Digital predistortion of optical field of a fast and high-fidelity entangling gate for trapped ions qubits

Tapped ions qubits are a leading quantum computing platform. In these systems, entangling gates are performed by driving the normal modes of motion of the ion chain, generating a spin-dependent force that mediates qubit-qubit interactions. 

In recent years, there have been many theoretical proposals and experimental demonstrations which have generalized this approach in order to increase the fidelity, robustness, and programmability of the entangling operation. These are all performed by carefully designing the electromagnetic fields which drive the ion chain. However, various components such as amplifiers and modulators, which are used to generate the required field modulations have inherent non-linear responses, resulting in an inaccurate and low-fidelity implementation of the entangling operations.

We propose a method to mitigate this degradation by using digital pre-distortion of the modulating waveform. Specifically, we measure the temporal and amplitude non-linear response of an acusto-optic modulator used to modulate the optical field driving the ion chain, and use it in a feed-forward correction of the desired waveform. 

We measure that the resulting optical field more closely resembles the desired spectrum. Moreover, we use the pre-distorted signal to generate a multi-tone two-qubit entangling gate described in Ref. [1]. We show that our method allows us to utilize all the available optical power in order to drive fast entanglement gates, without incurring fidelity loss due to unwanted non-linear effects. Our method is straightforward to implement, even in complicated waveform modulation, such as Refs. [2,3], which require many driving tones in order to generate multi-qubit robust entanglement gates.

[1] Phys. Rev. Lett. 121, 180502 (2018)

[2] Phys. Rev. A 101, 032330 (2020)

[3] Phys. Rev. Lett. 130, 030602 (2023)