How Microscopic Manufacturing Features Impact Quantum Computing
Key Summary Takeaways
• Averages Hide Weak Links: Quantum processors are bounded by their weakest qubits, making consistency across a chip essential.
• Nanometer Precision Matters: Features as small as 1 nanometer in oxide thickness can dramatically impact qubit performance.
• 3 Microscopic Culprits: Surface oxide thickness, sidewall edge slopes, and substrate trench depth directly dictate energy loss.
• Faster Screening: Advanced techniques like terahertz nanoimaging allow manufacturers to catch defect issues before cooling chips to absolute zero.
Overview
When tech companies talk about quantum computers, they usually share average performance metrics—such as two-qubit gate fidelities between 99.0 and 99.5 percent. However, averages hide a critical challenge in quantum engineering: on any given quantum chip, some qubits perform exceptionally well while others perform poorly. Because a quantum computer is only as strong as its weakest link, understanding why that variation occurs is essential for scaling up functional processors.
A groundbreaking study conducted at Fermilab's SQMS Center (Superconducting Quantum Materials and Systems Center) examined the spread behind these averages. The researchers traced performance differences directly to microscopic physical features measured in single nanometers—a scale tens of thousands of times smaller than a single strand of human hair. The peer-reviewed findings were published in Applied Physics Reviews based on collaborative research involving Northwestern University, Rigetti Computing, NIST, and Ames National Laboratory.
How a Transmon Qubit Works
The most common type of superconducting quantum bit is called a transmon. You can think of a transmon as a tiny electrical circuit made from a thin metal film (typically niobium) laid out on a base chip made of silicon or sapphire. The circuit consists of two primary components:
• Capacitor: Stores electrical energy across metal pads patterned on the surface.
• Josephson Junction: A razor-thin insulating barrier sandwiched between two superconductors. This junction gives the circuit non-linear, unevenly spaced energy levels, allowing engineers to isolate the two lowest energy states as the 0 and 1 of a qubit.
To function properly, the chip operates inside a dilution refrigerator at millikelvin temperatures (near absolute zero), where the metal loses all electrical resistance. Microwave pulses are then sent into the circuit to move the qubit between state 0 and state 1.
Understanding the T1 Lifetime & Energy Leaks
A fundamental metric of qubit quality is its T1 lifetime (energy relaxation time). This measures how long a qubit can remain in its excited state ('1') before releasing its energy and dropping back to the ground state ('0'). Because every quantum logic operation takes a set amount of time, a longer T1 allows a quantum computer to execute longer, more complex algorithms before stored information disappears.
A transmon's electric field is not completely confined inside the clean metal pads; it extends slightly into the chip surface, the underlying substrate, and the surrounding air. Energy stored in clean, defect-free material returns safely to the circuit. However, energy stored in surface materials with atomic defects gets absorbed and turned into heat or stray vibration. Engineers call this share of field energy sitting in lossy material the Energy Participation Ratio—and lower participation in lossy material means a longer-lasting qubit.
The Three Microscopic Culprits Behind Variance
By testing 22 transmon qubits across 7 multi-institution chips using a rigorous blinded protocol, the study identified three distinct manufacturing features that control how much energy leaks away:
1. Surface 'Rust' (Niobium Oxide Layer Thickness)
When niobium metal is exposed to ambient air during manufacturing, it naturally reacts with oxygen to form a thin surface oxide layer (primarily Nb₂O₅) a few nanometers thick. This oxide layer is naturally disordered and contains atomic defects that trap microwave energy. The study revealed that a variation of just 1 single nanometer in oxide thickness caused a noticeable difference in qubit lifetime, surprising even experienced chip fabricators.
2. Edge Slope (Sidewall Geometry & Footer Angle)
When chemical etching carves the metal film into qubit circuit patterns, it leaves an edge profile. The researchers compared sloped, tapered edges (about 30-degree angles) against near-vertical, sharper edges (10 to 15 degrees). Sloped edges increase the physical surface area where lossy oxide forms, exposing 20 to 30 percent more of the qubit's electric field to defective surface material and significantly lowering performance.
3. Moat Depth (Substrate Trenching Saturation)
To reduce electric field concentration at the chip surface, manufacturers etch shallow trenches into the substrate beside the metal pads. The study found that digging even a shallow trench under 20 nanometers produces an immediate, dramatic boost in qubit quality. However, once trench depth exceeds 70 nanometers, the benefit flattens out and saturates. Beyond 70 nm, deeper trenching no longer helps, and the sidewall edge angle takes over as the dominant factor driving energy loss.
Diagnostic Proxies: Testing Without the Deep Freeze
Cooling a quantum chip down to millikelvin temperatures inside a specialized dilution refrigerator takes days of setup and substantial operational expense. One of the study's key practical breakthroughs was identifying two room-temperature or mild-cold diagnostic tools as effective predictors of qubit quality:
• Terahertz Near-Field Nanoimaging (s-SNOM): Maps localized dielectric loss and oxide non-uniformities at nanometer resolution.
• Magneto-Optical Imaging: Screens local superconducting transition properties across the metal film.
By using these proxy techniques as a screening step on the cleanroom floor, fabrication teams can receive rapid feedback on each etch and oxidation step—catching bad chips before spending time and money on full cryogenic testing.
What This Means for Future Quantum Hardware
To scale up quantum processors from dozens of experimental qubits to thousands of fault-tolerant logical qubits, manufacturing consistency is everything. Eliminating nanometer-scale variations during chip fabrication will allow every qubit on a processor to perform like the best qubit on the chip.
An expanded section covering these device-to-device yield limits and nanometer-scale fabrication parameters will be included in Chapter 5 of the December 2026 third edition of Quantum from the Ground Up. You can download current book editions here.