Scalable Quantum Computing with Josephson Charge Qubits

A goal of quantum information technology is to control the quantum state of a system, including its preparation, manipulation, and measurement. However, scalability to many qubits and controlled connectivity between any selected qubits are two of the major stumbling blocks to achieve quantum computing (QC). Here we propose an experimental method, using Josephson charge qubits, to efficiently solve these two central problems. The proposed QC architecture is scalable since any two charge qubits can be effectively coupled by an experimentally accessible inductance. More importantly, we formulate an efficient and realizable QC scheme that requires only one (instead of two or more) two-bit operation to implement conditional gates.

Figure 1

Schematic diagram of the proposed scalable and switchable quantum computer. All Josephson charge-qubit structures are coupled by a common superconducting inductance. Here, each Cooper-pair box is operated both in the charging regime $E_{ck}\gg E_{Jk}^0$ and at low temperatures $k_{B}T\ll E_{ck}$. Moreover, the superconducting gap is larger than $E_{ck}$, so that quasiparticle tunneling is prohibited in the system.

Figure 2

(a) One-bit circuit with a Cooper-pair box connected to the inductance. (b) Two-bit structure where two Cooper-pair boxes are commonly connected to the inductance. Here, each SQUID connecting the superconducting island is represented by an effective Josephson junction.