Robust and Resource-Efficient Microwave Near-Field Entangling Be+9 Gate

Abstract

Microwave trapped-ion quantum logic gates avoid spontaneous emission as a fundamental source of decoherence. However, microwave two-qubit gates are still slower than laser-induced gates and hence more sensitive to fluctuations and noise of the motional mode frequency. We propose and implement amplitude-shaped gate drives to obtain resilience to such frequency changes without increasing the pulse energy per gate operation. We demonstrate the resilience by noise injection during a two-qubit entangling gate with 9 Be + ion qubits. In absence of injected noise, amplitude modulation gives an operation infidelity in the 10 −3 range.Trapped ions are a leading platform for scalable quantum logic [1, 2] and quantum simulations [3]. Major challenges towards larger-scale devices include the integration of tasks and components that have been so far only demonstrated individually, as well as single and multiqubit gates with the highest possible fidelity to reduce the overhead in quantum error correction. Microwave control of trapped-ion qubits has the potential to address both challenges [4,5] as it allows the gate mechanism, potentially including control electronics, to be integrated into scalable trap arrays. Because spontaneous emission as a fundamental source of decoherence is absent and microwave fields are potentially easier to control than the laser beams that are usually employed, microwaves are a promising approach for high fidelity quantum operations. In fact, microwave two-qubit gate fidelities seem to improve more rapidly than laser-based gates. However, observed two-qubit gate speeds of laser-based gates [6,7] are still about an order of magnitude faster than for microwave gates [8][9][10]. This makes gates more susceptible to uncontrolled motional mode frequency changes, as transient entanglement with the motional degrees of freedom is the key ingredient in multi-qubit gates for trapped ions. As other error sources have been addressed recently, this is of growing importance. Merely increasing Rabi frequencies may not be the most resource-efficient approach, as it will increase energy dissipation in the device. A more efficient use of available resources could be obtained using pulse shaping or modulation techniques. In fact, a number of recent advances in achieving highfidelity operations or long qubit memory times have been proposed or obtained by tailored control fields. Examples include pulsed dynamic decoupling [11], Walsh modulation [12], additional dressing fields to increase coherence times [13], phase [14], amplitude [15][16][17][18][19][20] and fre-quency modulation [21] as well multi-tone fields [22][23][24]. In many cases, these techniques lead to significant advantages. For multi-qubit gates, one mechanism is to optimize the trajectory of the motional mode in phase space for minimal residual spin-motional entanglement in case of experimental imperfections. This effectively reduces the distance between the origin and the point in phase space at which the gate terminates in case of errors.Here we propo...

Microwaves in Quantum Computing

“…A number of experimental demonstrations of high-fidelity microwave-based entangling gates between ions have been carried out [ 61 ], [ 77 ]–[ 80 ]. In general, these gates are slower (~ ms duration) than laser-based entangling gates, which can be performed in tens or hundreds of μ s (and some as fast as a few μ s [ 81 ]).…”

Quantum computing hardware in the cloud: Should a computational chemist care?

“…ZGP gates cannot be performed directly on the low-decoherence clock qubits, but are insensitive to the absolute magnetic field offset. Two-qubit gates have been performed both with lasers [56,57,59,62] and rf [80][81][82][83][84] as well as between ions of different elements [61,[85][86][87]. Due to the weak motional coupling rf multi-qubit gates are considerably slower than laser gates.…”

Numeric Optimization for Configurable, Parallel, Error‐Robust Entangling Gates in Large Ion Registers

“…Moreover, the addition of control permits the introduction of noise and drift‐robustness even in complex multi‐ion settings. [ 16–21 ]…”