Quantum nodes are key to building the quantum internet. These devices can store information for long durations of time, are capable of processing the information using local quantum gates between qubits, and can also send information to other distant nodes using photons, extending the network to larger distances.
Quantum nodes are platforms made of atoms, trapped ions, or solid-state systems, that use long-lived spin states to store data (quantum bits) and interactions between close-by qubits to perform quantum gates. For a quantum network to work we would need two things: long-lived spins (electronic or nuclear), and a way to interface these spins with photons. While the spins would have memory and processing capabilities, and efficient spin-photon interface would allow the photon to act as the carrier of this spin information among different nodes. So, the key is to find an optimal spin-photon interface.
Previous studies have shown that in order to have efficient spin-photon interfaces, a cavity is needed to channel and enhance the signal of the single quantum emitter, reaching the so-called Purcell enhancement regime. Importantly, to reach the large Purcell enhancement regime, the emitter must be placed in high-finesse and very small cavities. Quantum nodes with single rare-earth ions could provide such efficient spin-photon interfaces. Rare-earth ions can be doped into nanometer-scale particles, such that they can be embedded in small cavities allowing for large collection efficiencies and Purcell enhancement. Moreover, one of these rare-earth elements, erbium is promising for this task because the ions emit light directly at telecom wavelengths.
In a recent study published in Nature Communications, a team of researchers from the SQUARE research project of the Quantum Flagship has reported on being able to control the way light is emitted from a small ensemble of erbium ions doped into a nanoparticle. They were able to tune the Purcell effect, which determines the emission rate of photons, and achieved tuning rates more than 100 times faster than the natural decay of these ions, something never achieved before. The team of authors includes researchers Dr. Bernardo Casabone, Chetan Deshmukh, and Prof. Hugues de Riedmatten from ICFO, Dr. Shuping Liu, Dr. Diana Serrano, Dr. Alban Ferrier and Dr. Philippe Goldner from the Institut de Recherche de Chimie Paris, Dr. Thomas Hümmer from Ludwig-Maximilians- Universitat, and Prof. David Hunger from Karlsruher Institut für Technologie (KIT) – IQMT and coordinator of the SQUARE project.
The results of this work are a major step forward in quantum communications, in particular quantum nodes, because the control of the photon emission rate could lead to the generation of single photons with controllable waveform.The SQUARE Consortium
The experiment setup
The experiment was set up as follows. Chimie Paris Tech fabricated the erbium nanoparticles with a radius of 90nm, KIT fabricated the fiber cavities, and then these components were shipped to ICFO to carry out the experiment. The team of researchers at ICFO took one of the nanoparticles made up of several erbium ions and embedded it in a fiber-based micro-cavity, where the fiber mirror free to move in 3 dimensions and change the length of the micro-cavity. The setup was then placed inside a cryogenic fridge capable of cooling down to a few kelvins. Achieving the desired stability (tens of pm scale) and tunability of the setup inside the cryogenic fridge was one of the major technical achievements of the work which enabled the results. They then used a 1535 nm laser to resonantly excite the erbium ions and a 790 nm laser to control in real-time when the cavity could go into resonance with the light emitted by the ions by tuning the length of the cavity. They observed that when the cavity was not in resonance mode, the ions emitted light with their natural decay rate, which is rather long (12 ms). But when the cavity entered in resonance, the light was emitted at a much faster rate due to the Purcell effect. On resonance, it was shown that 10 % of the ions were emitting more than 70 times faster than the natural lifetimes. By controlling the length of the cavity at the sub-nm scale, the researchers could put the cavity on and off-resonance very fast, and could therefore control the emission rate on a time scale 100 times faster than the natural decay of the ions.
Being able to control the emission of light in such a tiny doped nano-particle device is of paramount importance because it has been shown theoretically that this system can serve as a quantum processor with tens of qubits at the nanoscale. Ions with different resonance frequencies within the nanoparticle could indeed in principle interact and be addressed independently to perform quantum gates. Controlling the emission rate would allow new functionalities such as being able to switch between a “read-out mode”, where an excited ion would emit a photon fast in the cavity mode, and a ” computing mode“, where the ion could be excited to perform two-qubit gates without emitting a photon. By controlling and tuning the length of the cavity at the sub-nm level using a piezo-electric actuator, they were able to control the Purcell enhancement effect and achieve these fast photon emission rates.
The results of this work are a major step forward in quantum communications, in particular quantum nodes, because the control of the photon emission rate could lead to the generation of single photons with a controllable waveform. It could also have applications in quantum networking and quantum computing. The challenge now lies in addressing the single ions individually of the erbium nanoparticle. This is something the team is working on and hoping to accomplish in another breakthrough since it has been achieved in different setups, but never in a system that allows this kind of fast tuning and high-Purcell enhancement.
Bernardo Casabone, Chetan Deshmukh, Shuping Liu, Diana Serrano, Alban Ferrier, Thomas Hümmer, Philippe Goldner, David Hunger & Hugues de Riedmatten. Dynamic control of Purcell enhanced emission of erbium ions in nanoparticles, Nature Communications, 2021.
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