Quantum computing platform IonQ said on Monday it will acquire cybersecurity company Elisity in an all-stock deal worth up to $1.3 billion, including up to $150 million in performance-based earnouts.

The move is designed to defend against and uncover attackers targeting generative artificial intelligence large-language models and AI agents, and to protect against emerging quantum threats, the companies said in a press release.

CEO Niccolo de Masi told CNBC's Morgan Brennan on Monday that the addition of Elisity's platform will help bolster his company's quantum security strategy.

"This means that we can protect not only critical infrastructure — we believe better than anyone in the world — but of course the Fortune 500," de Masi said.

The all-stock deal is expected to close in the fourth quarter, subject to regulatory approval, and would expand the quantum-computing company's cybersecurity portfolio.

IonQ shares fell more than 9% in afternoon trading. Other quantum computing stocks, including Rigetti Computing and D-Wave Quantum, also traded lower.

Elisity will retain its current leadership, including CEO James Winebrenner.

IonQ's stock has surged in recent months amid a buying spree. Most recently, the company bought semiconductor maker SkyWater Technology for $1.8 billion in July. In the past year and a half, IonQ has acquired several quantum companies, most notably U.K.-based quantum computing startup Oxford Ionics.

The deal follows two June executive orders from President Donald Trump aimed at building a quantum computer for scientific research by 2028 and protecting government systems from cyber threats. IonQ said in the release that the orders underscored the urgency of preparing for quantum computing threats.

De Masi told CNBC on Monday that the need for quantum-level security integration into government and business as outlined in the executive order will become all-important as "Q-Day" — the era where quantum computers can break today's standard encryption software — draws closer.

"Lab leaks" from frontier AI companies such as OpenAI's high-profile Hugging Face hack are just the beginning compared to cyber capabilities a year from now, de Masi said.

"Our technologies help you today. They also set you up, of course, as well as you can today, for an era where the bad guys will eventually get quantum computers. They will eventually be able to marry them, I fear, with LLMs, and so the world that we're in today is already dangerous," De Masi told CNBC, adding that LLM's "could do even more dastardly things."

"Frankly, people are not ready," he said.