We propose a multistage quantum pulse gate which efficiently mix multimode quantum states in the frequency domain. Furthermore we introduce a practicable quantum simulator for calculating Frank Condon factors.
We introduce the concept of a quantum pulse gate (QPG), a method for accessing the intrinsic broadband spectral mode structure of ultrafast quantum states of light. This mode structure can now be harnessed for applications in quantum information processing. We propose an implementation in a PPLN waveguide, based on spectrally engineered sum frequency generation (SFG). It allows us to pick well-defined spectral broadband modes from an ultrafast multi-mode state for interconversion to a broadband mode at another frequency. By pulse-shaping the bright SFG pump beam, different orthogonal broadband modes can be addressed individually and extracted with high fidelity. (C) 2011 Optical Society of America
Summary form only given. Optical linear circuit is one of the most important parts for optical quantum computing. The optical circuit, which consists of mode mixers and phase shifters, has been used in various quantum protocols: cluster state generation, Boson sampling, and quantum random walk. To realize a large-scale linear circuit, waveguide-based quantum circuits have experimentally been demonstrated by some groups [1, 2]. However, such a waveguide-based circuit cannot overcome issues of flexibility and loss.On the other hand, recently, a quantum pulse gate (QPG), which enables mode extraction from multimode quantum fields in the frequency domain, has been proposed and experimentally demonstrated [3]. For utilizing the frequency resource of photon field, parallel generations of single photons or multimode squeezed states in the frequency domain, namely time-frequency Schmidt modes, have been demonstrated. In such a wavelengthdivision-multiplexed quantum circuit, this mode extraction corresponds to one row of a linear unitary rotation of the Schmidt modes. It is achieved by sum frequency generation between a spectrally-shaped classical pulse and a quantum weak pulse. The arbitrary mode extraction can be designed by the pulse shaping with a high efficiency. In this report, we propose a multistage QPG which is a simple, flexible and highly efficient quantum linear optical circuit. We devised a novel multipass 4-f system to map the frequency multimode quantum states into the spatiotemporal domain with a single nonlinear crystal (Fig. 1(a)). Furthermore, we show its application to a quantum simulator for calculating molecular vibronic absorption spectra [4] (Fig. 1(b)).
We propose a large-scale quantum simulator by multiplexing optical quantum modes in the frequency domain and arbitrarily mixing them as a boson sampling optical circuit to obtain Franc Condon Factor of large molecules. By shaping 785-nm femtosecond pump laser pulses, we generate multimode squeezed states via spontaneous parametric down conversion with a type-II PPKTP. Sum frequency mixing using a type-II PPLN between the multimode squeezed states and gate pulses that are shaped to the arbitrary superposition of the multi-spectral mode functions acts as programmable quantum gating. We complete the quantum simulation by time-dependent photon number detection at the sum-frequency wavelength at 517 nm. We describe the advantage of quantum mode manipulation in the frequency domain and place a special emphasis on group velocity dispersion engineering required to achieve them.