Multimode Squeezed State for Reconfigurable Quantum Networks at Telecommunication Wavelengths

Abstract

Continuous variable encoding of quantum information requires the deterministic generation of highly correlated quantum states of light in the form of quantum networks, which, in turn, necessitates the controlled generation of a large number of squeezed modes. In this work, we present an experimental source of multimode squeezed states of light at telecommunication wavelengths. Generation at such wavelengths is especially important as it can enable quantum information processing, communication, and sensing beyond the laboratory scale. We use a single-pass spontaneous parametric down-conversion process in a non-linear waveguide pumped with the second harmonic of a femtosecond laser. Our measurements reveal significant squeezing in more than 21 frequency modes, with a maximum squeezing value exceeding 2.5 dB. We demonstrate multiparty entanglement by measuring the state's covariance matrix. Finally, we show the source reconfigurability by preparing few-node cluster states and measure their nullifier squeezing level. These results pave the way for a scalable implementation of continuous variable quantum information protocols at telecommunication wavelengths, particularly for multiparty, entanglement-based quantum communications. Moreover, the source is compatible with additional pulse-by-pulse multiplexing, which can be utilized to construct the necessary three-dimensional entangled structures for quantum computing protocols.

Figures (13)

• Figure 1: Experimental scheme for the generation of the multimode states. A telecom wideband pulsed laser is up converted to its second harmonic with a ppLN crystal and then coupled to a nonlinear ppKTP waveguide. Type 0 SPDC interaction generates independent squeezing in a set of frequency modes. Each mode is addressed individually with homodyne detection, where the local oscillator is shaped spectrally with the use of a pulse shaper. For more details see text.
• Figure 2: Experimental squeezing and anti-squeezing values for the Hermite-Gauss modes (left) and the flat modes defined in the text (right). An example of the HG$_0$ mode is also shown, indicating the spectral range common to all the modes.
• Figure 3: Expected values of the $\{\hat{x}_i\hat{x}_j\}$ and $\{\hat{p}_i\hat{p}_j\}$ quadrature components composing the covariance matrix of the multimode state, measured in the frexel basis using 8 frequency bands.
• Figure 4: The boxplots sum up the squeezing value for the first HG modes (left), and the nullifiers of different cluster state size and topologies. The numerical values indicate the mean nullifiers's squeezing value, the black line spread over the squeezing values of the different nullifiers while the blue boxes are related to their variance (they give the amount of values in the interval between the first and third quartiles for each nullifier dataset).
• Figure 5: Numerical simulation of the independent squeezed states. Left: JSA function, Right: First three frequency modes, approximating Hermite-Gauss modes, and the distribution of the Schmidt coefficients.