Correlation-induced suppression of decoherence in capacitively coupled Cooper-pair boxes

Charge fluctuations from gate bias and background traps severely limit the performance of a charge qubit in a Cooper-pair box (CPB). Here we present an experimentally realizable method to control the decoherence effects of these charge fluctuations using two strongly capacitively coupled CPBs. This coupled-box system has a low-decoherence subspace of two states. Our results show that the interbox Coulomb correlation can help significantly suppress decoherence of this two-level system, making it a promising candidate as a logical qubit, encoded using two CPBs.

Figure 1

(Color online) Strongly coupled Cooper-pair boxes. A bias voltage $V_i$ is applied to the $i\mathrm{th}$ charge box through a gate capacitance $C_i$, and a symmetric dc SQUID (with Josephson coupling energy $E_{Ji}^0$ and capacitance $C_{Ji}$ for each junction) is coupled to the box. Also, each box is connected to a detector via a probe junction (or a less invasive point contact). When a measurement is performed, the probe junction is biased with an appropriate voltage $V_{\mathrm{bi}}$. The two boxes are closely spaced long superconducting islands with sufficiently large mutual capacitance $C_m$, and the barrier between them is strong enough to prohibit the interbox Cooper-pair tunneling.

Figure 2

(Color online) Dependence of the energy levels of the coupled-box system on the reduced offset charge $n_{xL}$ for $n_{xR}=0.5$. Here $\Delta E_i=E_{ci}∕4$, and $E_{Ji}=E_{ci}∕10$, with $i=L,R$. The energy is in units of $E_c$. We choose ten two-box basis states $\mid m,n⟩$ that have the lowest electrostatic energy. The two lowest levels remain nearly unchanged in the vicinity of the degeneracy point $(n_{xL},n_{xR})=(\frac{1}{2},\frac{1}{2})$.