Higher-Order Process Matrix Tomography of a Passively-Stable Quantum Switch

Michael Antesberger, Marco Túlio Quintino, Philip Walther, Lee A. Rozema

The field of indefinite causal order (ICO) has seen a recent surge in interest. Much of this research has focused on the quantum switch, wherein multiple parties act in a superposition of different orders in a manner transcending the quantum circuit model. This results in a new resource for quantum protocols, and is exciting for its relation to issues in foundational physics. The quantum switch is also an example of a higher-order quantum operation, in that it transforms not only quantum states but also other quantum operations. To date, no quantum process without a definite causal order has been completely experimentally characterized. Indeed, past work on the quantum switch has confirmed its ICO by measuring causal witnesses or demonstrating resource advantages, but the complete process matrix has been described only theoretically. Here we report our performing higher-order quantum process tomography. However, doing so requires exponentially many measurements with a scaling worse than that of standard process tomography. We overcome this challenge by creating a new passively stable fiber-based quantum switch using active optical elements to deterministically generate and manipulate time-bin encoded qubits. Moreover, our new architecture for the quantum switch can be readily scaled to multiple parties. By reconstructing the process matrix, we estimate its fidelity and tailor different causal witnesses directly for our experiment. To achieve this, we measure a set of tomographically complete settings, which also spans the input operation space. Our tomography protocol allows the characterization and debugging of higher-order quantum operations with and without an ICO, while our experimental time-bin techniques could enable the creation of a new realm of higher-order quantum operations with an ICO.

Quantum Optics, Quantum Nanophysics and Quantum Information, Research Network Quantum Aspects of Space Time
External organisation(s)
Österreichische Akademie der Wissenschaften (ÖAW), Université Sorbonne-Paris-Cité
PRX Quantum
No. of pages
Publication date
Peer reviewed
Austrian Fields of Science 2012
103025 Quantum mechanics
ASJC Scopus subject areas
Electronic, Optical and Magnetic Materials, Computer Science(all), Mathematical Physics, Physics and Astronomy(all), Applied Mathematics, Electrical and Electronic Engineering
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