A quantum bus is a device which can be used to store or transfer information between independent qubits in a quantum computer, or combine two qubits into a superposition. It is the quantum analog of a classical bus.

There are several physical systems that can be used to realize a quantum bus, including trapped ions, photons, and superconducting 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 a microwave cavity on a chip, is another example of a quantum bus that has been successfully demonstrated in experiments.[1]

## History

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

## Mathematical description

A quantum bus for superconducting qubits can be built with a resonance cavity. The hamiltonian for a system with qubit A, qubit B, and the resonance cavity or quantum bus connecting the two is ${\displaystyle {\hat {H))={\hat {H))_{r}+\sum \limits _{j=A,B}{\hat {H))_{j}+\sum \limits _{j=A,B}hg_{i}\left({\hat {a))^{\dagger }{\hat {\sigma ))_{-}^{j}+{\hat {a)){\hat {\sigma ))_{\text{+))^{j}\right)}$ where ${\displaystyle {\hat {H))_{j}={\frac {1}{2))\hbar \omega _{j}{\hat {\sigma ))_{+}^{j}{\hat {\sigma ))_{-}^{j))$ 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 ${\displaystyle \hbar \omega _{j))$ is controlled by the amplitude of the D.C. and radio frequency flux bias.[6]

## References

1. ^ a b J. Majer; J. M. Chow; J. M. Gambetta; Jens Koch; B. R. Johnson; J. A. Schreier; L. Frunzio; D. I. Schuster; A. A. Houck; A. Wallraff; A. Blais; M. H. Devoret; S. M. Girvin; R. J. Schoelkopf (2007-09-27). "Coupling superconducting qubits via a cavity bus". Nature. 449 (7161): 443–447. arXiv:0709.2135. Bibcode:2007Natur.449..443M. doi:10.1038/nature06184. PMID 17898763. S2CID 8467224.
2. ^ M. A. Sillanpää; J. I. Park; R. W. Simmonds (2007-09-27). "Coherent quantum state storage and transfer between two phase qubits via a resonant cavity". Nature. 449 (7161): 438–42. arXiv:0709.2341. Bibcode:2007Natur.449..438S. doi:10.1038/nature06124. PMID 17898762. S2CID 4357331.
3. ^ "All Aboard the Quantum 'Bus'". 2007-09-27. Retrieved 2008-12-12.
4. ^ G.K. Brennen; D. Song; C.J. Williams (2003). "Quantum-computer architecture using nonlocal interactions". Physical Review A. 67 (5): 050302. arXiv:quant-ph/0301012. Bibcode:2003PhRvA..67e0302B. doi:10.1103/PhysRevA.67.050302. S2CID 118895065.
5. ^ Brooks, Michael (2012-12-06). Quantum Computing and Communications. Springer Science & Business Media. ISBN 978-1-4471-0839-9.
6. ^ Sillanpää, Mika A.; Park, Jae I.; Simmonds, Raymond W. (2007). "Coherent quantum state storage and transfer between two phase qubits via a resonant cavity". Nature. 449 (7161): 438–442. arXiv:0709.2341. Bibcode:2007Natur.449..438S. doi:10.1038/nature06124. PMID 17898762. S2CID 4357331.