Not to be outdone by Honeywell, who announced a quantum volume level of 64 in June, IBM has announced that they too have reached this level with their IBMQ_Montreal device which is based upon their Falcon processor family. The interesting thing about this improvement was that it was reached solely through the use of improved software. The IBMQ_Montreal device itself has been available since last July.
IBM achieved a quantum volume of 64 through a combination of four factors: improvements of the Qiskit compiler, refinements to the two-qubit gate and its calibration, addition of dynamical decoupling to mitigate noise affecting idle qubits, and introduction of excited state promoted readout. These software improvements were implemented both at the compiler level and also at the microwave pulse level using their OpenPulse interface which allows one to have pulse level control over the microwave pulses that control the qubits.
The specific quantum volume measurement of 64 is achieved using a square circuit configuration of 6 qubits with a gate depth of 6 levels (we will call it a 6×6 circuit, for short). The heavy output generaion test is applied multiple times to this configuration and a statistical measurement of 2/3rds or greater must be achieved. (For details, see IBM’s documentation here and here.) What’s noteworthy is that even through the IBMQ_Montreal device has a total of 27 qubits, 21 of them were not used for the quantum volume 64 test. If IBM is able make further improvements in their qubit quality performance, they may be able to pass the test with a 7×7 configuration and achieve a quantum volume of 128 (27=128).
August 20, 2020