Cold atoms and molecules have emerged as a leading platform for assembling highly coherent quantum matter -- the essential building block for quantum simulation, computation, communication, and metrology. Generating spin-based entanglement within an array of atoms and molecules requires spin-dependent interactions; however, the scalability of conventional approaches based on local motion-mediated spin exchange has proven to be difficult. Hence, platforms that exhibit non-local interactions have emerged as a promising alternative. My research pushes the frontier of such interactions by using dipolar couplings between polar molecules and highly-excited Rydberg atoms in low-entropy arrays, to entangle highly coherent spin states.
An ideal platform for generating highly-entangled quantum many-body systems is a defect-free array of particles that have long spin coherence times and sufficiently strong interactions over the nearest-neighbor spacing in the array or further. My research has pioneered two novel approaches to such a system, which I will now briefly overview.
Ultracold polar molecules in optical lattices
In Jun Ye's and the late Deborah Jin's groups at JILA, we demonstrated that the large 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 allow for entanglement generation over relatively large distances in an optical lattice. Future experiments in this direction require 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, and further cooling is now possible.
An alternative to cooling is to directly synthesize a low entropy sample. By tailoring the constituent atomic gases in an optical lattice prior to making molecules, efficient conversion of the atomic pairs into molecules generates a low-entropy array. Our investigation of this 'quantum synthesis' method led to a five-fold reduction in the entropy. A logical extension to this bottom-up approach would involve single-particle control.
Tweezer arrays of strontium atoms
This particle-by-particle approach can be realized with optical tweezers (tightly-focused dipole traps). In Manuel Endres' group at Caltech, we demonstrated the first detection and cooling of single strontium (Sr) atoms, and we employed large one- and two-dimensional (2D) arrays of optical tweezers (see also the Kaufman and Thompson groups). Further, we perform rearrangement in 1D of the occupied tweezers to create an assembled array of alkaline-earth (-like) atoms (AEAs) with nearly zero entropy for the first time. The extension of these tools to AEAs has profound ramifications for quantum computing, communication, simulation, and metrology.
AEAs offer access to ultranarrow (~mHz linewidth) optical 'clock' transitions that are employed in the most precise atomic clocks and novel quantum simulators. We have interrogated this ultranarrow transition with ~Hz linewidth and single-atom resolution, and constructed the first atomic array optical clock (see also the Kaufman group). Interactions between atoms in the 'clock' state can be engineered using Rydberg states. We have observed highly-coherent Rydberg excitation, and this combination of highly coherent clock and Rydberg state control at the single-atom level opens the door to significant advances in quantum computing, communication, simulation, and metrology.
I am involved in ongoing theory efforts at Caltech which are related to my past and present experimental research
Telecom-band quantum interfaces with alkaline-earth (-like) atoms
Generating atom-photon entanglement requires high cooperativity in a cavity QED context. Conventional approaches based on free-space Fabry-Perot resonators suffer from the difficulty in extracting the photon from the cavity for use in a quantum network. Atoms coupled to the evanescent field of nanophotonic crystal waveguides have emerged as a promising alternative, but a robust experimental demonstration is lacking.
In collaboration with Oskar Painter's group, we are extending this approach to AE atoms, which benefit from strong telecom-band transitions and long nuclear spin memories. Further, they allow short-wavelength optical tweezer trapping which enables a robust approach for coupling to the photonic crystal. The evanescent field has a larger spatial extent due to the longer wavelengths in the telecom window.
Operation in the telecom-band provides access to a wide variety of silicon-based photonic technologies and includes CMOS compatibility. Further, it allows for simplified fabrication of nanophotonic devices, which in some cases are even commercially available. It is therefore possible to integrate this system with silicon photonic circuits as a potential platform for all-silicon on-chip photonic quantum computing and quantum simulation.
Moreover, the telecom transitions can be employed for microwave-to-optical transduction, in which an AE atom is used as a nonlinear medium for four-wave-mixing. Strong electric dipole couplings at microwave frequencies can be obtained using highly-excited Rydberg states. We are studying this scenario in collaboration with Oskar Painter's group and with Mark Saffman. Such hybrid systems allow for the interconnectivity of disparate quantum platforms, such as superconducting qubits and cold atoms, into a quantum network.
In collaboration with Chris Greene's group, we are studying the possibilities when combining single-atom trapping and control of AE ions with neutral atoms. AE ions have similar structure to neutral alkalis, so optical trapping in optical tweezers or optical lattices is possible. Fluorescence detection of single ions is routine in ion trapping experiments, and techniques developed in that context, as well as for neutral alkalis in optical traps, can be applied.
It is possible to combine optical trapping of ions with Rydberg excitation of neutral AE atoms in the same system. If a neutral AE atom is within a few microns of an AE ion (Step I), subsequent excitation of the neutral atom to a highly excited Rydberg state (n > 100) will result in significant hybridization with molecular Rydberg states of AE_2+ (Step II). Specifically, we target a molecular state with equilibrium bond length equal to the spacing r between the ion cores in optical traps. This allows the electron to tunnel to the other ionic core (Step III) at a rate t_e related to the gerade-ungerade splitting of this molecular state, which could be ~MHz.
By dressing to such a Rydberg state, it may be possible to enable coherent electron tunneling between AE+ ionic cores. This is a new paradigm for cold atom physics, in which electrically conductive matter can be assembled atom-by-atom.
Coherent electron hopping between alkaline-earth (-like) atoms
Robust encoding of a qubit in a molecule
In collaboration with theorists at Caltech, we are studying quantum error correction protocols with polar molecules. Quantum systems described by continuous variables arise in many different laboratory settings. For example, a microwave resonator in a superconducting circuit or the motional degree of freedom of a trapped ion can be viewed as a harmonic oscillator with an infinite-dimensional Hilbert space. Such continuous-variable systems have potential applications to quantum information processing; however, quantum information encoded in an oscillator can be easily damaged by ubiquitous noise sources such as dissipation and diffusive motion in phase space.
Robustness against noise can be achieved more easily by encoding a protected finite-dimensional system within the infinite-dimensional Hilbert space of an oscillator. One method for doing so was proposed some years ago by Gottesman, Kitaev, and Preskill (GKP). A GKP code is a quantum error-correcting code designed to protect against noise that slightly shifts the position or momentum of an oscillator. The ideal basis states for the code space are "grid states'' supported on periodically spaced points in q or p space. By measuring the code's check operators, one can diagnose a shift error that may have occurred, without disturbing the encoded quantum information, and then correct the error (if the shift introduced by noise is not too large) by performing a compensating shift. These codes are expected to perform well against realistic noise, including dissipation, which typically acts locally in phase space. Construction of GKP grid states has recently been demonstrated experimentally.
In collaboration with theorists at Caltech, we are studying the connections between dipolar spin models with polar molecules in optical lattices and anyon fusion spaces. Spin-1 Heisenberg chains are known to map onto fusion spaces of Fibonacci anyons, and polar molecules can be used to realize such spin chains.
To this end we employ two novel aspects of polar molecule systems which are not widely appreciated. First, the anisotropic dipole-dipole interactions between polar molecules in an optical lattice can be larger than the external driving fields, thereby entering a 'dipolar blockade' regime analogous to Rydberg systems. This scenario emerges naturally for molecules with sufficiently large dipole moment, or are in a short-spacing optical lattice.
Second, we employ local spin-exchange interactions that only occur along one direction of an optical lattice. This can be accomplished by using a super-exchange process which is ubiquitous in cold atom experiments. The rate can be tuned with the lattice depth along that direction, and the sign can be tuned with the quantum statistics of the molecules. This process is particularly well suited for chemically stable molecules, and ideally even molecules with short-range van der Waals interactions, such as triplet ground state bi-alkali molecules or alkali-alkaline earth molecules.