One millionth of a volt. That is roughly the power source behind a new way to keep quantum bits stable. On October 1, Alice & Bob and researchers at École Normale Supérieure de Lyon unveiled a fix for the Alice & Bob cat qubit, the design behind the company’s push toward fault tolerant computing. The fix trades microwave gear for a steady trickle of DC voltage. Read the original writeup at The Quantum Insider.
I have followed cat qubits for a while because they take a different bet than IBM’s or Google’s qubits. They suppress one error type almost for free, which frees the correction budget for what is left.
Inside the Alice & Bob Cat Qubit Fix
Cat qubits need a steady nudge to hold their shape. Engineers call this stabilization, and Alice & Bob has normally used engineered microwave pulses for it. Those pulses work, but the generators, amplifiers, and wiring they need all add heat inside an already frigid refrigerator.
The new approach swaps that machinery for a DC biased SQUID, a superconducting loop fed by steady voltage instead of a pulsed signal. Cooper pairs tunneling across the loop release energy that shuffles photons inside the qubit. Turn the voltage knob and you get one, two, or four photon exchanges from one tiny device.
That last part matters most. Four photon exchanges make for sturdier cat qubit designs, and they are notoriously hard to pull off with microwaves.
“Four-to-one photon processes are very difficult to achieve with sufficient strength, but dc-biasing junctions seem to be a viable approach,” said Benjamin Huard, scientific advisor at Alice & Bob and a professor at ENS Lyon.
A few specific gains the team reported:
- A two photon exchange rate of 3 MHz, beating earlier microwave driven couplers
- No more parasitic nonlinearity, the frequency wobble that corrupts cat qubit data
- One voltage knob doing a job that used to take several microwave chains
Why Fewer Microwave Parts Could Make Quantum Computers Cheaper
Every microwave generator, amplifier, and cable adds cost and heat, and dilution refrigerators can only shed so much heat before they stop working. Cut the control hardware per qubit and you free up room and budget for more qubits on the same machine.
That matters for Alice & Bob’s stated goal of 100 logical qubits by 2030, since simpler hardware is one of the few realistic ways there. Other teams chase the same prize differently. QuEra and HPE wired fault tolerant machines into supercomputers. Infleqtion hit 30 logical qubits at the lowest overhead reported yet. This DC voltage trick is Alice & Bob’s answer to that same race.
What This Could Mean for You
Picture Dao, a biochemist at a small biotech startup outside Columbus, Ohio. Her team runs quantum simulations to design new antibiotics. But cloud quantum time already costs close to $500 an hour, so a careful budget buys only a few dozen hours a year.
Fewer microwave chains per qubit means lower manufacturing and upkeep costs, which tends to show up later as cheaper cloud access. If Dao’s lab affords twice the compute hours for the same money, her antibiotic work moves months faster. That head start matters to a parent awaiting a new treatment, or a rural hospital watching its budget.
None of this happens overnight. Alice & Bob still has to build a working chip, not just the math, and other labs will try to replicate it. Quantum Zeitgeist covered the theoretical groundwork back in April, work that took years to reach this stage.
Here is what I am watching next: whether Alice & Bob or another lab builds a physical chip with this coupler within the year. That is the real test of a microvolt doing a microwave generator’s job at scale. If it holds up, expect quieter, cheaper refrigerators packed with far more qubits.