@phdthesis{13106,
  abstract     = {Quantum entanglement is a key resource in currently developed quantum technologies. Sharing this fragile property between superconducting microwave circuits and optical or atomic systems would enable new functionalities, but this has been hindered by an energy scale mismatch of >104 and the resulting mutually imposed loss and noise. In this work, we created and verified entanglement between microwave and optical fields in a millikelvin environment. Using an optically pulsed superconducting electro-optical device, we show entanglement between propagating microwave and optical fields in the continuous variable domain. This achievement not only paves the way for entanglement between superconducting circuits and telecom wavelength light, but also has wide-ranging implications for hybrid quantum networks in the context of modularization, scaling, sensing, and cross-platform verification.},
  author       = {Sahu, Rishabh and Qiu, Liu and Hease, William J and Arnold, Georg M and Minoguchi, Y. and Rabl, P. and Fink, Johannes M},
  issn         = {1095-9203},
  keywords     = {Multidisciplinary},
  pages        = {718--721},
  publisher    = {American Association for the Advancement of Science},
  title        = {{Entangling microwaves with light}},
  doi          = {10.1126/science.adg3812},
  volume       = {380},
  year         = {2023},
}

@article{10924,
  abstract     = {Solid-state microwave systems offer strong interactions for fast quantum logic and sensing but photons at telecom wavelength are the ideal choice for high-density low-loss quantum interconnects. A general-purpose interface that can make use of single photon effects requires < 1 input noise quanta, which has remained elusive due to either low efficiency or pump induced heating. Here we demonstrate coherent electro-optic modulation on nanosecond-timescales with only 0.16+0.02−0.01 microwave input noise photons with a total bidirectional transduction efficiency of 8.7% (or up to 15% with 0.41+0.02−0.02), as required for near-term heralded quantum network protocols. The use of short and high-power optical pump pulses also enables near-unity cooperativity of the electro-optic interaction leading to an internal pure conversion efficiency of up to 99.5%. Together with the low mode occupancy this provides evidence for electro-optic laser cooling and vacuum amplification as predicted a decade ago.},
  author       = {Sahu, Rishabh and Hease, William J and Rueda Sanchez, Alfredo R and Arnold, Georg M and Qiu, Liu and Fink, Johannes M},
  issn         = {20411723},
  journal      = {Nature Communications},
  publisher    = {Springer Nature},
  title        = {{Quantum-enabled operation of a microwave-optical interface}},
  doi          = {10.1038/s41467-022-28924-2},
  volume       = {13},
  year         = {2022},
}

@phdthesis{12366,
  abstract     = {Recent substantial advances in the feld of superconducting circuits have shown its
potential as a leading platform for future quantum computing. In contrast to classical
computers based on bits that are represented by a single binary value, 0 or 1, quantum
bits (or qubits) can be in a superposition of both. Thus, quantum computers can store
and handle more information at the same time and a quantum advantage has already
been demonstrated for two types of computational tasks. Rapid progress in academic
and industry labs accelerates the development of superconducting processors which may
soon fnd applications in complex computations, chemical simulations, cryptography, and
optimization. Now that these machines are scaled up to tackle such problems the questions
of qubit interconnects and networks becomes very relevant. How to route signals on-chip
between diferent processor components? What is the most efcient way to entangle
qubits? And how to then send and process entangled signals between distant cryostats
hosting superconducting processors?
In this thesis, we are looking for solutions to these problems by studying the collective
behavior of superconducting qubit ensembles. We frst demonstrate on-demand tunable
directional scattering of microwave photons from a pair of qubits in a waveguide. Such a
device can route microwave photons on-chip with a high diode efciency. Then we focus
on studying ultra-strong coupling regimes between light (microwave photons) and matter
(superconducting qubits), a regime that could be promising for extremely fast multi-qubit
entanglement generation. Finally, we show coherent pulse storage and periodic revivals
in a fve qubit ensemble strongly coupled to a resonator. Such a reconfgurable storage
device could be used as part of a quantum repeater that is needed for longer-distance
quantum communication.
The achieved high degree of control over multi-qubit ensembles highlights not only the
beautiful physics of circuit quantum electrodynamics, it also represents the frst step
toward new quantum simulation and communication methods, and certain techniques
may also fnd applications in future superconducting quantum computing hardware.
},
  author       = {Redchenko, Elena},
  isbn         = {978-3-99078-024-4},
  issn         = {2663-337X},
  pages        = {168},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Controllable states of superconducting Qubit ensembles}},
  doi          = {10.15479/at:ista:12132},
  year         = {2022},
}

@article{7910,
  abstract     = {Quantum illumination uses entangled signal-idler photon pairs to boost the detection efficiency of low-reflectivity objects in environments with bright thermal noise. Its advantage is particularly evident at low signal powers, a promising feature for applications such as noninvasive biomedical scanning or low-power short-range radar. Here, we experimentally investigate the concept of quantum illumination at microwave frequencies. We generate entangled fields to illuminate a room-temperature object at a distance of 1 m in a free-space detection setup. We implement a digital phase-conjugate receiver based on linear quadrature measurements that outperforms a symmetric classical noise radar in the same conditions, despite the entanglement-breaking signal path. Starting from experimental data, we also simulate the case of perfect idler photon number detection, which results in a quantum advantage compared with the relative classical benchmark. Our results highlight the opportunities and challenges in the way toward a first room-temperature application of microwave quantum circuits.},
  author       = {Barzanjeh, Shabir and Pirandola, S. and Vitali, D and Fink, Johannes M},
  issn         = {23752548},
  journal      = {Science Advances},
  number       = {19},
  publisher    = {AAAS},
  title        = {{Microwave quantum illumination using a digital receiver}},
  doi          = {10.1126/sciadv.abb0451},
  volume       = {6},
  year         = {2020},
}

@article{8529,
  abstract     = {Practical quantum networks require low-loss and noise-resilient optical interconnects as well as non-Gaussian resources for entanglement distillation and distributed quantum computation. The latter could be provided by superconducting circuits but existing solutions to interface the microwave and optical domains lack either scalability or efficiency, and in most cases the conversion noise is not known. In this work we utilize the unique opportunities of silicon photonics, cavity optomechanics and superconducting circuits to demonstrate a fully integrated, coherent transducer interfacing the microwave X and the telecom S bands with a total (internal) bidirectional transduction efficiency of 1.2% (135%) at millikelvin temperatures. The coupling relies solely on the radiation pressure interaction mediated by the femtometer-scale motion of two silicon nanobeams reaching a <jats:italic>V</jats:italic><jats:sub><jats:italic>π</jats:italic></jats:sub> as low as 16 μV for sub-nanowatt pump powers. Without the associated optomechanical gain, we achieve a total (internal) pure conversion efficiency of up to 0.019% (1.6%), relevant for future noise-free operation on this qubit-compatible platform.},
  author       = {Arnold, Georg M and Wulf, Matthias and Barzanjeh, Shabir and Redchenko, Elena and Rueda Sanchez, Alfredo R and Hease, William J and Hassani, Farid and Fink, Johannes M},
  issn         = {2041-1723},
  journal      = {Nature Communications},
  keywords     = {General Biochemistry, Genetics and Molecular Biology, General Physics and Astronomy, General Chemistry},
  publisher    = {Springer Nature},
  title        = {{Converting microwave and telecom photons with a silicon photonic nanomechanical interface}},
  doi          = {10.1038/s41467-020-18269-z},
  volume       = {11},
  year         = {2020},
}

@article{8755,
  abstract     = {The superconducting circuit community has recently discovered the promising potential of superinductors. These circuit elements have a characteristic impedance exceeding the resistance quantum RQ ≈ 6.45 kΩ which leads to a suppression of ground state charge fluctuations. Applications include the realization of hardware protected qubits for fault tolerant quantum computing, improved coupling to small dipole moment objects and defining a new quantum metrology standard for the ampere. In this work we refute the widespread notion that superinductors can only be implemented based on kinetic inductance, i.e. using disordered superconductors or Josephson junction arrays. We present modeling, fabrication and characterization of 104 planar aluminum coil resonators with a characteristic impedance up to 30.9 kΩ at 5.6 GHz and a capacitance down to ≤ 1 fF, with lowloss and a power handling reaching 108 intra-cavity photons. Geometric superinductors are free of uncontrolled tunneling events and offer high reproducibility, linearity and the ability to couple magnetically - properties that significantly broaden the scope of future quantum circuits. },
  author       = {Peruzzo, Matilda and Trioni, Andrea and Hassani, Farid and Zemlicka, Martin and Fink, Johannes M},
  issn         = {23317019},
  journal      = {Physical Review Applied},
  number       = {4},
  publisher    = {American Physical Society},
  title        = {{Surpassing the resistance quantum with a geometric superinductor}},
  doi          = {10.1103/PhysRevApplied.14.044055},
  volume       = {14},
  year         = {2020},
}

@inproceedings{9001,
  abstract     = {Quantum illumination is a sensing technique that employs entangled signal-idler beams to improve the detection efficiency of low-reflectivity objects in environments with large thermal noise. The advantage over classical strategies is evident at low signal brightness, a feature which could make the protocol an ideal prototype for non-invasive scanning or low-power short-range radar. Here we experimentally investigate the concept of quantum illumination at microwave frequencies, by generating entangled fields using a Josephson parametric converter which are then amplified to illuminate a room-temperature object at a distance of 1 meter. Starting from experimental data, we simulate the case of perfect idler photon number detection, which results in a quantum advantage compared to the relative classical benchmark. Our results highlight the opportunities and challenges on the way towards a first room-temperature application of microwave quantum circuits.},
  author       = {Barzanjeh, Shabir and Pirandola, Stefano and Vitali, David and Fink, Johannes M},
  booktitle    = {IEEE National Radar Conference - Proceedings},
  isbn         = {9781728189420},
  issn         = {1097-5659},
  location     = {Florence, Italy},
  number       = {9},
  publisher    = {IEEE},
  title        = {{Microwave quantum illumination with a digital phase-conjugated receiver}},
  doi          = {10.1109/RadarConf2043947.2020.9266397},
  volume       = {2020},
  year         = {2020},
}