Jonas Bylander, Massachusetts Institute for Technology (MIT)
Abstract
Superconducting quantum circuits are “artificial atoms,” where a macroscopic circuit variable – in our case the electric current around a loop of wire – exhibits uniquely quantum phenomena such as superposition states. Superconducting quantum computation is presently limited by these qubits’ short coherence times due to noise.
We report measurements on a qubit with long relaxation time: T1 = 12 ms, over 10,000 times longer than a single control pulse, allowing substantial time to perform quantum logic gates or quantum error correction protocols. In contrast to energy relaxation, which is generally an irreversible process, dephasing is in principle reversible and can be refocused dynamically through the application of coherent control-pulse methods. We demonstrate that, by applying a sequence of up to 200 carefully adjusted microwave pulses (the CPMG protocol), we can “dynamically decouple” the qubit from the noise sources that lead to dephasing. This way we extend the coherence time T2 more than a factor 50 over its baseline value, the inhomogeneous T2*, and demonstrate coherence times limited by relaxation, with a dephasing lifetime Tj even exceeding 0.1 ms. We leverage the narrowband filtering property of the dynamical-decoupling technique to measure the noise that leads to dephasing, and find a 1/f-type flux-noise power spectral density (PSD) over the broad frequency range 0.2 – 20 MHz, a regime not accessed previously. We use relaxation spectroscopy to probe the PSD at higher frequencies, 5.4 – 21 GHz, and furthermore measure the very-low frequency spectrum, 0.1 mHz – 1 kHz, by monitoring the fluctuations of the transition frequency. In conjunction with the CPMG measurements, this enables us to reconstruct a substantial portion of the noise PSD for this long-lived device.
Nature Physics (in press); available at http://arxiv.org/abs/1101.4707