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تسجيل مستخدم جديدTransport-enabled entangling gate for trapped ions
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We implement a two-qubit entangling M{o}lmer-S{o}rensen interaction by transporting two co-trapped $^{40}mathrm{Ca}^{+}$ ions through a stationary, bichromatic optical beam within a surface-electrode Paul trap. We describe a procedure for achieving a constant Doppler shift during the transport which uses fine temporal adjustment of the moving confinement potential. The fixed interaction duration of the ions transported through the laser beam as well as the dynamically changing ac Stark shift require alterations to the calibration procedures used for a stationary gate. We use the interaction to produce Bell states with fidelities commensurate to those of stationary gates performed in the same system. This result establishes the feasibility of actively incorporating ion transport into quantum information entangling operations.
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Generating quantum entanglement in large systems on time scales much shorter than the coherence time is key to powerful quantum simulation and computation. Trapped ions are among the most accurately controlled and best isolated quantum systems with l
ow-error entanglement gates operated via the vibrational motion of a few-ion crystal within tens of microseconds. To exceed the level of complexity tractable by classical computers the main challenge is to realise fast entanglement operations in large ion crystals. The strong dipole-dipole interactions in polar molecule and Rydberg atom systems allow much faster entangling gates, yet stable state-independent confinement comparable with trapped ions needs to be demonstrated in these systems. Here, we combine the benefits of these approaches: we report a $700,mathrm{ns}$ two-ion entangling gate which utilises the strong dipolar interaction between trapped Rydberg ions and produce a Bell state with $78%$ fidelity. The sources of gate error are identified and a total error below $0.2%$ is predicted for experimentally-achievable parameters. Furthermore, we predict that residual coupling to motional modes contributes $sim 10^{-4}$ gate error in a large ion crystal of 100 ions. This provides a new avenue to significantly speed up and scale up trapped ion quantum computers and simulators.
We propose an optical scheme for generating entanglement between co-trapped identical or dissimilar alkaline earth atomic ions ($^{40}text{Ca}^+$, $^{88}text{Sr}^+$, $^{138}text{Ba}^+$, $^{226}text{Ra}^+$) which exhibits fundamental error rates below
$10^{-4}$ and can be implemented with a broad range of laser wavelengths spanning from ultraviolet to infrared. We also discuss straightforward extensions of this technique to include the two lightest Group-2 ions ($text{Be}^+$, $text{Mg}^+$) for multispecies entanglement. The key elements of this wavelength-insensitive geometric phase gate are the use of a ground ($S_{1/2}$) and a metastable ($D_{5/2}$) electronic state as the qubit levels within a $sigma^z sigma^z$ light-shift entangling gate. We present a detailed analysis of the principles and fundamental error sources for this gate scheme which includes photon scattering and spontaneous emission decoherence, calculating two-qubit-gate error rates and durations at fixed laser beam intensity over a large portion of the optical spectrum (300 nm to 2 $mu text{m}$) for an assortment of ion pairs. We contrast the advantages and disadvantages of this technique against previous trapped-ion entangling gates and discuss its applications to quantum information processing and simulation with like and multispecies ion crystals.
Entangling gates in trapped-ion quantum computing have primarily targeted stationary ions with initial motional distributions that are thermal and close to the ground state. However, future systems will likely incur significant non-thermal excitation
due to, e.g., ion transport, longer operational times, and increased spatial extent of the trap array. In this paper, we analyze the impact of such coherent motional excitation on entangling-gate error by performing simulations of Molmer-Sorenson (MS) gates on a pair of trapped-ion qubits with both thermal and coherent excitation present in a shared motional mode at the start of the gate. We discover that a small amount of coherent displacement dramatically erodes gate performance in the presence of experimental noise, and we demonstrate that applying only limited control over the phase of the displacement can suppress this error. We then use experimental data from transported ions to analyze the impact of coherent displacement on MS-gate error under realistic conditions.
We present a general theory for laser-free entangling gates with trapped-ion hyperfine qubits, using either static or oscillating magnetic-field gradients combined with a pair of uniform microwave fields symmetrically detuned about the qubit frequenc
y. By transforming into a `bichromatic interaction picture, we show that either ${hat{sigma}_{phi}otimeshat{sigma}_{phi}}$ or ${hat{sigma}_{z}otimeshat{sigma}_{z}}$ geometric phase gates can be performed. The gate basis is determined by selecting the microwave detuning. The driving parameters can be tuned to provide intrinsic dynamical decoupling from qubit frequency fluctuations. The ${hat{sigma}_{z}otimeshat{sigma}_{z}}$ gates can be implemented in a novel manner which eases experimental constraints. We present numerical simulations of gate fidelities assuming realistic parameters.
Control over physical systems at the quantum level is a goal shared by scientists in fields as diverse as metrology, information processing, simulation and chemistry. For trapped atomic ions, the quantized motional and internal degrees of freedom can
be coherently manipulated with laser light. Similar control is difficult to achieve with radio frequency or microwave radiation because the essential coupling between internal degrees of freedom and motion requires significant field changes over the extent of the atoms motion. The field gradients are negligible at these frequencies for freely propagating fields; however, stronger gradients can be generated in the near-field of microwave currents in structures smaller than the free-space wavelength. In the experiments reported here, we coherently manipulate the internal quantum states of the ions on time scales of 20 ns. We also generate entanglement between the internal degrees of freedom of two atoms with a gate operation suitable for general quantum computation. We implement both operations through the magnetic fields from microwave currents in electrodes that are integrated into the micro-fabricated trap structure and create an entangled state with fidelity 76(3) %. This approach, where the quantum control mechanism is integrated into the trapping device in a scalable manner, can potentially benefit quantum information processing, simulation and spectroscopy.
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