Protocols for designing and manipulating qubits with ultracold alkali atoms in 3D optical lattices are introduced. These qubits are formed from two-atom spin superposition states that create a decoherence-free subspace immune to stray magnetic fields, dramatically improving coherence times while still enjoying the single-site addressability and Feshbach resonance control of state-of-the-art alkali atom systems. The protocol requires no continuous driving or spin-dependent potentials, and instead relies upon the population of a higher motional band to realize naturally tunable in-site exchange and cross-site superexchange interactions. As a proof-of-principle example of their utility for entanglement generation for quantum computation, it is shown that the cross-site superexchange interactions can be used to engineer 1D cluster states. Explicit protocols for experimental preparation and manipulation of the qubits are also discussed, as well as methods for measuring more complex quantities such as out-of-time-ordered correlation functions (OTOCs).
Protocols for designing and manipulating qubits with ultracold alkali atoms in 3D optical lattices are introduced. These qubits are formed from two-atom spin superposition states that create a decoherence-free subspace immune to stray magnetic fields, dramatically improving coherence times while still enjoying the single-site addressability and Feshbach resonance control of state-of-the-art alkali atom systems. The protocol requires no continuous driving or spin-dependent potentials, and instead relies upon the population of a higher motional band to realize naturally tunable in-site exchange and cross-site superexchange interactions. As a proof-of-principle example of their utility for entanglement generation for quantum computation, it is shown that the cross-site superexchange interactions can be used to engineer 1D cluster states. Explicit protocols for experimental preparation and manipulation of the qubits are also discussed, as well as methods for measuring more complex quantities such as out-of-time-ordered correlation functions (OTOCs).
Measurement-based quantum computation, an alternative paradigm for quantum information processing, uses simple measurements on qubits prepared in cluster states, a class of multiparty entangled states with useful properties. Here we propose and analyze a scheme that takes advantage of the interplay between spin-orbit coupling and superexchange interactions, in the presence of a coherent drive, to deterministically generate macroscopic arrays of cluster states in fermionic alkaline earth atoms trapped in three dimensional (3D) optical lattices. The scheme dynamically generates cluster states without the need of engineered transport, and is robust in the presence of holes, a typical imperfection in cold atom Mott insulators. The protocol is of particular relevance for the new generation of 3D optical lattice clocks with coherence times >10 s, 2 orders of magnitude larger than the cluster state generation time. We propose the use of collective measurements and time reversal of the Hamiltonian to benchmark the underlying Ising model dynamics and the generated many-body correlations.
We present a protocol to prepare decoherence-free cluster states using ultracold atoms loaded in a two dimensional superlattice. The superlattice geometry leads to an array of $2\ifmmode\times\else\texttimes\fi{}2$ plaquettes, each of them holding four spin-1∕2 particles that can be used for encoding a single logical qubit in the twofold singlet subspace, insensitive to uniform magnetic field fluctuations in any direction. Dynamical manipulation of the supperlattice yields distinct inter- and intraplaquette interactions and permits us to realize one qubit and two qubit gates with high fidelity, leading to the generation of universal cluster states for measurement based quantum computation. Our proposal based on inter- and intraplaquette interactions also opens the path to study polymerized Hamiltonians which support ground states describing arbitrary quantum circuits.
Gate operations composed in quantum circuits form the basis for digital quantum simulation and quantum processing. While two-qubit gates generally operate on nearest neighbors, many circuits require nonlocal connectivity and necessitate some form of quantum information transport. Yet, connecting distant nodes of a quantum processor still remains challenging, particularly for neutral atoms in optical lattices. Here, we create singlet pairs of two magnetic states of fermionic potassium-40 atoms in an optical lattice and use a bidirectional topological Thouless pump to transport, coherently split, and separate the pairs, as well as to demonstrate interaction between them via tuneable ( SWAP ) α -gate operations. We achieve pumping with a single-shift fidelity of 99.78(3)% over 50 lattice sites and split the pairs within a decoherence-free subspace. Gates are implemented by superexchange interaction, allowing us to produce interwoven atomic singlets. For readout, we apply a magnetic field gradient, resulting in single- and multifrequency singlet-triplet oscillations. Our work shows avenues to create complex patterns of entanglement and new approaches to quantum processing, sensing, and atom interferometry.
We propose a scalable quantum-computing architecture based on cold atoms confined to sites of a tight optical lattice. The lattice is placed in a nonuniform magnetic field and the resulting Zeeman sublevels define qubit states. Microwave pulses tuned to space-dependent resonant frequencies are used for individual addressing. The atoms interact via magnetic-dipole interactions allowing implementation of a universal controlled-NOT gate. The resulting gate operation times for alkalis-metals are on the order of milliseconds, much faster then the anticipated decoherence times. Single qubit operations take about $10\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}s}$. Analysis of motional decoherence due to NOT operations is given. We also comment on the improved feasibility of the proposed architecture with complex open-shell atoms, such as Cr, Eu, and metastable alkaline-earth atoms with larger magnetic moments.
Over the past decade, quantum information science has progressed from few-qubit demonstrations to experiments with hundreds of qubits. Neutral atom processors based on individual atoms trapped in arrays of optical tweezers have emerged as a leading candidate due to their inherent scalability and flexible connectivity. Advanced protocols like measurement-based state preparation and quantum error correction, however, require efficient crosstalk-free mid-circuit operations for the readout, reset, and replenishment of a subset of qubits. In typical single-species atom arrays, where all atoms are identical and on-resonance, such operations necessitate complex overheads from single-site addressing, coherent transport, or shielding techniques, to mitigate the qubit decoherence induced from photons scattered by neighboring atoms. To address this challenge of engineering scalable quantum architectures compatible with efficient control techniques, we introduce and implement the first dual-species atom array platform, and explore new dynamics inaccessible in single-species arrays. In this approach, two atomic species are addressed independently due to the spectral separation between their control wavelengths. We engineer two-dimensional arrays from individual rubidium (Rb) and cesium (Cs) atoms in up to 512 optical tweezers, and show that the two species can be independently trapped, cooled, and imaged with state-of-the-art performance metrics. Harnessing this intrinsic crosstalk-free control, we establish the key primitives of mid-circuit readout, real-time processing and feed-forward, and coherent mid-circuit reloading. By integrating these capabilities, we realize in-sequence, real-time correction of correlated phase errors via a spectator qubit protocol. Next, we excite atoms to Rydberg states in the Förster regime, accessing hitherto unobserved enhanced interspecies Rydberg interactions, and program them to implement the first interspecies CNOT gate. We combine this multispecies entanglement with mid-circuit readout to achieve quantum non-demolition measurement of a Rb-qubit using an auxiliary Cs-qubit, circumventing physical qubit transport. Finally, leveraging the two qubit modalities, we present the first experimental demonstration of quantum cellular automata in atom arrays using only global addressing, and prepare a 17-qubit Cs cluster state with Rb-mediated interactions. The dual-species platform and techniques established in this thesis pave the way toward measurement-based algorithms and real-time feedback control in large-scale quantum systems.
Optical lattices with one atom on each site and interacting via cold controlled collisions provide an efficient way to entangle a large number of qubits with high fidelity. It has already been demonstrated experimentally that this approach is especially suited for the generation of cluster states [O. Mandel et al., Nature 425, 937 (2003)] which reduce the resource requirement for quantum computing to the ability to perform single-qubit rotations and qubit read out. In this paper, we describe how to implement these rotations in one-dimensional and two-dimensional optical lattices without having to address the atoms individually with a laser field.