Sharing information coherently between nodes of a quantum network is at the foundation of distributed quantum information processing. In this scheme, the computation is divided into subroutines and performed on several smaller quantum registers connected by classical and quantum channels. A direct quantum channel, which connects nodes deterministically rather than probabilistically, is advantageous for fault-tolerant quantum computation because it reduces the threshold requirements and can achieve larger entanglement rates. Here, we implement deterministic state transfer and entanglement protocols between two superconducting qubits [1] fabricated on separate chips [2] and connected by about one meter of coaxial cable with well-characterized loss [3]. Superconducting circuits constitute a universal node capable of sending, receiving, storing, and processing quantum information. Our implementation is based on an all-microwave cavity-assisted Raman process, which entangles or transfers the qubit state of a transmon-type artificial atom with a time-symmetric itinerant single photon [4]. We transfer qubit states at a rate of 50 kHz using the emitted photons, which are absorbed at the receiving node with a probability of 98 % achieving a transfer process fidelity of 80 %. We also prepare on demand remote entanglement with a fidelity as high as 79 %. Our results are in excellent agreement with numerical simulations based on a master equation description of the system. Deterministic state transfer protocols have the potential to be used as a backbone of surface code quantum error correction across different nodes of a cryogenic network to realize large-scale fault-tolerant quantum computation. It is also interesting to consider augmenting the methods presented in this work by quantum-non-demolition detection of single photons [5].
[1] P. Kurpiers et al., Nature 558, 264-267 (2018)
[2] T. Walter et al., Phys. Rev. Applied 7, 054020 (2017)
[3] P. Kurpiers et al., EPJ Quantum Technology 4, 8 (2017)
[4] M. Pechal et al., Phys. Rev. X 4, 041010 (2014)
[5] J.-C. Besse et al., Phys. Rev. X 8, 021003 (2018)
Quantum information processing and computing , Quantum communication , Superconducting circuits
