Title: Reduction of Cavity Photon Dephasing for Superconducting Qubits

Abstract: Since the first demonstration of quantum coherent operations on superconducting qubits, coherence times have improved by five orders of magnitude, up to 0.1 ms and beyond. With these improvements, the thermalization of the device, and its isolation from noise sources has become more critical. In particular, superconducting transmon qubits embedded in a cQED architecture can be very sensitive to dephasing from fluctuations in the number of photons as small as 0.001 photons in the read-out cavity [1]. To reduce the generation of thermal photons from the input driveline filters, we have designed and fabricated wide bandwidth microwave attenuators for millikelvin temperatures. Using these attenuators in conjunction with transmon qubits, a dephasing time greater than Tφ > 0.14 ms was demonstrated corresponding to an effective noise temperature of less than 55 mK [2]. To increase the interaction volume of the hot electrons with the cold phonons and thus improve the cooling power of these attenuators, we have divided the dissipative elements into smaller lengths and interleaved 10 μm thick conducting hot-electron heat sinks. With these improvements, we demonstrate that our 20 dB filters can dissipate up to 100 nW of microwave power while maintaining a noise temperature Tn below 100 mK.