We report results of a ground-state entanglement protocol for a pair of Cs atoms separated by 6 μ m, combining the Rydberg blockade mechanism with a two-photon Raman transition to prepare the Bell state with a loss-corrected fidelity of 0.81(5), equal to the best demonstrated fidelity for atoms trapped in optical tweezers but without the requirement for dynamically adjustable interatomic spacing. Qubit state coherence is also critical for quantum information applications, and we characterise both ground-state and ground-Rydberg dephasing rates using Ramsey spectroscopy. We demonstrate transverse dephasing times ms and s for the qubit levels and achieve long ground-Rydberg coherence times of as required for implementing high-fidelity multi-qubit gate sequences where a control atom remains in the Rydberg state while applying local operations on neighbouring target qubits.
We report results of a ground-state entanglement protocol for a pair of Cs atoms separated by 6 μ m, combining the Rydberg blockade mechanism with a two-photon Raman transition to prepare the Bell state with a loss-corrected fidelity of 0.81(5), equal to the best demonstrated fidelity for atoms trapped in optical tweezers but without the requirement for dynamically adjustable interatomic spacing. Qubit state coherence is also critical for quantum information applications, and we characterise both ground-state and ground-Rydberg dephasing rates using Ramsey spectroscopy. We demonstrate transverse dephasing times ms and s for the qubit levels and achieve long ground-Rydberg coherence times of as required for implementing high-fidelity multi-qubit gate sequences where a control atom remains in the Rydberg state while applying local operations on neighbouring target qubits.
Neutral atoms provide an excellent resource for quantum information processing, combining the long atomic coherence times of the hyperfine ground-state with strong dipole-dipole interactions of highly excited Rydberg states for generating deterministic entanglement between qubits separated by < 10 µm [1]. Scalable long-range interactions can be obtained by coupling the atomic array to a superconducting microwave cavity enabling hybrid quantum information processing where the cavity-mediated entanglement allows atoms to be coupled over cm length scales. We present the first steps towards such an experiment incorporating high fidelity readout using an sCMOS camera [2] and the ability to drive fast, optically addressable rotations of the hyperfine-encoded qubits to the Rydberg state. Using our sub-kHz cavitystabilised laser system [3] we demonstrate coherent control of single Rydberg atoms, performing Ramsey spectroscopy to determine coherence time and to generate entanglement between a pair of atoms separated by 6 µm. Combining this excitation scheme with our ground-state Raman lasers we show progress towards the implementation of a mesoscopic Rydberg gate based on electromagnetically induced transparency (EIT) offering robust entanglement of multi-atom ensembles [4].
We present experimental results on two-qubit Rydberg-blockade quantum gates and entanglement in a two-dimensional qubit array. Without postselection against atom loss we achieve a Bell state fidelity of $0.73\ifmmode\pm\else\textpm\fi{}0.05$. The experiments are performed in an array of single Cs atom qubits with a site to site spacing of $3.8\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Using the standard protocol for a Rydberg-blockade CZ gate together with single qubit operations we create Bell states and measure their fidelity using parity oscillations. We analyze the role of ac Stark shifts that occur when using two-photon Rydberg excitation and show how to tune experimental conditions for optimal gate fidelity.
We report the implementation of universal two- and three-qubit entangling gates on neutral-atom qubits encoded in long-lived hyperfine ground states. The gates are mediated by excitation to strongly interacting Rydberg states and are implemented in parallel on several clusters of atoms in a one-dimensional array of optical tweezers. Specifically, we realize the controlled-phase gate, enacted by a novel, fast protocol involving only global coupling of two qubits to Rydberg states. We benchmark this operation by preparing Bell states with fidelity F≥95.0(2)%, and extract gate fidelity ≥97.4(3)%, averaged across five atom pairs. In addition, we report a proof-of-principle implementation of the three-qubit Toffoli gate, in which two control atoms simultaneously constrain the behavior of one target atom. These experiments demonstrate key ingredients for high-fidelity quantum information processing in a scalable neutral-atom platform.
This thesis reports progress towards using hyperfine-encoded neutral atomic ensembles for quantum information applications. Ensemble qubits are alternative building blocks to process quantum information, while circumventing the challenges preparing a low-entropy single-atom qubit register. While ensemble approach inherits the single-atom quantum gate protocols, cooperative atom-light interaction makes it favorable platform to implement atom-photon interface, such as quantum memory. We have demonstrated semi-deterministic preparation of W-state, which we use as an ensemble qubit. Observed 50–60% preparation fidelity shows discrepancy to expected > 80% fidelity including known imperfections. We suspect Ground-Rydberg and Rydberg-Rydberg molecular interactions within the ensemble modify the dynamics. State tomography and Ramsey experiment clearly identified that majority of constituents in an ensemble are indeed entangled. We report the first measurement of W-state coherence time as 2.6ms for 7.6 atoms, which is long enough to perform 2600 Rydberg-mediated quantum gate operations. The first demonstration of Rydberg blockade between two ensembles happened in the same apparatus. We observe blockade fidelity of 0.89 (uncorrected) and near unity when post-selected on successful N = 1 Fock state preparation of Control ensemble. For weakly blockaded cases, Target ensemble remained as a perturber inhibiting Control ensemble from de-excitation. We discuss the efficacy of optical lattice insertion to suppress molecular interactions that might have been affecting the Rydberg dynamics. High fidelity, parallel, low loss fluorescence detection of hyperfine ground state of rubidium atoms is presented. Enforced cycling transition scattering minimally alters the internal states. We achieve mean state detection fidelity of 97% (uncorrected) and 98.7% (SPAM error corrected), with atom loss rate < 2% and hyperfine-changing rate < 2%. This scalable technique is also compatible with ensemble qubits and error correcting schemes. Technical developments on apparatus, lasers, supporting hardware and software are discussed. Automatic beam alignment system is implemented to correct long term drift of tightly focused addressing beams. Achieved pointing stability of 60 nm. Two home-made, high performing, cavity-enhanced frequency doubling systems were constructed to meet experimental demand. We observe 960/480 system output as high as 180 mW with 600 mW fundamental, and 1540/770 system output as high as 13.97 W from 19 W fundamental.
For neutral-atom qubits, two-qubit gates are typically realized via the Rydberg-blockade effect, which hints at the special status of Rydberg levels compared with regular qubit register states. However, the experimental and theoretical studies that have been done to reveal how the ground-Rydberg coherence of the control-qubit atom affects the fidelity of a two-qubit gate are not yet adequate, especially when all experimental two-qubit gates feature a single atom that stays in the Rydberg state for a certain time. Here, we perform a careful investigation of decoherence in ground-Rydberg transitions in a single atom with various pulse sequences. In particular, based on the commonly used controlled-Z (CZ) gate sequence proposed by D. Jaksch et al., Phys. Rev. Lett. 85, 2208 (2000), we observe that a control-qubit atom subject to a two-qubit entangling process with the H-CZ controlled-not gate scheme experiences a process that is essentially similar to ground-Rydberg Ramsey interference. Then, we build a straightforward theoretical model to link the decoherence time ${\ensuremath{\tau}}_{\mathrm{gr}}$ of the control qubit (extracted from Ramsey fringes of the ground-Rydberg transition) to this CZ pulse sequence, and also analyze the typical origins of decoherence effects. Finally, we discuss the loss of fidelity of the CZ gate due to the limits imposed by the ground-Rydberg coherence properties, and the prospect of improving fidelity with refined gate protocols.
Abstract Multi-qubit quantum gates are the most important building blocks for quantum computation. In this article, we propose a scheme for realizing the Toffoli gate in a linear neutral atom array. Two ground-state hyperfine levels of the atoms are considered as qubit states. Our method relies on the strong and long-range interactions between atoms due to Rydberg excitations and the occurrence of dark states in the target qubit, with both control and target qubits being individually addressed with laser pulses. Our gate protocol enables precise control over the quantum states of individual qubits, effectively suppressing undesirable transitions to ensure high-fidelity gate performance. We also implement two-atom Rydberg antiblockade mechanism to avoid any unwanted population blockage in the control atoms to get the best fidelity. At zero temperature, the gate fidelity is estimated to be about 99.16% for realistic laser power omitting the motional state of the atoms when the interatomic distance ( l ) 6.55 μ m. We construct a C n NOT gate with n > 2 in a 2D atomic array by exploiting the n -atom Rydberg antiblockade (RAB) mechanism, which allows n atoms within the blockade radius to be simultaneously excited to the Rydberg states. In particular, we simulate C 3 NOT gate with about 98.37 % fidelity. We further analyze the gate error by decreasing the interatomic distance and find that gate fidelity improves more rapidly for larger n with increased interaction potential. We find that more than 99 % fidelity can be achieved for C 3 NOT gate for l < 6.2 μ m, implying that our proposed scheme promising and reliable for multi-qubit controlled NOT gates. For a finite temperature ( ∼ 30 μ K), including the Doppler dephasing, we simulate our gates to achieve 98.90% and 98.54 % fidelity for the Toffoli gate and C 3