INRS researchers have achieved a breakthrough in a light-weight photonic system created using on-chip devices and off-the-shelf telecommunications components. They demonstrated that photons can become an accessible and powerful quantum resource when generated in the form of color-entangled quDits.

The system uses a small and cost-effective photonic chip fabricated through processes similar to those used for integrated electronics. With an on-chip micro-ring resonator excited by a laser, photons are emitted in pairs that share a complex quantum state. The photons are constructed in a state featuring a number of superimposed frequency components: The photons have several colors simultaneously, and the colours of each photon in a pair are linked (entangled), regardless of their separation distance.

With each frequency —or color— representing a dimension, the photons are generated on-chip as a high-dimensional quantum state (quDit). Thus far, quantum information science has largely focused on the exploitation of qubits, based on two-dimensional systems where two states are superimposed (for example, 0 AND 1 at the same time, in contrast to classical bits, which are 0 OR 1 at any time). Working in the frequency domain allows the superposition of many more states (for example, a high-dimensional photon can be red AND yellow AND green AND blue, although the photons used here were infrared for telecommunications compatibility), enhancing the amount of information in a single photon.

Professor Roberto Morandotti, who leads the INRS research team, confirms the realization of a quantum system with at least one hundred dimensions using this approach, and the technology developed is readily extendable to create two-quDit systems with more than 9,000 dimensions (corresponding to 12 qubits and beyond, comparable to the state of the art in significantly more expensive/complex platforms).

The use of the frequency domain for such quantum states enables their easy transmission and manipulation in optical fibre systems. “By merging the fields of quantum optics and ultrafast optical processing, we have shown that high-dimensional manipulation of these states is indeed possible using standard telecommunications elements like modulators and frequency filters,” stresses telecommunications system expert Professor José Azaña, co-supervisor of the conducted research.

Until now, advances in established technologies for the telecommunications sector were targeted for the manipulation of classical signals. This research is a game-changer: The advances can be immediately transferred to quantum science and will directly enable fundamental investigations of high-dimensional quantum state characteristics, applications in large-alphabet fibre-based quantum communications, and the future development of frequency-domain, high-dimensional quantum logic gates and other applications.

Leading authors Michael Kues and Christian Reimer note that a highlight of the demonstrated platform is its accessibility: It is easy to build and exploits components used in standard telecommunication systems that are commercially available everywhere. Thus, in the short term, researchers around the world will be able to incorporate and push this technology forward, enabling a leap in the development of practical quantum applications.

Nature – On-chip generation of high-dimensional entangled quantum states and their coherent control

“On-chip generation of high-dimensional entangled quantum states and their coherent control” in Nature (June 29, 2017, DOI: 10.1038/nature22986) was carried out by Michael Kues, Christian Reimer, Piotr Roztocki, Luis Romero Cortés, Stefania Sciara, Benjamin Wetzel, Yanbing Zhang, Alfonso Cino, Sai T. Chu, Brent E. Little, David J. Moss, Lucia Caspani, José Azaña, and Roberto Morandotti.

Qubits, the quantum version of bits, are constructed from two-level quantum systems, but in principle a quantum information processor could exploit higher-dimensional quantum systems for operation. These systems with an arbitrary number of levels are often termed qudits and can be generated, for example, from photons. Using qudits instead of qubits can increase sensitivity in quantum imaging and can boost quantum communication schemes. Here, Michael Kues et al. generate two entangled qudits on an integrated photonic chip using a four-wave mixing process. Each qudit encodes a 10-dimensional state, enabling the realization of a quantum system with 100 dimensions. This technique could find application in fibre-based quantum communications.

Optical quantum states based on entangled photons are essential for solving questions in fundamental physics and are at the heart of quantum information science1. Specifically, the realization of high-dimensional states (D-level quantum systems, that is, qudits, with D over 2) and their control are necessary for fundamental investigations of quantum mechanics, for increasing the sensitivity of quantum imaging schemes, for improving the robustness and key rate of quantum communication protocols, for enabling a richer variety of quantum simulations, and for achieving more efficient and error-tolerant quantum computation. Integrated photonics has recently become a leading platform for the compact, cost-efficient, and stable generation and processing of non-classical optical states7. However, so far, integrated entangled quantum sources have been limited to qubits (D = 2). Here we demonstrate on-chip generation of entangled qudit states, where the photons are created in a coherent superposition of multiple high-purity frequency modes. In particular, we confirm the realization of a quantum system with at least one hundred dimensions, formed by two entangled qudits with D = 10. Furthermore, using state-of-the-art, yet off-the-shelf telecommunications components, we introduce a coherent manipulation platform with which to control frequency-entangled states, capable of performing deterministic high-dimensional gate operations. We validate this platform by measuring Bell inequality violations and performing quantum state tomography. Our work enables the generation and processing of high-dimensional quantum states in a single spatial mode.

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