I’ve written about qubits in the past - the basic unit of quantum information that can exist in a
superposition of both 0 and 1 states simultaneously until measured, unlike a classical bit which is always either 0 or 1. Let’s take a closer look on how a qubit can be made - there are three ways:
· Superconducting Qubits
· Trapped Ion Qubits
· Photonic Qubits
Superconducting qubits are what IBM and Google use and I’ll cover in this post. They're tiny electrical circuits that only work at temperatures colder than deep space.
What Makes Them Quantum
At room temperature, these are just pieces of metal. Cool them to 15 millikelvin and they become superconductors. Electricity flows without resistance. Electrons move in perfect sync, acting like one quantum wave instead of individual particles.
The circuit has specific energy levels, like rungs on a ladder. Ground state is 0, first excited state is 1. The quantum effect lets the circuit be in both states at once until you measure it.
The Josephson Junction
The heart of a superconducting qubit is the Josephson junction. Two pieces of aluminum separated by an insulating barrier 1 to 2 nanometers thick. About 10 atoms wide.
At cryogenic temperatures, electrons quantum tunnel through that barrier even though classical physics says they can't. This creates a nonlinear inductance. Combined with a capacitor, you get unequally spaced energy levels.
Why does that matter? A regular circuit has evenly spaced levels. You can't use it as a qubit because when you try to flip between 0 and 1, you accidentally excite higher levels too. The Josephson junction creates anharmonicity. The gap between 0 and 1 differs from the gap between 1 and 2. This lets you address just the first two levels with microwave pulses.
The Fabrication Process
defects.
Deposit aluminum: Vacuum chamber, molecular beam epitaxy. Deposit 100 to 200 nanometers of aluminum in ultra-high vacuum to prevent oxidation.
Pattern the circuit: Photolithography for the basic shapes. Electron beam lithography for the junction because you need nanometer precision. An electron beam writes the pattern point by point.
Create the junction: The Dolan bridge technique works well. Evaporate aluminum at an angle, deposit the oxide barrier, evaporate more aluminum at a different angle. The two layers overlap slightly with oxide between them. The overlap area is your junction.
Getting the oxide thickness right is critical. Too thick and electrons can't tunnel. Too thin and you get leakage. You're aiming for 1 to 2 nanometers with sub-nanometer precision.
Add control circuitry: Microwave transmission lines, coupling capacitors, and resonators. The readout resonator is a microwave cavity whose frequency shifts depending on qubit state. Send in a microwave pulse, measure the reflected signal. The tiny frequency shift tells you if the qubit is 0 or 1.
Why It's Hard
Junction uniformity: A 5% variation in junction area changes qubit frequency by hundreds of megahertz. Hitting the right target across a whole chip is brutal.
Material defects: Impurities create two-level systems that absorb energy and cause decoherence. You need ultra-pure materials and ultra-clean fabrication. Cosmic rays passing through can disrupt qubits.
Yield: When IBM makes a chip with 50 qubits, maybe 30 to 40 work well. The rest have junction defects, frequency problems, or excessive noise. No way to repair a bad qubit.
Coherence times: Superconducting qubits lose their quantum state in 100 to 500 microseconds. Some designs reach milliseconds but that took years of material science improvements.
Different Designs
Transmon: Most common. Large capacitor reduces sensitivity to charge noise. Coherence around 100 microseconds. Relatively easy to make.
Flux qubit: Superconducting loop with Josephson junctions. Sensitive to magnetic flux. Harder to isolate from noise.
Fluxonium: Long chain of Josephson junctions as a superinductor. Can hit 1 millisecond coherence but harder to fabricate and control.
The Support System
Dilution refrigerator: Pumps helium-3 and helium-4 to reach millikelvin temperatures. Takes 24 hours to cool down. Costs approx. $2 million.
Microwave control: Room temperature electronics generate 4 to 8 gigahertz pulses. Signals travel down coaxial cables into the fridge. A 50 qubit chip needs 100+ cables.
Magnetic shielding: Mu-metal around the refrigerator, sometimes superconducting shields at the coldest stage. Stray fields from power lines or passing cars disrupt qubits.
Signal processing: Low-noise amplifiers, high electron mobility transistor amplifiers, fast analog-to-digital converters. Software extracts qubit states from noisy signals.
Current Performance
Best superconducting qubits today:
- 100 to 500 microsecond coherence
- 20 nanosecond gate operations
- 99%+ two-qubit gate fidelity
- 99.9%+ single-qubit gate fidelity
You can do roughly 1,000 to 10,000 operations before decoherence. Not enough for most useful algorithms yet, which need millions of operations. That's why quantum error correction is critical.
Scaling Challenges
Wiring: Can't run a million coax cables into a fridge. Need multiplexing and cryogenic control electronics inside the refrigerator.
Crosstalk: Packed qubits interfere with each other. Control signals leak to neighbors.
Uniformity: Making 1,000 nearly identical qubits pushes fabrication limits.
Materials: Better materials with fewer defects would improve coherence directly.
Five years ago, 50 microsecond coherence was state of the art. Now it's 500 microseconds. Ten years ago, chips had 5 qubits. Now they have hundreds.
The physics works. Scaling remains unsolved. I'll describe trapped ion and photonic qubits here in future posts.
No comments:
Post a Comment