Controlling the quantum entanglement between parts of a many-body system is key to unlocking the power of quantum technologies such as quantum computation, high-precision sensing, and the simulation of many-body physics. The spin degrees of freedom of ultracold neutral atoms in their ground electronic state provide a natural platform for such applications thanks to their long coherence times and the ability to control them with magneto-optical fields. However, the creation of strong coherent coupling between spins has been challenging. Here we demonstrate a strong and tunable Rydberg-dressed interaction between spins of individually trapped caesium atoms with energy shifts of order 1 MHz in units of Planck's constant. This interaction leads to a ground-state spin-flip blockade, whereby simultaneous hyperfine spin flips of two atoms are inhibited owing to their mutual interaction. We employ this spin-flip blockade to rapidly produce single-step Bell-state entanglement between two atoms with a fidelity ≥81(2)%. P ristine quantum control of many-body correlations is fundamental to realizing the power of quantum information processors. Steady progress has continued in various platforms ranging from solid-state spintronics 1 and superconductors 2,3 to nanophotonics 4 and ultracold trapped atoms, both ionic 5-7 and neutral 8-10 . Cold neutral atoms are particularly attractive as the ability to create entanglement between atoms would allow for greatly increased precision of interferometers for applications in clocks 11-13 , and force sensors 14-16 . In addition, cold atoms provide a natural platform for quantum simulation of condensed-matter physics 17,18 and scalable digital quantum computers 19-21 . Controlled entanglement of neutral atoms, however, has been challenging, particularly if one seeks tunable interactions that are strong, coherent and long-range (∼µm).One mechanism to achieve strong, long-range coupling is the Rydberg blockade 22 . This has been successfully employed for implementing controlled entangling interactions between atoms 9,10,23 and quantum logic gates 24 . In the standard protocol, short pulses excite the population of one atom to the Rydberg state and optical excitation of a second atom is blockaded because of the electric dipole-dipole interaction 21 (EDDI). An alternative protocol is to adiabatically dress the ground state with the excited Rydberg state 25-27 . This Rydberg-dressed interaction enables tunable, anisotropic interactions that open the door to quantum simulations of a variety of exotic quantum phases 26,28,29 . In addition, it allows for quantum control of interacting atoms based solely on microwave/radiofrequency fields whose phase coherence is easily maintained. Applications include spin-squeezing for metrology 13,25 , and quantum computing 30,31 . Although the promise of Rydbergdressed interactions is great, experimental demonstration has been elusive. We present here a clear measurement of this interaction between two Rydberg-dressed atoms and employ coherent control in the ...
We study a scheme for implementing a controlled-Z (CZ) gate between two neutral-atom qubits based on the Rydberg blockade mechanism in a manner that is robust to errors caused by atomic motion. By employing adiabatic dressing of the ground electronic state, we can protect the gate from decoherence due to random phase errors that typically arise because of atomic thermal motion. In addition, the adiabatic protocol allows for a Doppler-free configuration that involves counterpropagating lasers in a σ+/σ− orthogonal polarization geometry that further reduces motional errors due to Doppler shifts. The residual motional error is dominated by dipole-dipole forces acting on doubly-excited Rydberg atoms when the blockade is imperfect. For reasonable parameters, with qubits encoded into the clock states of 133 Cs, we predict that our protocol could produce a CZ gate in < 10 µs with error probability on the order of 10 −3 .
We study an architecture for implementing adiabatic quantum computation with trapped neutral atoms. Ground state atoms are dressed by laser fields in a manner conditional on the Rydberg blockade mechanism, thereby providing the requisite entangling interactions. As a benchmark we study the performance of a Quadratic Unconstrained Binary Optimization (QUBO) problem whose solution is found in the ground state spin configuration of an Ising-like model. We model a realistic architecture, including details of the atomic implementation, with qubits encoded into the clock states of 133Cs, effective B-fields implemented through stimulated Raman transitions, and atom-atom coupling achieved by excitation to the 100P3/2 Rydberg level. Including the fundamental effects of photon scattering, we find the fidelity of two-qubit implementation to be on the order of 0.99, with higher fidelities possible with improved laser sources.Comment: 5 pages, 3 figure
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