A preeminent goal for quantum engineering is to interface a modular quantum computer, sensor, or clock with a quantum bus capable of conveying quantum information between modules. Such an architecture holds promise for both scalable quantum computing, and for building a quantum network for quantum communication and metrology.

Rydberg atom arrays are quickly becoming a competitive platform for quantum computing, with alkali atoms recently achieving 20-qubit entangled states. We have shown that AEAs have competitive or superior Rydberg properties, yet they can also be used in high-precision atomic array optical clocks as we have demonstrated. However, Yb in particular offers an incredible additional feature that we have studied in two recent proposals: strong transitions in the telecom wavelength band (~1.25-1.65 um) where loss in optical fibers is minimal. These transitions allow for direct, optical connectivity of quantum computers or clocks as well as quantum key distribution over meter or kilometer length scales without microwave-to-optical transduction or optical wavelength conversion.

We will develop a modular platform for quantum entanglement in an Yb tweezer array mediated by Rydberg interactions, as well as between atoms and a remote telecom-band photon using a Fabry-Perot cavity (see left). We are particularly interested in developing a long-distance quantum repeater network, as well as exploring a modular quantum computing architecture in which arrays of ~10-20 atoms are linked together via photons. Finally, this platform could naturally lead to an elementary quantum network of atomic clocks that will provide both a quantum-encrypted global time standard, and a Heisenberg-limited quantum sensor network operating over length and timescales relevant for fundamental physics and geodesy.

The level structure of 171Yb is sufficient rich and complex to allow us to perform readout operations or other controlled dissipative processes on individual qubits while maintaining the coherence of the others. We aim to leverage this capability to realize wide classes of quantum circuits with programmable dissipation, or openness, where "read enable" operations determine which qubits are measured and then subsequently reset on each layer of the circuit.

This capability is needed for real-time feedback in protocols like quantum error correction. Figure 2 shows a schematic of the so-called surface code, where stabilizers are encoded and measured with ancilla qubits. More generally, programmable openness will allow us to explore a "phase diagram" of random quantum circuits. As the amount of dissipation is varied, it is believed that there is a so-called measurement-induced phase transition between circuits with area- and volume-law entanglement growth. Interestingly, it has been predicted that real-time feedback (i.e. adaptive circuits) can be utilized to "steer" the system in the vicinity of such a phase transition.

Bipartite many-body systems with entanglement between and within the two regions offer access to a surprising range of physical models. In particular, bipartite many-body states with finite temperature are holographically dual to gravitational systems. The so-called thermofield double state -- the finite-temperature analog of the Bell or GHZ state -- is holographically dual to a wormhole between a pair of black holes. Therefore, by teleporting quantum information between the two regions in the bipartite finite-temperature system, we may be able to access in the laboratory the dual picture: traversing the wormhole.

Finite-temperature many-body systems are fascinating in their own right, and are often poorly understood. While most quantum simulations hope to identify the ground state, the low temperature region of the many-body spectrum is typically far more complex in terms of both its static and dynamic properties. Like the gravitational dual model, low-temperature many-body systems can be created from the thermofield double state -- in this case by using one region of the bipartite system as a thermal reservoir for the other. The system can be isolated from the reservoir by tracing over the reservoir degrees of freedom -- a feat that can be accomplished by measurement of those qubits. Therefore, the toolbox of tunable dissipation described above places a crucial role here as well.

This effort is mostly focused on "YbII", which features the 171Yb atom array in an optical cavity. Among other things, the cavity can be used for weak measurements of the collective state of the system, thereby generating spin squeezing. Such states hold great promise to improve optical atomic clocks below the so-called "standard quantum limit" set by quantum projection noise. We aim to explore the production and quantification of such squeezing at the atom-by-atom level with the hopes of achieving deeper squeezing. In the few-atom limit, the protocol is similar to photon-mediated quantum gates, and full state tomography is possible. In the many-atom limit, atom-atom correlations over arbitrary length scales are readily detectable, helping to characterize the many-body state and its deviation from the expected fully symmetric behavior.

Moreover, this "atom array optical clock" platform offers the ability to perform local single- or two-qubit gates that may complement the global, symmetric interactions with the cavity field. Indeed, spin-squeezed states could serve as an effective initial condition for generating a richer class of many-body entangled states that is metrologically useful. Generally speaking, we believe that having access to both global interactions mediated by the cavity and short-range interactions mediated by Rydberg gates offers a wealth of new possibilities for metrology and other areas of quantum information science. 

Through a novel approach to single-atom control employed in Manuel Endres' group at Caltech, we demonstrated the first detection and cooling of single strontium (Sr) atoms in tightly-focused optical tweezers, and the generation of large one- and two-dimensional arrays (see also Kaufman and Thompson groups). Alkaline-earth (-like) atoms (AEAs) such as Sr offer access to ~mHz-linewidth optical 'clock' transitions that are used in the most precise atomic clocks. We have interrogated the clock transition with ~Hz linewidth and constructed the first atomic array optical clock (see also Kaufman group). Moreover, interactions between atoms can be engineered using Rydberg states, for which AEAs offer several advantages including higher excitation rates and detection fidelities. We have recently observed highly-coherent Rydberg excitation as well as high-fidelity preparation and detection, opening the door to advances in quantum computing, simulation, and metrology with individually controlled AEAs. 

In the groups of Jun Ye and the late Deborah Jin at JILA, we demonstrated that the electric dipole moments of polar molecules are an excellent tool for spin-dependent interactions. We showed for the first time that long-lived rotational states combined with long-range dipole-dipole interactions give rise to many-body dynamics in an optical lattice. Future experiments in this direction required an advancement in experimental capabilities, such as large electric fields and high-resolution optics. I spearheaded the design and construction of a next generation apparatus with these tools, and we have used it to realize the first quantum degenerate gas of polar molecules. In a complementary experiment, we directly synthesized a low-entropy molecular array in a lattice by tailoring the constituent atomic gases in the lattice prior to making molecules.