  Faithfully transferring the quantum state is essential for quantum
information processing. Here we demonstrate a fast (in 84 ns) and high-fidelity
(99.2%) transfer of arbitrary quantum states in a chain of four superconducting
qubits with nearest-neighbor coupling. This transfer relies on full control of
the effective couplings between neighboring qubits, which is realized only by
our parametrically modulating the qubits without increasing circuit complexity.
Once the couplings between qubits fulfill a specific ratio, perfect quantum
state transfer can be achieved in a single step, and is therefore robust to
noise and accumulation of experimental errors. This quantum state transfer can
be extended to a larger qubit chain and thus adds a desirable tool for future
quantum information processing. The demonstrated flexibility of the coupling
tunability is suitable for quantum simulation of many-body physics, which
requires different configurations of qubit couplings.
