@article{21449,
  abstract     = {Three-dimensional (3D) crystals offer a route to scaling up trapped-ion systems for quantum sensing and quantum simulation applications; however, engineering coherent spin-motion couplings and effective spin-spin interactions in large crystals poses technical challenges associated with decoherence and prolonged timescales to generate appreciable entanglement. Here, we explore the possibility of speeding up these interactions in 3D crystals via parametric amplification. For this purpose, we derive a general Hamiltonian for the parametric amplification of spin-motion coupling that is broadly applicable to normal modes with motion transverse to or along the spatial extent of the crystal. Unlike in lower-dimensional crystals, we find that the ability to faithfully (uniformly) amplify the spin-spin interactions in 3D crystals depends on the physical implementation of the spin-motion coupling. We consider the light-shift gate, and the so-called phase-insensitive and phase-sensitive Mølmer-Sørensen (MS) gates, and we find that only the phase-sensitive MS gate can be faithfully amplified in general 3D crystals. We discuss a situation where nonuniform amplification can be advantageous. We also reconsider the effect of counter-rotating terms on parametric amplification and find that they are not as detrimental as previous studies suggest.},
  author       = {Hawaldar, Samarth and Nikhil, N. and Rey, Ana Maria and Bollinger, John J. and Shankar, Athreya},
  issn         = {2331-7019},
  journal      = {Physical Review Applied},
  number       = {3},
  publisher    = {American Physical Society},
  title        = {{Parametric amplification of spin-motion coupling in three-dimensional trapped-ion crystals}},
  doi          = {10.1103/h1m9-h3yw},
  volume       = {25},
  year         = {2026},
}

@unpublished{21870,
  abstract     = {Superconducting qubits are a leading candidate for utility-scale quantum computing due to their fast gate speeds and steadily decreasing error rates. The requirement for millikelvin operating temperatures, however, creates a significant scaling bottleneck. Modular architectures using optical fiber links could bridge separate cryogenic nodes, but superconducting circuits do not have coherent optical transitions and microwave-to-optical conversion has not been shown for any non-classical photon state. In this work, we demonstrate the on-demand generation and tomographic reconstruction of itinerant single microwave photons at 8.9 GHz from a superconducting qubit. We upconvert this non-Gaussian state with a transducer added noise below 0.012 quanta and count the converted telecom photons at 193.4 THz with a signal-to-noise ratio of up to 5.1$\pm$1.1. We characterize the trade-offs between throughput and noise, and establish a viable path toward heralded entanglement distribution and gate teleportation. Looking ahead, these results empower existing superconducting devices to take a key role in distributed quantum technologies and heterogeneous quantum systems.},
  author       = {Werner, Thomas and Riyazi, Erfan and Hawaldar, Samarth and Sahu, Rishabh and Arnold, Georg M and Paul Falthansl-Scheinecker, Paul Falthansl-Scheinecker and Naranjo, Jennifer A. Sánchez and Loi, Dante and Kapoor, Lucky N. and Zemlicka, Martin and Qiu, Liu and Militaru, Andrei and Fink, Johannes M},
  booktitle    = {arXiv},
  title        = {{Electro-optic conversion of itinerant Fock states}},
  doi          = {10.48550/arXiv.2602.00928},
  year         = {2026},
}

@article{22637,
  abstract     = {The distribution of entanglement across distant qubits is a central challenge for the operation of scalable quantum computers and large-scale quantum networks. Existing approaches rely on deterministic state transfer, or probabilistic protocols that require active control or measurements and postselection. Here, we demonstrate a fundamentally different, fully autonomous process, where two remote qubits are entangled through their coupling to a quantum-correlated photonic reservoir. In our experiment, a Josephson parametric converter produces a Gaussian, continuous-variable entangled state of propagating microwave fields that drives two spatially separated superconducting transmon qubits into a stationary, discrete-variable entangled state. We also show how qubit tomography unlocks a direct and sensitive verification of two-mode squeezing in the microwave domain. These results establish networks of qubits interfaced with distributed continuous-variable entangled states as a powerful platform for foundational studies and quantum-technology applications.},
  author       = {Andres Juanes, Alejandro and Agustí, J. and Sett, Riya and Redchenko, Elena and Kapoor, Lucky and Hawaldar, Samarth and Rabl, P. and Fink, Johannes M},
  issn         = {2160-3308},
  journal      = {Physical Review X},
  number       = {3},
  publisher    = {American Physical Society},
  title        = {{Distributing stationary qubit entanglement through a nonlocal squeezed reservoir}},
  doi          = {10.1103/r4jt-j39w},
  volume       = {16},
  year         = {2026},
}

@article{20976,
  abstract     = {We present an experimental demonstration of an impedance-engineered Josephson parametric amplifier (IEJPA) fabricated in a single-step lithography process. Impedance-engineering is implemented using a lumped-element series LC circuit. We use a simpler lithography process where the entire device—impedance transformer and Josephson parametric amplifier (JPA)—is patterned in a single electron beam lithography step, followed by a double-angle Dolan-bridge technique for Al–AlOx–Al deposition. We observe amplification with 18 dB gain over a wide 400 MHz bandwidth centered around 5.3 GHz with added noise approaching the quantum limit, and a saturation power of −114 dBm. To accurately explain our experimental results, we extend existing theories for IEJPAs to incorporate the full sine nonlinearity of both the JPA and the transformer. Our work provides a route to simpler realization of broadband JPAs and a theoretical foundation for a regime of JPA operation that has been less explored in literature.},
  author       = {Patel, Lipi and Hawaldar, Samarth and Panikkar, Aditya and Shankar, Athreya and Suri, Baladitya},
  issn         = {1077-3118},
  journal      = {Applied Physics Letters},
  number       = {25},
  publisher    = {AIP Publishing},
  title        = {{Impedance-engineered Josephson parametric amplifier with single-step lithography}},
  doi          = {10.1063/5.0290636},
  volume       = {127},
  year         = {2025},
}

@article{19617,
  abstract     = {In this article, we propose a method for generating single microwave photons in superconducting circuits. We theoretically show that pure single microwave photons can be generated on demand and tuned over a large frequency band by making use of Landau-Zener transitions under a rapid sweep of a control parameter. We devise a protocol that enables fast control of the frequency of the emitted photon over two octaves, without requiring extensive calibration. Additionally, we make theoretical estimates of the generation efficiency, tunability, purity, and linewidth of the photons emitted using this method for both charge- and flux-qubit-based architectures. We also provide estimates of the optimal device parameters required for these architectures to realize the device.},
  author       = {Hawaldar, Samarth and Khaire, Siddhi Satish and Delsing, Per and Suri, Baladitya},
  issn         = {2331-7019},
  journal      = {Physical Review Applied},
  number       = {4},
  publisher    = {American Physical Society},
  title        = {{On-demand single-microwave-photon source in a superconducting circuit with wideband frequency tunability}},
  doi          = {10.1103/physrevapplied.23.044042},
  volume       = {23},
  year         = {2025},
}

@article{17477,
  abstract     = {Trapped-ion systems are a leading platform for quantum information processing, but they are currently limited to 1D and 2D arrays, which imposes restrictions on both their scalability and their range of applications. Here, we propose a path to overcome this limitation by demonstrating that Penning traps can be used to realize remarkably clean bilayer crystals, wherein hundreds of ions self-organize into two well-defined layers. These bilayer crystals are made possible by the inclusion of an anharmonic trapping potential, which is readily implementable with current technology. We study the normal modes of this system and discover salient differences compared to the modes of single-plane crystals. The bilayer geometry and the unique properties of the normal modes open new opportunities—in particular, in quantum sensing and quantum simulation—that are not straightforward in single-plane crystals. Furthermore, we illustrate that it may be possible to extend the ideas presented here to realize multilayer crystals with more than two layers. Our work increases the dimensionality of trapped-ion systems by efficiently utilizing all three spatial dimensions, and it lays the foundation for a new generation of quantum information processing experiments with multilayer 3D crystals of trapped ions.},
  author       = {Hawaldar, Samarth and Shahi, Prakriti and Carter, Allison L. and Rey, Ana Maria and Bollinger, John J. and Shankar, Athreya},
  issn         = {2160-3308},
  journal      = {Physical Review X},
  number       = {3},
  publisher    = {American Physical Society},
  title        = {{Bilayer crystals of trapped ions for quantum information processing}},
  doi          = {10.1103/PhysRevX.14.031030},
  volume       = {14},
  year         = {2024},
}