In 2022, all the R&D projects of Alice&Bob were focussed on the logical qubit demonstrator with cat-qubits. Since the bit-flip error is corrected at the single cat-qubit level, a logical qubit consists of a 1D chain of cat-qubit that corrects phase-flips. The minimal logical qubit demonstrator consists of 3 data and 2 ancillary cat-qubits. To operate the logical qubit, the ancillary qubits have to be prepared and measured in the X-basis and CNOT gates have to be performed between the data and the ancillary qubits. As for any error correction scheme, these operations have to be performed with sufficient accuracy otherwise, running the repetition code produces more errors than it corrects. When the amount of errors coming from both native errors and imperfect operation of the device equals the amount of errors corrected the device is said to be operating at threshold. As soon as a device is below threshold, error correction is useful and the remaining errors can be scaled down exponentially by increasing the length of the cat-qubit chain. Our 2022 roadmap was divided in 3 main experimental projects :
-Scaling-up the cat-qubit count on a single chip
-Preparing and measuring in the X-basis with high fidelity
-Running the error correction sequence on a multi cat-qubit chip
First, we redesigned the cat qubit to make it more compact to fit on a fixed size chip. This was done by fully redesigning the microwave layout of the cat. The compactness also enabled to increase the non-linearity of the device and its stabilization rate, a critical ingredient for fast gates. As the qubit count increases, our design and microwave simulation tools had to scale-up. Second, the cryogenic set-up was also scaled-up in particular the sample holder shielding the chip and the number of microwave lines available in A&B’s cryostats. For next generation chips, we ramped up our cryogenic power by purchasing several cryostats. Finally, the room-temperature microwave control set-up was up-graded to enable the control of several cat-qubits.
The second project required implementing a set of state of the art techniques and to choose relevant parameter regimes that allow fast and accurate control of the microwave resonator that hosts the cat-qubit. These control techniques involve a intermediate transmon qubit on the chip. We reached state of the art fidelity in the control and readout of this intermediate qubit heavily used in the field. This allowed us to attain a high level of fidelity in the preparation and measurement of cat-states, mostly limited by the finite lifetime of the hosting microwave resonator and finite coherence time of the intermediate transmon qubit.
The third project had two distinct milestones for 2022. The first one was to demonstrate that cat-qubit bit-flip suppression was efficient on a chip comprising several cat-qubits. The second milestone was to work on the implementation and improvement of a CNOT gate between two consecutive cat qubits of the 1D chain. Embedded in the repetition code, this gate maps phase-flips error of the data cat-qubit to the ancillary (the state of the ancillary system is then analysed by the tools developed during the second project).
We also worked on improving single cat-qubits performance for faster gates, using a detuned Kerr approach. This strategy builds on the Kerr cat paradigm where the bit-flip protection comes from Hamiltonian confinement but solves a leakage issue (which eventually leads to bit-flips). A second project designed a parameter regime aimed at improving the repetition code performances. By having some asymmetry between the data and the ancilla cat-qubits one can gain significantly on the error correction threshold.
Finally, we developed our relationship with key distributors of quantum computing power. With Microsoft, for example, we are aligning our efforts towards a shared goal : increased awareness on fault tolerant quantum computation. As a result:
-Microsoft is publishing a tool called "resource estimator" that allows to quantify how qubit noise characteristic impact computation feasibility. It is obviously of great value to our positioning. We were in the private beta group for the tool and published an article on its usage.
-We are now officially member of the Azure Quantum Network.
