Observation of Three-Photon Spontaneous Parametric Down-Conversion in a Superconducting Parametric Cavity

Abstract

Spontaneous parametric downconversion (SPDC) has been a key enabling technology in exploring quantum phenomena and their applications for decades. For instance, traditional SPDC, which splits a high energy pump photon into two lower energy photons, is a common way to produce entangled photon pairs. Since the early realizations of SPDC, researchers have thought to generalize it to higher order, e.g., to produce entangled photon triplets. However, directly generating photon triplets through a single SPDC process has remained elusive. Here, using a flux-pumped superconducting parametric cavity, we demonstrate direct three-photon SPDC, with photon triplets generated in a single cavity mode or split between multiple modes. With strong pumping, the states can be quite bright, with flux densities exceeding 60 photon/s/Hz. The observed states are strongly non-Gaussian, which has important implications for potential applications. In the single-mode case, we observe a triangular star-shaped distribution of quadrature voltages, indicative of the longpredicted "star state". The observed star state shows strong third-order correlations, as expected for a state generated by a cubic Hamiltonian. By pumping at the sum frequency of multiple modes, we observe strong three-body correlations between multiple modes, strikingly, in the absence of secondorder correlations. We further analyze the third-order correlations under mode transformations by the symplectic symmetry group, showing that the observed transformation properties serve to "fingerprint" the specific cubic Hamiltonian that generates them. The observed non-Gaussian, thirdorder correlations represent an important step forward in quantum optics and may have a strong impact on quantum communication with microwave fields as well as continuous-variable quantum computation.

Entangling Superconducting Qubits through an Analogue Wormhole

“…Another experimental challenge would be the preparation of a pure initial quantum vacuum for the field. However, for realistic temperatures of 30 mK [69] the number of thermal photons at 5 mK is as low as 3 • 10 −4 so the effect of thermal noise should be negligible. In order to test the effects of effective curved spacetime on the extraction of vacuum entanglement, the experiment should be repeated with and without external bias (wormhole) and the comparison of the results should follow our predictions.…”

Progress toward third-order parametric down-conversion in optical fibers

“…It realizes a three-mode squeezing operator that differs greatly from the two-mode squeezing operator that generates Gaussian squeezed states. Photon triplet states generated directly through the cubic interaction were observed only at microwave frequencies [15]. The photon triplet states generated so far at optical frequencies were mediated by the second-order susceptibility [16,17].…”

Quantifying tripartite entanglement with entropic correlations

“…Here, it is clear that there exist genuinely tripartite entangled states that are nonetheless separable when considering only two out of three parties [such as the GHZ state shown in Eq. (15)]. For a carefully, concisely, and clearly laid out discussion of the hierarchy of multiparite entanglement and partial separability, see Ref.…”

“…[10] using quantum entropies obtained from experimental correlations, as this measure has the greatest potential to be a resource-based measure with further theoretical development. We show how this strategy works both for three systems entangled in either discrete or continuous-variable degrees of freedom (e.g., photon triplets generated in third-order parametric down-conversion [14,15]).…”