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1. Erasure conversion in a high-fidelity Rydberg quantum simulator

   Scholl, Pascal; Shaw, Adam; Tsai, Richard; Finkelstein, Ran; Choi, Joonhee; Endres, Manuel (October 2023, Nature)

   Minimizing and understanding errors is critical for quantum science, both in noisy intermediate scale quantum (NISQ) devices1 and for the quest towards fault-tolerant quantum computation2,3. Rydberg arrays have emerged as a prominent platform in this context4 with impressive system sizes5,6 and proposals suggesting how error-correction thresholds could be significantly improved by detecting leakage errors with single-atom resolution7,8, a form of erasure error conversion9,10,11,12. However, two-qubit entanglement fidelities in Rydberg atom arrays13,14 have lagged behind competitors15,16 and this type of erasure conversion is yet to be realized for matter-based qubits in general. Here we demonstrate both erasure conversion and high-fidelity Bell state generation using a Rydberg quantum simulator5,6,17,18. When excising data with erasure errors observed via fast imaging of alkaline-earth atoms19,20,21,22, we achieve a Bell state fidelity of ≥0.9971−13+10, which improves to ≥0.9985−12+7 when correcting for remaining state-preparation errors. We further apply erasure conversion in a quantum simulation experiment for quasi-adiabatic preparation of long-range order across a quantum phase transition, and reveal the otherwise hidden impact of these errors on the simulation outcome. Our work demonstrates the capability for Rydberg-based entanglement to reach fidelities in the 0.999 regime, with higher fidelities a question of technical improvements, and shows how erasure conversion can be utilized in NISQ devices. These techniques could be translated directly to quantum-error-correction codes with the addition of long-lived qubits7,22,23,24. 

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2. Long-lived Bell states in an array of optical clock qubits

   https://doi.org/10.48550/arXiv.2111.14653  

   Schine, N.; Young, A.W.; Eckner, W.J.; Martin, M. J.; Kaufman, A. M. (July 2022, ArXivorg)

   The generation of long-lived entanglement on an optical clock transition is a key requirement to unlocking the promise of quantum metrology. Arrays of neutral atoms constitute a capable quantum platform for accessing such physics, where Rydberg-based interactions may generate entanglement between individually controlled and resolved atoms. To this end, we leverage the programmable state preparation afforded by optical tweezers along with the efficient strong confinement of a 3d optical lattice to prepare an ensemble of strontium atom pairs in their motional ground state. We engineer global single-qubit gates on the optical clock transition and two-qubit entangling gates via adiabatic Rydberg dressing, enabling the generation of Bell states, |ψ⟩=12√(|gg⟩+i|ee⟩), with a fidelity of F=92.8(2.0)%. For use in quantum metrology, it is furthermore critical that the resulting entanglement be long lived; we find that the coherence of the Bell state has a lifetime of τbc=4.2(6) s via parity correlations and simultaneous comparisons between entangled and unentangled ensembles. Such Bell states can be useful for enhancing metrological stability and bandwidth. Further rearrangement of hundreds of atoms into arbitrary configurations using optical tweezers will enable implementation of many-qubit gates and cluster state generation, as well as explorations of the transverse field Ising model and Hubbard models with entangled or finite-range-interacting tunnellers. 

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3. Erasure conversion in a high-fidelity Rydberg quantum simulator

   https://doi.org/10.1038/s41586-023-06516-4  

   Scholl, Pascal; Shaw, Adam L.; Tsai, Richard Bing-Shiun; Finkelstein, Ran; Choi, Joonhee; Endres, Manuel (October 2023, Nature)

   Abstract Minimizing and understanding errors is critical for quantum science, both in noisy intermediate scale quantum (NISQ) devices¹and for the quest towards fault-tolerant quantum computation^2,3. Rydberg arrays have emerged as a prominent platform in this context⁴with impressive system sizes^5,6and proposals suggesting how error-correction thresholds could be significantly improved by detecting leakage errors with single-atom resolution^7,8, a form of erasure error conversion^9–12. However, two-qubit entanglement fidelities in Rydberg atom arrays^13,14have lagged behind competitors^15,16and this type of erasure conversion is yet to be realized for matter-based qubits in general. Here we demonstrate both erasure conversion and high-fidelity Bell state generation using a Rydberg quantum simulator^5,6,17,18. When excising data with erasure errors observed via fast imaging of alkaline-earth atoms^19–22, we achieve a Bell state fidelity of$$\ge 0.997{1}_{-13}^{+10}$$ $\ge 0.9971_{-13}^{+10}$, which improves to$$\ge 0.998{5}_{-12}^{+7}$$ $\ge 0.9985_{-12}^{+7}$when correcting for remaining state-preparation errors. We further apply erasure conversion in a quantum simulation experiment for quasi-adiabatic preparation of long-range order across a quantum phase transition, and reveal the otherwise hidden impact of these errors on the simulation outcome. Our work demonstrates the capability for Rydberg-based entanglement to reach fidelities in the 0.999 regime, with higher fidelities a question of technical improvements, and shows how erasure conversion can be utilized in NISQ devices. These techniques could be translated directly to quantum-error-correction codes with the addition of long-lived qubits^7,22–24. 

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4. A universal quantum gate set for transmon qubits with strong ZZ interactions

   Junling Long, Tongyu Zhao (March 2021, ArXivorg)

   null (Ed.)

   High-fidelity single- and two-qubit gates are essential building blocks for a fault-tolerant quantum computer. While there has been much progress in suppressing single-qubit gate errors in superconducting qubit systems, two-qubit gates still suffer from error rates that are orders of magnitude higher. One limiting factor is the residual ZZ-interaction, which originates from a coupling between computational states and higher-energy states. While this interaction is usually viewed as a nuisance, here we experimentally demonstrate that it can be exploited to produce a universal set of fast single- and two-qubit entangling gates in a coupled transmon qubit system. To implement arbitrary single-qubit rotations, we design a new protocol called the two-axis gate that is based on a three-part composite pulse. It rotates a single qubit independently of the state of the other qubit despite the strong ZZ-coupling. We achieve single-qubit gate fidelities as high as 99.1% from randomized benchmarking measurements. We then demonstrate both a CZ gate and a CNOT gate. Because the system has a strong ZZ-interaction, a CZ gate can be achieved by letting the system freely evolve for a gate time tg=53.8 ns. To design the CNOT gate, we utilize an analytical microwave pulse shape based on the SWIPHT protocol for realizing fast, low-leakage gates. We obtain fidelities of 94.6% and 97.8% for the CNOT and CZ gates respectively from quantum progress tomography. 

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5. Preparation and control of neutral atom ensemble qubits using Rydberg interactions

   Young, Christopher; Kwon, Minho; Yang, Peiyu; Huft, Preston; Radzom, Brandon; Ebert, Matthew; Walker, Thad; Saffman, Mark (May 2019, DAMOP19 Meeting of The American Physical Society)

   Ensemble qubits with strong coupling to photons and resilience against single atom loss are promising candidates for building quantum networks. We report on progress towards high fidelity preparation and control of ensemble qubits using Rydberg blockade. Our previous demonstration of ensemble qubit preparation at a fidelity <60% was possibly limited by Rydberg blockade leakage due to uncontrolled short range atom pair separation. We show progress towards ensembles with a blue-detuned 1-D lattice on top of the existing red-detuned dipole trap, which will suppress unwanted Rydberg interactions by imposing constraints on the atomic separation. We study the effect of lattice insertion on the fidelity of ensemble state preparation and Rydberg-mediated gates. Studies of cooperative scattering from a 1D atomic array will also be presented. 

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