Coherence control for qubits

Abstract

We study the influence of an external driving field on the coherence properties of a qubit subject to bit-flip noise. In the presence of driving, two paradigmatic cases are considered: (i) a field that results for a suitable choice of the parameters in so-called coherent destruction of tunneling and (ii) one that commutes with the static qubit Hamiltonian. In each case, we give for high-frequency driving a lower bound for the coherence time. This reveals the conditions under which the external fields can be used for coherence stabilization.

Intrinsic phonon decoherence and quantum gates in coupled lateral quantum-dot charge qubits

“…If the density operator ρ describes an almost pure state, i.e., if the purity is always close to the ideal value 1, it is possible to estimate the purity loss during the gate operation from its decay rate along the lines of Ref. [36]. Thereby, one first evaluates the decay of (d/dt)trρ 2 for an arbitrary pure qubit state ρ = |ψ ψ|.…”

Improving the Purity of One‐ and Two‐qubit Gates

“…The unavoidable coupling to external degrees of freedom and the thereby caused decoherence still presents a main obstacle for the realization of a quantum computer. Several proposals to overcome the ensuing decoherence have been put forward, such as the use of decoherence free subspaces [8][9][10][11][12], coherence-preserving qubits [13], quantum Zeno subspaces [14], optimized pulse sequences [15,16], dynamical decoupling [17][18][19][20][21], and coherent destruction of tunneling [22,23]. Theoretical studies of decoherence of two-level systems have been extended to gate operations in the presence of an environment in [24][25][26][27][28][29].…”

Decoherence control: Universal protection of two-qubit states and two-qubit gates using continuous driving fields

“…For example, there is no longer any concern about pulse timings or pulse-sequence engineering, and the higher driving frequencies that we can achieve with continuous fields are naturally expected to eliminate higher frequency noise sources [44]. Indeed, continuous DD schemes to protect a single qubit have attracted considerable interest [44][45][46][47]. Among the known features of single-qubit continuous DD, most relevant here is the fact that continuous control fields may be constructed to protect a quantum state and implement a gate at the same time, without the use of any overhead [46,47] (thus forming a type of DCG [41,42]).…”

“…This is a pertinent question to ask because, compared with pulsed DD, continuous DD has some advantages from a practical point of view. For example, there is no longer any concern about pulse timings or pulse-sequence engineering, and the higher driving frequencies that we can achieve with continuous fields are naturally expected to eliminate higher frequency noise sources [44]. Indeed, continuous DD schemes to protect a single qubit have attracted considerable interest [44][45][46][47].…”