Multimode Squeezed State for Reconfigurable Quantum Networks at Telecommunication Wavelengths

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.

💡 Research Summary

In this work the authors demonstrate a deterministic source of multimode continuous‑variable (CV) squeezed states operating in the telecom C‑band (≈1550 nm), a wavelength regime directly compatible with existing fiber‑optic infrastructure. The source is based on a single‑pass type‑0 spontaneous parametric down‑conversion (SPDC) process in a periodically poled potassium titanyl phosphate (ppKTP) waveguide. A broadband femtosecond laser centered at 1560 nm (55 nm bandwidth, 57 fs pulses, 100 MHz repetition) is frequency‑doubled in a periodically poled lithium niobate (ppLN) crystal to generate a 780 nm pump. The pump is coupled into the ppKTP waveguide, where it creates a broadband multimode squeezed vacuum spanning the entire C‑band.

The multimode structure is described by a Schmidt decomposition of the joint spectral amplitude, yielding a set of orthogonal frequency modes {h_k(ω)} with associated Schmidt coefficients λ_k. Under the Hamiltonian H = ∑_k λ_k A_k†² + h.c., each mode A_k experiences independent squeezing. By shaping the local oscillator (LO) with a 4f pulse shaper and spatial light modulator, the authors can project the output onto arbitrary mode bases and perform homodyne detection of individual modes.

Two families of modes are investigated. First, Hermite‑Gauss (HG) modes are defined, with HG0 having a 45 nm spectral width. The authors report squeezing and anti‑squeezing for the first 21 HG modes, with a maximum measured squeezing of > 2.5 dB (≈0.57 × shot noise) and corresponding anti‑squeezing limited by optical losses. The main loss sources are imperfect mode matching between signal and LO (visibility ≈ 77 %) and bandwidth truncation at the edges of the pulse shaper due to finite mirror size. Second, “flat” modes—orthogonal, spectrally flattened combinations of the HG basis—are measured, yielding > 2 dB of squeezing for the first four flat modes, indicating that appropriate mode engineering can improve observable squeezing.

To verify genuine quantum correlations, the authors partition the spectrum into eight equally spaced “frexel” bands (≈7 nm each) and reconstruct the full covariance matrix of the quadrature operators x_i and p_i. The off‑diagonal elements reveal strong inter‑frequency correlations. Applying the Positive‑Partial‑Transpose (PPT) criterion to all possible bipartitions, they observe PPT violations for 94 % of them, confirming multipartite entanglement across the multimode state. Diagonalization of the covariance matrix yields the experimental super‑mode basis, which matches the theoretical HG modes with only a modest spectral broadening.

The reconfigurability of the source is showcased by generating small‑scale CV cluster states. By programming the pulse‑shaper mask, the authors implement different adjacency matrices V_ij, thereby defining the nullifiers δ_i = p_i − ∑_j V_ij x_j for various graph topologies. They experimentally produce four‑node linear, four‑node square, and up to eight‑node linear cluster states. The nullifier variances are measured to be below shot noise for all generated graphs, with the eight‑node linear cluster still showing measurable squeezing despite a slight degradation compared to the raw HG mode squeezing—attributed to residual correlations among the selected HG modes. This demonstrates that the same physical hardware can be rapidly reprogrammed to produce different entanglement structures without altering the underlying optical setup.

The paper emphasizes that the demonstrated multimode squeezed source is compatible with additional pulse‑by‑pulse multiplexing techniques, opening a route toward three‑dimensional cluster states required for fault‑tolerant CV quantum computing. The authors discuss that further improvements—such as longer waveguides with lower propagation loss, higher pump powers, optimized mode‑matching optics, and faster electro‑optic switching—could push squeezing beyond 10 dB and increase the number of usable modes into the hundreds.

In summary, the work provides (1) a compact, fiber‑compatible source of > 20 simultaneously squeezed frequency modes at telecom wavelengths, (2) a thorough characterization of the multimode entanglement via covariance matrix reconstruction and PPT analysis, and (3) a proof‑of‑principle demonstration of on‑demand generation of various CV cluster states using programmable spectral shaping. These results constitute a significant step toward scalable, room‑temperature quantum networks, multipartite quantum communication protocols, and large‑scale continuous‑variable quantum computing architectures that can be directly interfaced with existing telecom infrastructure.