An improved quantum watermarking scheme using small-scale quantum circuits and color scrambling

In order to solve the problem of embedding the watermark into the quantum color image, in this paper, an improved scheme of using small-scale quantum circuits and color scrambling is proposed. Both color carrier image and color watermark image are represented using novel enhanced quantum representation. The image sizes for carrier and watermark are assumed to be $$2^{n+1}\times 2^{n+2}$$2n+1×2n+2 and $$2^{n}\times 2^{n}$$2n×2n, respectively. At first, the color of pixels in watermark image is scrambled using the controlled rotation gates, and then, the scrambled watermark with $$2^n\times 2^n$$2n×2n image size and 24-qubit gray scale is expanded to an image with $$2^{n+1}\times 2^{n+2}$$2n+1×2n+2 image size and 3-qubit gray scale. Finally, the expanded watermark image is embedded into the carrier image by the controlled-NOT gates. The extraction of watermark is the reverse process of embedding it into carrier image, which is achieved by applying operations in the reverse order. Simulation-based experimental results show that the proposed scheme is superior to other similar algorithms in terms of three items, visual quality, scrambling effect of watermark image, and noise resistibility.