In announcement at the APS March 2018 conference, Google announced it has created a 72 qubit chip called Bristlecone which will serve as a basis for its demonstration of quantum supremacy. Google has discussed their efforts to create such a chip starting with a talk at the Adiabatic Quantum Computing Conference in June, 2016, an arXiv article titled “Characterizing Quantum Supremacy in Near-Term Devices” submitted in July 2016 and frequent mentions at other technical conferences over the past year and a half.
However, all their previous discussion on this chip characterized it as having 49 qubits in a 7×7 2D lattice which would be enough to demonstrate quantum supremacy for their chosen problem. So it came as a surprise when it was finally announced that the chip had 72 qubits. Perhaps Google made a late decision to modify their design after IBM announced its 50 qubit chip, Intel described their 49 qubit chip, and IBM also disclosed that they had successfully simulated a 56 qubit design on a classical computer.
Besides increasing the qubit count, Google has placed a high priority on increasing the level of qubit quality. The goal for this chip would be to maintain or improve upon the error rates of their previous 9-qubit chip which showed 1% for readout, 0.1% for single-qubit gates 0.6% for two-qubit gates. Additionally, they have apparently improved qubit connectivity because each qubit can connect with four nearest neighbors in the 2D array versus only two neighbors in the previous 9-qubit linear array.
Google will be using this device to not only demonstrate quantum supremacy, but to also investigate error-correction methods using the surface code, optimize the systems engineering for best performance and to facilitate quantum algorithm development on actual hardware.
For more information on this new chip, you can read the Google blog posting here.