Aquantum bus is a device which can be used to store or transfer information between independentqubits in aquantum computer, or combine two qubits into asuperposition. It is thequantum analog of aclassical bus.

There are several physical systems that can be used to realize a quantum bus, includingtrapped ions,photons, andsuperconducting qubits. Trapped ions, for example, can use the quantized motion of ions (phonons) as a quantum bus, while photons can act as a carrier of quantum information by utilizing the increased interaction strength provided by cavity quantum electrodynamics.Circuit quantum electrodynamics, which uses superconducting qubits coupled to amicrowave cavity on a chip, is another example of a quantum bus that has been successfully demonstrated in experiments.^[1]

The concept was first demonstrated by researchers atYale University and theNational Institute of Standards and Technology (NIST) in 2007.^[1][2][3] Prior to this experimental demonstration, the quantum bus had been described by scientists atNIST as one of the possible cornerstone building blocks inquantum computing architectures.^[4][5]

A quantum bus forsuperconducting qubits can be built with aresonance cavity. Thehamiltonian for a system with qubit A, qubit B, and the resonance cavity or quantum bus connecting the two is${\displaystyle \hat{H}={\hat{H}}_r+\sum _{j=A,B}{\hat{H}}_j+\sum _{j=A,B}h{g}_i\left({\hat{a}}^{†}{\hat{\sigma }}_{-}^j+\hat{a}{\hat{\sigma }}_{\text{+}}^j\right)}$ where${\displaystyle {\hat{H}}_j=\frac{1}{2}\hslash {\omega }_j{\hat{\sigma }}_{+}^j{\hat{\sigma }}_{-}^j}$ is the single qubit hamiltonian,${\displaystyle {\hat{\sigma }}_{+}^j{\hat{\sigma }}_{-}^j}$ is the raising or lowering operator for creating or destroying excitations in the${\displaystyle j}$th qubit, and${\displaystyle \hslash {\omega }_j}$ is controlled by the amplitude of theD.C. andradio frequencyflux bias.^[6]

• Quantum information science
• Quantum electronics

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