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Neutral atoms interacting with single photons are a fascinating experimental platform that has been explored in the last decades for a series of applications in quantum science. A highlight in the history of this research was the awarding of the Nobel Prize to Serge Haroche in 2012. Over the last years, it turned out that neutral atoms trapped at the center of an optical cavity are a promising platform for the implementation of a novel kind of communication network that is based on the laws of quantum mechanics. The practical implementation of this quantum internet is an open challenge tackled by many research groups worldwide. The general idea is to employ the strong atom-light interaction provided by the cavity to implement an efficient interface between atoms and photons. This allows processing of quantum information within the network nodes. Additionally, the photons may also propagate to other nodes in the network and can therefore be employed as flying carriers of quantum information. Landmark experiments have been performed with this experimental platform, but the performance of state-of-the-art setups was limited because -due to a stochastic atom-loading procedure- it was practically not possible to work with more than two individually controlled neutral atoms trapped in each of the network nodes.
Fortunately, there is a solution to this challenge provided by optical tweezers which were pioneered by Nobel laureate Arthur Ashkin. Optical tweezers are tightly focused laser beams in which single atoms can be trapped and positioned with high precision. Injecting many tweezers into a reservoir of cold atoms from a magneto-optical trap allows for stochastic loading of a tweezer array with a success probability of about 50% per tweezer trap. Taking a picture of the array then allows identifying the empty tweezer traps. The occupied traps are rearranged to an ordered array which is continuously refilled from a spatially separated reservoir. Combining the optical tweezer platform with an optical cavity will combine the advantages of both experimental platforms and enable working with N>2 individually controlled atoms in the cavity mode. This is the mission of the Quantum Network Node group. The envisioned system is shown in Fig. 1 and comprises a set of atoms coupled to the cavity mode. Outside of the cavity mode resides the reservoir which allows replacing intracavity atoms in case one of them is lost from its respective tweezer trap.
The Quantum Network Node Group will focus on the implementation of this experimental system. We want to employ it as a versatile building block in a quantum network. The neutral atoms trapped at the center of the cavity are used as stationary carriers of quantum information while photons serve as flying carriers of quantum information. In addition to the tweezer beams arranging the atoms within the cavity mode, we will implement single-atom addressing to control the atomic qubits without affecting the neighboring atoms. This will enable individual qubit control and the intra-cavity qubits can interact via a bus provided by the commonly shared cavity mode.
Once available, the new system will open the route towards a series of exciting experiments. We plan to perform photon-mediated quantum information processing with the atomic qubits. The photons that trigger the inter-atomic quantum gates are injected into the cavity mode and can potentially originate from another network node. In particular one of our research goals is the implementation of an N-qubit Toffoli gate mediated by the reflection of a single photon from the cavity. This mechanism is independent of the number N of intra-cavity atoms. Additionally, we will focus on the generation of highly entangled optical cluster states which are versatile resource states in quantum communication and one-way quantum computing. Another research goal is the generation of optical Gottesman-Kitaev-Preskill (GKP) states. These states can be employed as a useful resource in one-way quantum repeater protocols and have not been generated with protocols based on deterministic principles.
Fig. 1: Envisioned experimental system: A set of atoms is assembled in the optical cavity with tweezer beams. Outside of the cavity mode, a reservoir of atoms is stored. In case an intra-cavity atom is lost, it can quickly be replaced with an atom from the reservoir.
As a long-term vision, we plan to connect several such systems to a quantum network link on the campus in Stuttgart. A picture showing the envisioned quantum network is shown in Fig. 2.
Fig. 2: Vision of a quantum network. Several network nodes are connected with glass fibers. Each of the nodes hosts neutral atoms assembled with optical tweezers. The nodes can be employed as quantum repeater stations or generators of useful resource states like photonic cluster states of GKP states.
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