Ramsey fringes. (a) Coherent charge oscillations are directly observed by measuring the conductance of a local quantum point contact charge detector. (b) A larger pulse amplitude results in faster Ramsey fringes.

A single electron trapped in a double well potential forms a charge qubit [1]. We are studying charge coherence in gate defined GaAs/AlGaAs double quantum dots (DQD). The DQD provides a fully tunable Hamiltonian, with a gate-voltage-tunable energy level detuning and interdot tunnel barrier height [2]. The charge degree of freedom interacts strongly with the environment, leading to short 300 ps inhomogenous dephasing times [3]. While the short coherence times are impractical for quantum computing, we take advantage of the sensitivity of the charge qubit to probe the local solid state environment. We use the charge qubit as a quantum sensor to probe 1/f noise in the sample and quantitatively study electron-phonon coupling in quantum confined systems.

A typical charge qubit experiment begins by biasing the double well potential such that the single electron is localized in the right quantum dot. We then apply a non-adiabatic voltage pulse to bring the system to zero detuning, where the left and right dot charge states are hybridized by interdot tunnel coupling. Here the system freely precesses about the x-axis of the Bloch sphere, leading to coherent rotations, with an oscillation frequency set by the strength of the interdot tunnel coupling. Readout is performed by strongly biasing the double well potential and using a local quantum point contact charge detector to determine if the charge is localized in the right or left side of the double well potential.

Double quantum dot charge qubits are sensitive to charge noise since the level detuning is set by electrostatic gate voltages. By studying the decay of the Larmor oscillations we can determine the strength of the charge noise at the sample [4]. At zero detuning, the level splitting is first order insensitive to charge noise, leading to quantum oscillations that persist for many cycles. More sophisticated gate voltage pulse sequences can be used to observe "Ramsey Fringes" and a "Charge Echo". From these measurements we determine that the charge noise experienced by the qubit has a rms magnitude of approximately 4 μeV. Charge noise has significant implications for quantum computing since many quantum gates rely on an electrically tunable interaction.

[1] Y. Nakamura

[2] T. Hayashi

[3] J. R. Petta

[4] J. M. Martinis

J. Stehlik, Y. Dovzhenko, J. R. Petta, J. R. Johansson, F. Nori, H. Lu, A. C. Gossard

Phys. Rev. B (RC) **86**, 121303 (2012)

Y. Dovzhenko, J. Stehlik, K. D. Petersson, J. R. Petta, H. Lu, and A. C. Gossard

Phys. Rev. B (RC) **84**, 161302 (2011)

K. D. Petersson, J. R. Petta, H. Lu, A. C. Gossard

Phys. Rev. Lett. **105**, 246804 (2010)