Rare-earth ions could pave the way to quantum networks and memories
Quantum technologies make use of some strange aspects of quantum physics to encode and transfer information. These are quantum superposition (where particles can simultaneously exist in two different quantum states (qubits)), and quantum entanglement where particles share their quantum state in a way that is impossible in classical systems. Qubits are delicate entities that must be kept carefully isolated from external forces. If not, the fragile superposition will decohere into a traditional computing state – a one or a zero. Atomic systems such as rare-earth elements in nanocrystals offer a way of generating qubits with very low levels of noise, and therefore maintain the quantum coherence required for quantum applications. “Rare-earth ions in bulk materials can retain the optical and spin coherence for a long time. We showed that this unique property is preserved to a large extent at the nanoscale,” explains Philippe Goldner, coordinator of the EU-funded NanOQTech. To test their theory, researchers synthesised crystals of different sizes doped with europium, praseodymium and erbium, and measured the coherence times of their optical and spin quantum states. The team reported, amongst other findings, the longest optical coherence times, namely 3 µs and 6 µs, for europium-doped crystals of sizes 60 nm and 100 nm, respectively.
Rare-earth ions see the light
The main goal of NanOQTech was to build nanoscale hybrid quantum devices that efficiently couple to light. “Thanks to the long coherence times of their quantum states, rare-earth qubits coupled to microcavities can function as hybrid quantum systems, with several potential applications in quantum information science,” notes Goldner. To be useful for quantum applications, rare-earth ions and microcavities need to be maintained at very low (cryogenic) temperatures. The project team achieved efficient coupling between a few rare-earth ions and a fibre-based microcavity in closed-cycle cryostats. The NanOQTech researchers’ experiment demonstrated the potential to achieve optical communication to a single rare-earth ion inside the fibre-based microcavity, a phenomenon of significant interest for quantum information storage. Although the project did not succeed in detecting a single ion in the cavity, it explored how a strong Purcell effect (Purcell factors of up to 150) enhanced optical interaction with 10 rare-earth ions. Ultimately, project researchers fabricated hybrid systems that can interact with rare-earth ions and therefore could exploit the transferred quantum properties to enable new functionalities. “Rare-earth ions can form a quantum interface between light and other systems, in our case, graphene plasmons and mechanical oscillators,” observes Goldner. The project established rare-earth-doped nanostructures as a new platform for optical quantum technologies. The nanoparticles developed during NanOQTech demonstrated unmatched coherence times of their optical and spin quantum states. Project work paves the way to building quantum networks that transmit information through solid-state qubits. Compared to solid-state qubits based on atom-like defects in diamond or quantum dots, rare-earth qubits live longer, are more stable, and have strong potential as scalable quantum systems.
Keywords
NanOQTech, rare-earth ion, coherence, solid-state qubits, quantum network, spin quantum state, fibre-based microcavity, quantum information storage, graphene, mechanical resonators
