Effects of external driving on the coherence time of a Josephson junction qubit in a bath of two-level fluctuators

Abstract

We study the effect of external driving on the two level systems (TLSs) assumed to be a major obstacle in increasing the coherence time of solid state Josephson-junction qubits. We find, by use of a Bloch-Redfield approach, that external driving has two major effects on the TLS. The first is increased fluctuations between the two states of the TLS, the significance of this effect compared to thermal fluctuations depend on the energy splitting of the TLS compared to temperature. The second effect is a reduction in the intensity of the noise spectrum at low frequencies, and at the same time an increase in intensity around the renormalized Rabi frequency of the TLS, the driving frequency and at beatings between these two frequencies. Finally we study the ensemble averaged noise spectrum for a typical distribution of TLSs known to give origin to ∝ 1/f noise. We find that strong driving leads to reduced noise at low frequencies, and therefore to an increased dephasing time T Q 2 of the qubit. However this effect is exponentially suppressed when the driving frequency is large compared to temperature, as we typically find for Josephson qubits. We suggest that external driving at frequencies much lower than the qubit frequency might be used in order to enhance the the qubit coherence time.

Evolution of inhomogeneously broadened spin-noise spectrum with ac drive

“…On the other hand, from Eq. (22) we see that this area comes from the domain (ω − ω ⊥dr ) ∼ ω ⊥dr δ e , i.e. the area conservation is ensured locally.…”

“…22 where the noise of a driven two-level system was considered. It was demonstrated 22 that the drive-induced additional harmonics in the dynamics of the two-level system manifest themselves as additional peaks in the noise spectrum.…”

Spin noise at electron paramagnetic resonance

“…While most of early experimental studies benefit from the fact than the noise spectroscopy requires no excitation of an electron system and allows one to study the system in the conditions close to thermal equilibrium, the modern spin noise spectroscopy goes beyond the measurement of linear response 9,10 . In particular, the study of spin noise in the presence of a radio-frequency (RF) field provides an additional insight into the physics of fluctuations and spin dephasing processes [11][12][13][14][15][16] . A scheme to measure the spin fluctuations via optical Faraday rotation with an oscillating magnetic field instead of a static magnetic field commonly used in experiments was discussed in Ref.…”

Decoherence of a qubit due to either a quantum fluctuator, or classical telegraph noise

“…However, in systems such as glasses this inequality is usually expected to hold, and the TLS can be treated effectively as a classical fluctuator, 32 with an exception if the system is subject to an external AC field. 42 The pointer states of a quantum system are defined as the pure states that are the least affected by environmental decoherence. 1,3 It is generally believed that when the dynamics of the system is dominated by the interaction with the environment, the pointer states are the eigenstates of the interaction Hamiltonaian.…”