What are spin qubits?

IBM just signed a definitive agreement to acquire HRL Laboratories, and the reasoning behind it says a lot about where the company thinks quantum computing needs to go next. HRL has spent years building expertise in silicon-spin qubits, a different approach to quantum hardware than the superconducting qubits IBM has bet on for years. Rather than treating this as a rival technology, IBM is framing it as a complementary one, something that could extend its long-term plan to build genuinely useful quantum computers and, eventually, a quantum computing internet.

The pitch here isn't about spin qubits replacing superconducting qubits. It's about the overlap. Both modalities get fabricated using standard chip manufacturing processes, both live inside cooling systems, and both get controlled with external signals that apply gates to coupled qubits. What differs is how the quantum information itself gets stored. Spin qubits lean on the spin of an electron, a basic quantum property that comes in one of two values, up or down, which naturally maps onto the 0s and 1s of computation.

HRL's specific method is where things get interesting. Instead of trying to control a single electron's spin directly, which is notoriously hard to engineer precisely, HRL uses what's called an exchange-only qubit: three electrons spread across three quantum dots made from layered silicon and germanium. Two of those electrons determine whether the qubit reads as 0 or 1 based on how their spins sum together, while the third electron functions almost like a stabilizer, keeping the whole arrangement in line. Gates get applied as electrical pulses that nudge the electrons closer together or further apart.```

There are practical upsides to this approach beyond the elegance of the physics. Spin qubits can be built with existing semiconductor fabrication tools, which matters if manufacturability at scale is the goal. They've also shown high coherence times, low error rates, and are comparatively easier to control. And notably, they operate at 1 Kelvin, a temperature that's dramatically less demanding than the 0.015 Kelvin required for superconducting qubit systems. HRL has already demonstrated a digitally controlled silicon-spin quantum computer with 54 quantum dots arranged across three rails, supporting up to 18 qubits, and has run one- and two-qubit gates along with small-scale error-detecting codes.

IBM's Jay Gambetta framed the acquisition as reinforcing multiple fronts at once, not just computation. HRL brings quantum sensing and quantum networking capabilities into the mix too, including precision sensors aimed at healthcare, navigation, and defense applications, along with ongoing materials research that could eventually produce sturdier qubits and better semiconductors across the board.