Decoherence in superconducting quantum bits by phonon radiation
L. B. Ioffe, V. B. Geshkenbeǐn, G. Blatter, L.D. LANDAU · APS March Meeting Abstracts · 2004
We discuss a fundamental limitation for the coherent operation of superconducting quantum bits originating from phonon radiation generated in the Josephson junctions of the device. The time dependent superconducting phase across the junction produces an electric field that couples to the underlying crystal lattice via the piezoelectric effect. We determine the radiation resistance of the junction due to phonon emission and derive substantial decoherence rates for the quantum bits, which are compatible with quality factors measured in recent experiments. Decoherence is the main adversary of the unitary time evolution governing the quantum systems that provide the hardware for a future quantum information technology. Solid state implementations of such ‘‘quantum hardware’’ based on superconducting structures have undergone an amazing development during the last years [1‐7], thus underlying their potential for the construction of quantum information processors. Key elements in this type of hardware are the Josephson junctions with their dynamics driving the quantum fluctuations in these devices. In this Letter, we analyze the phonon radiation emitted from these Josephson junctions and determine the associated energy relaxation rate leading to the decoherence of the qubit’s quantum state. Superconducting qubits come in three main varieties: charge qubits [1,2,8,9] store the quantum information in the charge states of a small Cooper pair box, while superpositions of macroscopic ring currents with opposite circulation assume this role in flux/phase qubits [3‐5,10 ‐ 12]. Finally, Josephson junction qubits [6,7] store the information in the internal state of a current biased Josephson junction. The operation of these devices is governed by two energy scales, the Josephson energy EJ � � hIc=2e associated with the current flow and the