1. Optimal preparation of a Gaussian quantum network with an ultrafast frequency comb
- We have implemented a pulse shaper for the pump beam to generate a multimode squeezed vacuum state and another pulse shaper for the local oscillator to measure the generated state. The pulse shaper for the pump, for example, can shift the phase of a half of the pump spectrum, and as a result, we observed that the covariance matrix of the Gaussian quantum network is qualitatively changed. The pulse shaper for the local oscillator enables us to access an individual node in a quantum network. Moreover, it allows to accessing multiple nodes as a superposition, which is equivalent with having a quantum network in a different topology. Thanks to such versatility, we can generate a Gaussian quantum network optimized for a given quantum information task.
2. Generation of photonic quantum network by employing single-photon control
- We have implemented a single-photon subtractor based on nonlinear interaction between an input beam and an auxiliary gate beam. The spectral mode of the gate beam defines the mode of single-photon subtraction at the input beam, which is required for single-photon subtraction in a quantum network. Note that the conventional method of photon subtraction cannot be used in multimode quantum states such as a Gaussian quantum network because the resultant state becomes a mixed quantum state, while our method can be used to subtract a single photon at a desired mode or at a superposition of desired modes by maintaining quantum properties. To control the spectrum of the gate beam, we have implemented a pulse shaper, which can tune the amplitude and the phase at each frequency band. We have fully characterized the implemented device based on quantum process tomography. An important figure of merit of a single-photon subtractor is a mode selectivity, and the implemented one typically exhibits a high value amounting to 0.9. This result led to publication in Phys. Rev. X 7, 031012 (2017).
3. Full characterization of a photonic quantum network
- We have successfully generated a non-Gaussian quantum network based on the single-photon subtractor implemented in the aforementioned paragraph. This is, in fact, the main goal of this project, which combines a Gaussian quantum network and single-photon control to develop a photonic quantum network. We have experimentally observed that photon subtraction results in a non-Gaussian statistics at a desired mode or at a superposition of different modes. We have also reconstructed the generated quantum states by quantum state tomography.
4. Measurement based quantum computing using a photonic quantum network
- To fully exploit the photonic quantum network in quantum protocols (e.g. measurement based quantum computing), one needs simultaneous accesses to multiple modes, which requires multimode homodyne detection. We have implemented multimode homodyne detection for multiple frequency bands of light. In more detail, it is constructed by mixing a spectrally broadband local oscillator with a quantum state to be measured at a beam splitter; the two output beams are diffracted by optical gratings, which are subsequently focused by micro-lens arrays at multipixel photodiodes; a pair of photodiodes each from the two multipixel photodiodes construct a individual homodyne detection. This device will be useful to efficiently characterize a photonic quantum network and to conduct quantum information tasks.
