@article{21564,
  abstract     = {Multimode quantum light is enticing for several applications, spanning imaging, spectroscopy, communication, and more. Parametric nonlinear processes have been vital in realizing squeezed and other quantum states of light. However, most work exploiting these processes has focused on generating multimode squeezed vacua and squeezing in mode superpositions (supermodes). Bright squeezing in multiple discrete frequency modes, if realized, could unlock novel applications in quantum-enhanced spectroscopy and optical quantum computing. Here, we show how dissipation engineering of a multimode nonlinear cavity with cascaded three-wave-mixing processes allows us to shape above-threshold frequency combs that feature strong single-mode output amplitude noise squeezing over 10 dB below the shot-noise limit, tunable across the comb. In addition, we demonstrate squeezing for multiple discrete frequency modes above threshold. This bright squeezing arises from enhancement of the (noiseless) nonlinear rate relative to decay rates in the system due to the cascaded generation of photons in a single idler “bath” mode. A natural consequence of the strong nonlinear coupling in our system is the creation of an effective cavity in the synthetic frequency dimension that sustains Bloch oscillations in the modal energy distribution. Bloch mode engineering could provide an opportunity to better control nonlinear energy flow in the synthetic frequency dimension, with exciting applications in quantum random walks and topological photonics. Lastly, we show evidence of long-range correlations in amplitude noise between discrete frequency modes, enabling long-range entanglement in a synthetic frequency dimension and providing a new resource for quantum communication.},
  author       = {Pontula, Sahil and Salamin, Yannick and Roques-Carmes, Charles and Soljačić, Marin},
  issn         = {2691-3399},
  journal      = {PRX Quantum},
  number       = {4},
  publisher    = {American Physical Society},
  title        = {{Shaping quantum noise through cascaded nonlinear processes in a dissipation-engineered multimode cavity}},
  doi          = {10.1103/prxquantum.5.040345},
  volume       = {5},
  year         = {2024},
}

@article{11471,
  abstract     = {Variational quantum algorithms are promising algorithms for achieving quantum advantage on nearterm devices. The quantum hardware is used to implement a variational wave function and measure observables, whereas the classical computer is used to store and update the variational parameters. The optimization landscape of expressive variational ansätze is however dominated by large regions in parameter space, known as barren plateaus, with vanishing gradients, which prevents efficient optimization. In this work we propose a general algorithm to avoid barren plateaus in the initialization and throughout the optimization. To this end we define a notion of weak barren plateaus (WBPs) based on the entropies of local reduced density matrices. The presence of WBPs can be efficiently quantified using recently introduced shadow tomography of the quantum state with a classical computer. We demonstrate that avoidance of WBPs suffices to ensure sizable gradients in the initialization. In addition, we demonstrate that decreasing the gradient step size, guided by the entropies allows WBPs to be avoided during the optimization process. This paves the way for efficient barren plateau-free optimization on near-term devices. },
  author       = {Sack, Stefan and Medina Ramos, Raimel A and Michailidis, Alexios and Kueng, Richard and Serbyn, Maksym},
  issn         = {2691-3399},
  journal      = {PRX Quantum},
  keywords     = {General Medicine},
  number       = {2},
  publisher    = {American Physical Society},
  title        = {{Avoiding barren plateaus using classical shadows}},
  doi          = {10.1103/prxquantum.3.020365},
  volume       = {3},
  year         = {2022},
}

@article{9114,
  abstract     = {Microwave photonics lends the advantages of fiber optics to electronic sensing and communication systems. In contrast to nonlinear optics, electro-optic devices so far require classical modulation fields whose variance is dominated by electronic or thermal noise rather than quantum fluctuations. Here we demonstrate bidirectional single-sideband conversion of X band microwave to C band telecom light with a microwave mode occupancy as low as 0.025 ± 0.005 and an added output noise of less than or equal to 0.074 photons. This is facilitated by radiative cooling and a triply resonant ultra-low-loss transducer operating at millikelvin temperatures. The high bandwidth of 10.7 MHz and total (internal) photon conversion
efficiency of 0.03% (0.67%) combined with the extremely slow heating rate of 1.1 added output noise photons per second for the highest available pump power of 1.48 mW puts near-unity efficiency pulsed quantum transduction within reach. Together with the non-Gaussian resources of superconducting qubits this might provide the practical foundation to extend the range and scope of current quantum networks in analogy to electrical repeaters in classical fiber optic communication.},
  author       = {Hease, William J and Rueda Sanchez, Alfredo R and Sahu, Rishabh and Wulf, Matthias and Arnold, Georg M and Schwefel, Harald G.L. and Fink, Johannes M},
  issn         = {2691-3399},
  journal      = {PRX Quantum},
  number       = {2},
  publisher    = {American Physical Society},
  title        = {{Bidirectional electro-optic wavelength conversion in the quantum ground state}},
  doi          = {10.1103/prxquantum.1.020315},
  volume       = {1},
  year         = {2020},
}