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arXiv:1808.10462 · Physical Review Applied

Phase-Modulated Entangling Gates Robust to Static and Time-Varying Errors

Alistair R. Milne, Claire Edmunds, Cornelius Hempel, Federico Roy, Sandeep Mavadia, Michael J. Biercuk

Published Feb 11, 2020• 85 citations
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Abstract

In quantum computing systems where entangling logic gates are mediated via bosonic oscillator modes ($e.g.$ trapped ions), residual coupling between qubits and oscillator is a dominant source of gate infidelity. This work shows how discrete phase modulation of the field mediating the entangling operation ensures that the system of qubits is decoupled from multiple oscillator modes, even in the presence of common time-varying sources of noise and hardware instability. The results demonstrate the capabilities of quantum control to drive major performance advances in quantum computing.

DOI
10.1103/physrevapplied.13.024022

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In quantum computing systems where entangling logic gates are mediated via bosonic oscillator modes (e.g.e.g.e.g. trapped ions), residual coupling between qubits and oscillator is a dominant source of gate infidelity. This work shows how discrete phase modulation of the field mediating the entangling operation ensures that the system of qubits is decoupled from multiple oscillator modes, even in the presence of common time-varying sources of noise and hardware instability. The results demonstrate the capabilities of quantum control to drive major performance advances in quantum computing.
We present a scheme designed to suppress the dominant source of infidelity in entangling gates between quantum systems coupled through intermediate bosonic oscillator modes. Such systems are particularly susceptible to residual qubit-oscillator entanglement at the conclusion of a gate period that reduces the fidelity of the target entangling operation. We demonstrate how the exclusive use of discrete shifts in the phase of the field moderating the qubit-oscillator interaction is sufficient to both ensure multiple oscillator modes are decoupled and to suppress the effects of fluctuations in the driving field. This approach is amenable to a wide variety of technical implementations including geometric phase gates in superconducting qubits and the Molmer-Sorensen gate for trapped ions. We present detailed example protocols tailored to trapped-ion experiments and demonstrate that our approach has the potential to enable multiqubit gate implementation with a significant reduction in technical complexity relative to previously demonstrated protocols.
Abstract Robust qubit-qubit interactions mediated by bosonic modes are central to many quantum technologies. Existing proposals combining fast oscillator-mediated gates with dynamical decoupling require strong pulses or fast control over the qubit-boson coupling. Here, we present a method based on dynamical decoupling techniques that leads to faster-than-dispersive entanglement gates with low-intensity pulses. Our method is general, i.e., it is applicable to any quantum platform that has qubits interacting with bosonic mediators via longitudinal coupling. Moreover, the protocol provides robustness to fluctuations in qubit frequencies and control fields, while also being resistant to common errors such as frequency shifts and heating in the mediator as well as crosstalk effects. We illustrate our method with an implementation for trapped ions coupled via magnetic field gradients. With detailed numerical simulations, we show that entanglement gates with infidelities of 10−3 or 10−4 are possible with current or near-future experimental setups, respectively.
We investigate an approach to universal quantum computation based on the modulation of longitudinal qubit-oscillator coupling. We show how to realize a controlled-phase gate by simultaneously modulating the longitudinal coupling of two qubits to a common oscillator mode. In contrast to the more familiar transversal qubit-oscillator coupling, the magnitude of the effective qubit-qubit interaction does not rely on a small perturbative parameter. As a result, this effective interaction strength can be made large, leading to short gate times and high gate fidelities. We moreover show how the gate infidelity can be exponentially suppressed with squeezing and how the entangling gate can be generalized to qubits coupled to separate oscillators. Our proposal can be realized in multiple physical platforms for quantum computing, including superconducting and spin qubits.
We introduce a scheme that combines phonon-mediated quantum logic gates in trapped ions with the benefits of continuous dynamical decoupling. We demonstrate theoretically that a strong driving of the qubit decouples it from external magnetic-field noise, enhancing the fidelity of two-qubit quantum gates. Moreover, the scheme does not require ground-state cooling, and is inherently robust to undesired ac Stark shifts. The underlying mechanism can be extended to a variety of other systems where a strong driving protects the quantum coherence of the qubits without compromising the two-qubit couplings.
A novel two-qubit entangling gate for trapped-ion quantum processors is proposed theoretically and demonstrated experimentally. During the gate, double-dressed quantum states are created by applying a phase-modulated continuous driving field. The speed of this quantum gate is an order of magnitude higher than that of previously demonstrated rf controlled two-qubit entangling gates in static magnetic field gradients. At the same time, the field driving the gate dynamically decouples the qubits from amplitude and frequency noise, increasing the qubits’ coherence time by 3 orders of magnitude. The gate requires only a single continuous rf field per qubit, making it well suited for scaling a quantum processor to large numbers of qubits. Implementing this entangling gate, we generate the Bell states | Φ + ⟩ and | Ψ + ⟩ in less than or equal to 313 μ s with fidelities up to 98 − 3 + 2 % in a static magnetic gradient of only 19.09 T / m . At higher magnetic field gradients, the entangling gate speed can be further improved to match that of laser-based counterparts.
The performance of a quantum information processor depends on the precise control of phases introduced into the system during quantum gate operations. As the number of operations increases with the complexity of a computation, the phases of the gates at different locations and different times must be controlled, which can be challenging for optically driven operations. We circumvent this issue by demonstrating an entangling gate between two trapped atomic ions that is insensitive to the optical phases of the driving fields while using a common master reference clock for all coherent qubit operations. Such techniques may be crucial for scaling to large quantum information processors in many physical platforms.