In the race to build a fault-tolerant quantum computer, the choice of qubit platform is arguably the most consequential engineering decision a company can make. Different physical systems — superconducting circuits, trapped ions, photons, silicon spin qubits, and germanium spin qubits — each come with distinct trade-offs in fidelity, scalability, operating temperature, and compatibility with existing manufacturing infrastructure.
At Groove Quantum, we have built our entire platform around germanium hole-spin qubits. This is not an accident of circumstance or a second-best choice. It reflects a deliberate, physics-grounded argument that germanium offers the most credible path to scalable, fault-tolerant quantum hardware.
What Is a Hole-Spin Qubit?
A conventional spin qubit encodes quantum information in the spin state of a single electron trapped in a semiconductor quantum dot. Hole-spin qubits do the same, but with a hole — the absence of an electron in the semiconductor valence band. Holes in germanium have a fundamentally different band structure from electrons in silicon: they experience strong spin-orbit coupling, which is a quantum mechanical interaction that links a particle's spin to its momentum.
This might sound like an obscure detail, but it has enormous practical consequences. In silicon electron spin qubits, you need an oscillating magnetic field (a microwave antenna) delivered locally to each qubit to flip its spin state. At the scale of thousands or millions of qubits, routing microwave signals to every qubit becomes an engineering nightmare. Germanium hole-spin qubits solve this problem elegantly: because of strong spin-orbit coupling, the spin state can be controlled entirely with electric fields applied via the same gate electrodes that define the quantum dot itself.
All-Electric Control: A Manufacturing Advantage
All-electric spin control is a pivotal feature. It means that the control lines for each qubit are standard voltage signals on metal gate electrodes — exactly the kind of signals that semiconductor electronics have been routing and amplifying at cryogenic temperatures for decades. This is fundamentally compatible with cryo-CMOS control electronics, which can be integrated on the same chip or in very close proximity to the qubit array.
The contrast with superconducting qubits is stark. Superconducting qubits operate at millikelvin temperatures, require microwave coaxial cables running from room temperature, and currently scale poorly beyond a few hundred qubits before wiring complexity becomes a bottleneck. Germanium hole-spin qubits are also cold — they operate at around 50 millikelvin — but their control interface is the same gate electrode architecture already proven in classical transistors.
CMOS Compatibility: Leveraging 70 Years of Semiconductor Engineering
Perhaps the single most important argument for germanium is its compatibility with standard CMOS semiconductor manufacturing. Germanium is a group-IV semiconductor, closely related to silicon, and has been part of the semiconductor industry since its very beginnings. Modern germanium quantum well heterostructures can be grown epitaxially on silicon wafers using industry-standard CVD and MBE tools. The gate electrode patterning uses standard electron-beam or deep-UV lithography — the same techniques used to manufacture every transistor in your laptop.
This means Groove Quantum can, in principle, use existing semiconductor foundries to manufacture its quantum processors. No exotic materials, no custom deposition chambers, no proprietary processes available only in a single research lab. The path to high-volume production is not blocked by manufacturing barriers — only by continued progress in qubit quality and control systems.
Fidelity: The Number That Matters
All of this scalability is meaningless if the qubits themselves are too noisy to be useful. Quantum error correction can in principle correct errors, but only if the underlying physical error rates are below a fault-tolerance threshold — typically in the range of 0.1% to 1% per gate operation. Achieving this requires meticulous engineering of the qubit environment, the control pulses, and the readout mechanism.
In our most recent Science publication (2024), we demonstrated a 10-qubit germanium array achieving two-qubit gate fidelities exceeding 99%. This is not just a record for germanium — it is competitive with the best-reported results across all qubit platforms. Reaching this fidelity on a multi-qubit array, rather than an isolated pair of qubits, is the critical milestone that validates the scalability of the platform.
Looking Forward
The quantum computing industry is entering a phase where the initial demonstration games are over. What matters now is not whether you can make a few qubits work in a lab, but whether you can scale to the thousands of logical qubits required for commercially relevant algorithms. That scaling challenge is fundamentally a manufacturing and systems engineering problem — exactly the kind of problem the semiconductor industry has solved repeatedly over the past seven decades.
Germanium hole-spin qubits sit at the intersection of world-class qubit physics and world-class semiconductor manufacturing. That is why we chose this platform, and that is why we believe it represents the most credible path to fault-tolerant quantum computing at commercial scale.