Research

The unifying theme for our work is entanglement distribution between remote, compute-capable quantum nodes spanning distances > 10 km. For compute we focus on the 171Yb+ hyperfine qubit with laser-mediated gates. On the photonic side we are developing ion-photon hybrid interfaces compatible with commercial optical fiber networks. This work supports distributed quantum computing, optical clock comparisons and field sensing.

Direct generation of ion-entangled 1650 nm photons

Contemporary approaches to quantum networking with ions involve co-trapping two species. For example, 43Ca+ and 88Sr+ (link) or 171Yb+ and 138Ba+ (link) where one species serves as qubit and the other as quantum emitter. This approach has a successful track record but is complicated by the need for quantum frequency conversion (to reach telecom colors) and a two-species swap gate (to map coherences from the emitter to the qubit). Our lab explores an approach where the same ion species serves as both qubit and photon emitter. This exploits a peculiarity in several high-mass ions which simultaneously exhibit optical, metastable and ground state qubits (OMG, link).

Our approach uses the low-lying, metastable 2D3/2 and 2D5/2 manifolds in Yb+. The level diagram to the right shows the states involved in one class of experiment. Here we spontaneously generate a 1650 nm photon using a 3+1 level scheme: a 435 nm pulse is followed by a 1345 nm pulse. This has several advantages:

1650 nm light is directly emitted
emission of two 1650 nm photos in a single cycle is highly unlikely
only a single ion species is required

When we add an optical cavity, the cavity-assisted Raman interaction to reduces scattering to the ground state, deterministically generates 1650 nm photons and permits photon wavepacket shaping. Note that the near-IR color is also well matched to silicon-based photonic integrated circuits (PICs).

Cryogenic cavity-coupled Yb+ ion traps

We built two ultra low vibration cryogenic systems that enable rapid prototyping of photonic structures with trapped ions. Our first ion-integrated photonic structure is a fiber-coupled mesoscopic Fabry-Perot cavity.

At cryogenic temperatures ion qubits remains trapped for extended periods (our record is 5 weeks) and motional heating is substantially reduced. A key enabling feature of our cryostats is rapid prototyping on the trap and photonic structure -- a typical turn around time from room temperature to ion trapping is 2-3 days.

Modeling Field-Deployed Optical Fiber

Optical fibers are a critical tool for a number of active research areas, such as quantum networks. Across many of these applications, the polarization of the light being guided is crucial. For instance, in quantum networks, polarization is a common choice for encoding quantum information. And, in sensors that depend on interferometry, the two interfering signals should have the same polarization for maximum sensitivity. However, commercial single-mode optical fibers are subject to time-varying polarization mode dispersion (PMD) which complicates these aims.

Historical models of PMD were phenomenological and under-constrained. We have developed a software infrastructure called BIFROST (github) that calculates PMD and phase delay from first principles. Soon we will extend the model to include spontaneous Raman scattering and polarization-dependent loss.

Future applications of the BIFROST toolkit include the following.

Explore the fundamental limits of wavelength division multiplexing for co-existence of quantum and classical signals on a single fiber.
Use machine learning and polarization-sensitive optical time domain reflectometry (PS-OTDR) to reverse-engineering the path shape of deployed fiber.
Support NIST & IEEE in the development of standards for fiber deployment for quantum networking.

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