length since it provides negligible optical path length differences among all possible optical trajectories. It can be a useful building block for implementing large-scale quantum walks on complex graph networks.

Shuto Osawa, David S. Simon, Alexander V. Sergienko

All existing optical quantum walk approaches are based on the use of beamsplitters and multiple paths to explore the multitude of unitary transformations of quantum amplitudes in a Hilbert space. The beamsplitter is naturally a directionally biased device: the photon cannot travel in the reverse direction. This causes rapid increases in the optical hardware resources required for complex quantum walk applications, since the number of options for the walking particle grows with each step. Here we present the experimental demonstration of a directionally-unbiased linear-optical multiport, which allows reversibility of the photon direction. An amplitude-controllable probability distribution matrix for a unitary three-edge vertex is reconstructed with only linear-optical devices. Such directionally-unbiased multiports allow direct execution of quantum walks over a multitude of complex graphs and in tensor networks. This approach would enable simulation of complex Hamiltonians of physical systems and quantum walk applications in a more efficient and compact setup, substantially reducing the required hardware resources.

Subhashish Banerjee, Arun M. Jayannavar

Shuto Osawa, David S. Simon, Alexander V. Sergienko

The optical beam splitter is one of the main building blocks in photonics-based quantum information processing. Traditionally, beam splitters are used in feed-forward configurations that reflect their natural directional-bias, meaning a photon cannot leave the scattering element through the entrance port. However, this directional-bias constraint can be circumvented by designing new linear-optical configurations that include mirrors in addition to the beam splitter. This directionality removal restores a full symmetry in the scattering element and allows the input photon to leave the system also from the input port. Such a system can be seen as a scattering center, enabling execution of quantum walks on rather complex optical graph networks with great savings in hardware resources when compared with existing approaches utilizing directionally-biased devices. The directionallyunbiased configuration can be realized using different optical systems. Analysis of some originally directional optical devices and the basic principles of their conversion into directionally-unbiased systems form the base of this paper. Several quantum walk procedures executed on graph networks constructed using directionallyunbiased nodes are discussed.

Ehlotzky, Fritz