  Non-deterministic noiseless amplification of a single mode can circumvent the
unique challenges to amplifying a quantum signal, such as the no-cloning
theorem, and the minimum noise cost for deterministic quantum state
amplification. However, existing devices are not suitable for amplifying the
fundamental optical quantum information carrier, a qubit coherently encoded
across two optical modes. Here, we construct a coherent two-mode amplifier, to
demonstrate the first heralded noiseless linear amplification of a qubit
encoded in the polarization state of a single photon. In doing so, we increase
the transmission fidelity of a realistic qubit channel by up to a factor of
five. Qubit amplifiers promise to extend the range of secure quantum
communication and other quantum information science and technology protocols.
