Security Lead - Member of Technical Staff

About Us We’re living through a Cambrian explosion of intelligence: new models and new chips, each specialised for different tasks, are arriving all at once. The result is a new era for AI, one of radical heterogeneity. Callosum is the Intelligent Systems Company. We believe the next generation of AI won't be defined by any single model or chip, but by intelligent systems in which hardware and intelligence co-evolve. We are building the infrastructure that unifies heterogeneous compute across the full stack. This opens a new axis of scaling intelligence: a dynamic system that tailors itself to what each workload actually needs, whether that's speed, cost, precision, or whatever unit comes next. The last era scaled on a different bet: one bigger model, more of the same chip, more data. That bet is running into structural limits. Frontier models offer extraordinary capability at unsustainable cost, one that today's monolithic infrastructure was never designed to serve. Our founding principle is that intelligence comes from many specialised systems working together, not from any single component. We build the software orchestration layer that co-evolves models, workflows and silicon into one system, delivering inference tailored to every workload, and demonstrating orders-of-magnitude leaps in capability and cost. Because our software spans the full stack, our engineering team works directly with heterogeneous accelerators and frontier silicon, including Cerebras, d-Matrix, Intel, NVIDIA, AMD, Normal Computing, Tenstorrent, GreatSky, and Mixx. We are not stopping at today's chips: each new generation of silicon unlocks algorithms that couldn't run before, and we intend to be first to them, every time. If we get it right, it will belong to everyone building on it - not to any single vendor. In our latest funding round, we raised $100M, led by Atomico with participation from Plural, DCVC and the UK Sovereign AI Fund’s first investment. With this, we are building the infrastructure for the next era of intelligence. We are engineers and scientists based in London, working across the full depth of the stack. We are curious, intellectually honest, and building what doesn't exist yet. If you thrive on uncharted territory and are energised by the scale of the challenge, we'd love to hear from you. About the Role Callosum operates across a broad technical surface: APIs used by customers directly, orchestration software deployed across heterogeneous clusters, agentic systems with access to tools and data, and physical compute infrastructure that we operate ourselves. These products run in very different environments and for customers with very different security requirements, from shared cloud infrastructure through to sovereign, on-premise, and isolated deployments. This role owns how security is designed and assured across that landscape. You will set the security architecture, translate customer and deployment requirements into concrete technical expectations for engineering teams, and ensure that the guarantees we make can be demonstrated in practice. You will lead security engagements across customers, infrastructure partners, and external technical reviewers, while building and managing the team responsible for security across Callosum. What You'll Do - Define security architecture and requirements across our API, orchestration stack, agentic systems, and physical infrastructure - Translate complex security requirements into specific engineering work, measurable guarantees, and verifiable evidence - Lead security assurance for sensitive deployments from initial requirements through technical review and approval - Build and lead the security team across architecture, threat modelling, adversarial evaluation, incident response, and operations What Sets You Apart - Experience personally owning a demanding security assurance or sensitive deployment process end-to-end - Strong technical foundations in infrastructure or security engineering, with the depth to challenge architectural decisions - Experience securing sovereign, on-premise, air-gapped, multi-tenant, or other high-assurance environments - Experience building and leading security teams, with strong judgement around risk, prioritisation, and engineering trade-offs What We Offer - Competitive Salary, determined by skills and experience - Equity & Ownership - Private healthcare - We offer Visa sponsorship and relocation benefits to hire the best in the world - We work in person at our London office. You'll have the tools, space and setup to do your best work, and if you have specific needs, just tell us We're committed to building an inclusive workplace where everyone feels welcome, and believe in equal opportunities for all.