  We propose a linear-optical scheme for an efficient amplification of a
photonic qubit based on interaction of the signal mode with a pair of entangled
ancillae. In contrast to a previous proposal for qubit amplifier by Gisin et
al., [Phys Rev. Lett. 105, 070501 (2010)] the success probability of our device
does not decrease asymptotically to zero with increasing gain. Moreover we show
how the device can be used to restore entanglement deteriorated by transmission
over a lossy channel and calculate the secure key rate for device-independent
quantum key distribution.
