I will discuss the cavity-mediated transfer of quantum information between ions and photons, with the goal of linking distant qubits in a quantum network, focusing on multi-ion encodings and adiabatic passage along channels.
Trapped ions are a promising platform for local quantum information processing. In order to distribute this quantum information over long distances, we can take advantage of optical cavities, which ofier a coherent interface between matter and light, enabling the transfer of quantum information from stationary qubits such as ions onto photons. We demonstrate such an interface by coupling trapped ions to a cavity and have recently shown that a quantum state can be faithfully transferred from a single ion onto a single photon. In particular, this transfer can be improved by taking advantage of a collective effect between multiple ions, namely, superradiant emission into the cavity. In this proof-of-principle experiment, we tune the phase of a two-ion entangled state between sub- and superradiance. The superradiant coupling is then used to enhance the transfer of quantum information onto a photon from a logical qubit encoded in the two ions. Finally, prospects for linking together distant ions in cavities via a quantum network are discussed. Toward this goal, we outline a fiber-based ion-cavity experiment which allows access to the single-ion strong-coupling regime.
Quantum networking exploits features of quantum mechanics to provide ultrasecure networks that are both tamper-proof and tamper-evident. Such networks can be implemented as distant memory nodes connected via photon-based interfaces. Trapped ions are nearly ideal quantum network nodes due to the precise control possible over both their internal and external degrees of freedom as well as for their superior performance as long-term quantum memories. Photon-based qubits are the natural choice to transfer information within the network due to their ability to transmit quantum information over long distances and the capability to process information ”on-the-fly” between the memory nodes. We present the quantum research being done at the Air Force Research Laboratory (AFRL) with a focus on trapped ion qubits, the short- and long-term goals of the lab, and some of the unique resources we have access to at AFRL.
Performing a faithful transfer of an unknown quantum state is a key challenge for enabling quantum networks. The realization of networks with a small number of quantum links is now actively pursued, which calls for an assessment of different state transfer methods to guide future design decisions. Here, we theoretically investigate quantum state transfer between two distant qubits, each in a cavity, connected by a waveguide, e.g., an optical fiber. We evaluate the achievable success probabilities of state transfer for two different protocols: standard wave packet shaping and adiabatic passage. The main loss sources are transmission losses in the waveguide and absorption losses in the cavities. While special cases studied in the literature indicate that adiabatic passages may be beneficial in this context, it remained an open question under which conditions this is the case and whether their use will be advantageous in practice. We answer these questions by providing a full analysis, showing that state transfer by adiabatic passage—in contrast to wave packet shaping—can mitigate the effects of undesired cavity losses, far beyond the regime of coupling to a single waveguide mode and the regime of lossless waveguides, as was proposed so far. Furthermore, we show that the photon arrival probability is in fact bounded in a trade-off between losses due to non-adiabaticity and due to coupling to off-resonant waveguide modes. We clarify that neither protocol can avoid transmission losses and discuss how the cavity parameters should be chosen to achieve an optimal state transfer.