Session M08: Quantum Computation and Simulation with Molecules

Quantum annealing with 2Σ molecules

Quantum annealing (QA) has been considered as a practical quantum algorithm for NP-complete and NP-hard optimization problems. While most of the QA devices currently in use are based on superconducting circuits using stoquastic Hamiltonians, Quantum Monte Carlo algorithms can efficiently simulate stoquastic dynamics and therefore no exponential speed-up is expected from these systems. Ultracold molecules in optical lattices have been recently considered as platforms for quantum simulation. ²Σ molecules display an avoided crossing in the spin-rotational energy levels when placed in superimposed DC electric and magnetic fields. Dipole-dipole interactions between molecules in rotational states close to the avoided crossing lead to an XXZ model which can be tuned from an XY model at the crossing to an Ising model away from the crossing. In this work, we propose two architectures for quantum annealers based on ²Σ molecules placed in superimposed electric and magnetic fields. Qubits can be designed using pairs of molecules in combined DC fields or molecules in microwave fields. The latter results in a non-stoquastic Hamiltonian during the annealing.   

Rotational Coherence of Polar Molecules in an Optical Tweezer

An optical tweezer array of polar molecules is an appealing quantum computing platform. Qubit gate operations using rotational states are reliant upon dipolar interactions between individual molecules. We demonstrate a rotational qubit coherence time of \tau ~100 ms for laser cooled CaF molecules in an optical tweezer. Application of spin echo pulses result in a coherence time up to 500 ms.  In order to achieve these long \tau, we engineer the anisotropic interaction of the molecules with the tweezer light. By tuning the tweezer light polarization and magnetic field to a “magic angle” condition, we reduce the differential polarizability and the inhomogeneous broadening due to the thermal motion of molecules in the trap. \tau could be increased with further cooling of the trapped molecule. \tau can be compared to dipolar gate times in this system, which are predicted to be on the microsecond to millisecond timescale. Thus, our observed 100 ms rotational coherence time is long enough to demonstrate the initial feasibility of this platform for quantum computing. We also note that the qubit states we use are generic to a large category of laser coolable molecules, including polyatomic species.   

Toward quantum-logic spectroscopy of single molecular ions in a cryogenic ion trap

Quantum state control of molecules has applications including tests of fundamental physics and quantum information processing, but molecules’ additional degrees of freedom make the control required for these applications more challenging than for atoms. In our group, the application of quantum-logic spectroscopy (QLS) in an ion trap has enabled preparation and coherent manipulation of pure molecular quantum states of a single CaH⁺ ion [1]. Moving forward, we would like to expand the variety of molecular ions we can study and reduce the detrimental effects of background gas collisions and black-body radiation seen in a room-temperature environment. Here, we present progress in the construction and operation of a cryogenic ion trap apparatus for loading a broad range of molecular ions and achieving better control of their states. Molecules are injected via a molecular beam source to be ionized and co-trapped with an atomic ion for QLS. We discuss ongoing investigation into versatile methods of ionization and the forthcoming integration of a closed-cycle cryostat for cryogenic operation.

[1] C.-W. Chou et al., Nature 545, 203 (2017).   

Observation of the Hanbury Brown and Twiss Effect with Ultracold Molecules

Measuring the statistical correlations of individual quantum objects provides an excellent way to study complex quantum systems. Ultracold molecules represent a powerful platform for quantum simulation and quantum computation due to their rich and controllable internal degrees of freedom. However, the detection of correlations between single molecules in an ultracold gas had not been previously demonstrated. We report on the observation of the Hanbury Brown and Twiss effect in a gas of bosonic NaRb, enabled by the realization of a quantum gas microscope for molecules. We detect the characteristic bunching correlations in the density fluctuations of a 2D molecular gas released from and subsequently recaptured in an optical lattice. The quantum gas microscope allows us to extract the positions of individual molecules with single-site resolution. As a result, we obtain a high-contrast two-molecule interference pattern with a visibility of 54(13)%. Our work paves the way toward realizing other quantum optical phenomena with molecules of increasing complexity.   

Quantum gas microscopy of polar molecules

Ultracold molecules are a promising platform for quantum simulation of spin physics due to their long-range interactions and large set of internal states. However, to understand the complex many-body states that emerge in these systems in and out of equilibrium, new experimental techniques are needed to probe molecule correlations in the strongly-interacting regime. Here we study the site-resolved dynamics of spin correlations in a gas of ultracold NaRb molecules in a 2D optical lattice. We first form NaRb Feshbach molecules in the lattice before transferring them to the ground state via STIRAP with 85% one-way efficiency. We operate at near-magic trapping conditions where we prepare long-lived superpositions of the ground and first excited rotational states. The molecules realize a 2D quantum XY model with long-range interactions. Using a site-resolved Ramsey interferometric technique, we detect oscillations in nearest- and next-nearest-neighbor correlations due to spin interactions. The correlations are measured by dissociating the molecules and detecting the corresponding Rb atoms with single-site resolution using a quantum gas microscope. The techniques presented here open new doors for probing quantum correlations in complex many-body systems of ultracold molecules.   

Optical trapping of a polyatomic molecule

Polyatomic molecules have unique features not found in atoms or diatomic molecules that make them ideally suited for novel applications in quantum simulation, quantum computation, and precision measurements. However, the rotational and vibrational degrees of freedom that give rise to these features also present a challenge to realizing the high degree of quantum state control that is achieved with optically trapped atoms and, more recently, diatomic molecules. We will present progress towards achieving such control with a polyatomic molecule, calcium monohydroxide (CaOH). Starting with magneto-optically trapped CaOH molecules, we demonstrate deep laser cooling to temperatures of ~20 μK and discuss recent progress on loading the cooled CaOH molecules into an optical dipole trap. The long lifetimes afforded by the optical dipole trap makes it an ideal platform for investigating the suitability of CaOH for quantum simulation and quantum computation applications.   

A Novel Imaging Technique for Optical Tweezer Arrays of CaF Molecules

We report on a novel imaging scheme that offers nearly background-free high-fidelity detection of CaF molecules in optical tweezer traps.  Our scheme relies on two distinct electronic transitions, one for laser cooling and one for fluorescent detection, and consequently provides low detection backgrounds. We have quantified the imaging losses and have characterized many of the possible loss channels. We have also identified methods that can mitigate some of these losses. In addition to further improving the detection fidelity, these methods may also help with slowing and MOT loading.   

Seconds-scale coherence on nuclear spin transitions of ultracold NaRb polar molecules in 3D optical lattices

Ultracold polar molecules (UPMs) are emerging as a novel and powerful platform for fundamental applications in quantum science. Here, we report characterization of the coherence between nuclear spin levels of ultracold ground-state sodium-rubidium molecules loaded into a 3D optical lattice with a nearly photon scattering limited trapping lifetime of 9(1) seconds. After identifying and compensating the main sources of decoherence, we achieve a maximum nuclear spin coherence time of  T_2^* = 3.3(6) s with two-photon Ramsey spectroscopy. Furthermore, based on the understanding of the main factor limiting the coherence of the two-photon Rabi transition, we obtain a Rabi lineshape with linewidth below 0.8 Hz. The simultaneous realization of long lifetime and coherence time, and ultra-high spectroscopic resolution in our system unveils the great potentials of UPMs in quantum simulation, computation, and metrology.   

A Reconfigurable Optical Tweezer Array of Fully Quantum State Controlled Ultracold Dipolar Molecules

Ultracold dipolar molecules represent a promising platform for near-term quantum information and simulation experiments, by virtue of their strong, long-range dipolar interactions and ther long-lived ground states, which can be used to robustly encode quantum information. In this talk, I will discuss our preparation of an optical tweezer array of single NaCs molecules, starting from arrays of laser-cooled Na and Cs atoms. Using adiabatic assembly of the molecules followed by microwave control of their rotation, we achieve full control over the rovibrational and hyperfine degrees of freedom of the molecule. With full control of these single molecules achieved, we are now working towards controlling dipole-dipole interactions between them and using the interaction to engineer entangling quantum gates between molecular qubits. I will further discuss our strategy to generate densely filled arrays of molecules through dynamic reconfiguration of the tweezer array.   

Microwave dressed polar molecules in two-dimensional optical lattices

A gas of ultracold polar molecules trapped in an optical lattice provides a powerful platform for realizing lattice spin models and Hubbard models with longer-range interactions. In this context, realizing the most interesting phases requires a sample with low entropy and collisional stability. However, the inelastic collisions between molecules prevents efficient preparation of such samples. Here we report on loading microwave dressed ²³Na⁴⁰K molecules into a few layers of two-dimensional optical lattices. The microwave-dressed molecules are not only shielded from two-body inelastic collisions, but also feature strong elastic dipolar collisions.These properties allows for coherent tunneling and thermalization in the lattices, thus improve the loading efficiency.  We achieved more than 15% filling fraction and observe 10 seconds lifetime of molecules in a deep lattice. Our results lay the groundwork for future studies of many-body physics with polar molecules in optical lattices.