@phdthesis{21863,
  abstract     = {Atoms and photons, two things so different but yet so alike. The former, the building block of matter, something we learn about in school and imagine it as some tiny marbles encircled by other tinier marbles. The latter, an electromagnetic wave, a light particle or an excitation of the electromagnetic field. Quantum mechanics tells us about the properties of these two entities. And even if it sounds, looks and writes counter-intuitive, it has proven right for over a century now.

In this work, I elaborate on how we tested the laws of quantum mechanics and how we used them learn more about the tiny building blocks of nature and the fields they use to talk to each other. The atoms we use, are artificial. Superconducting qubits, small electrical circuits with quantized energy levels behave like electrons that transition between different orbitals in an atom. One of the qubits' advantages, is also a big disadvantage. We design the circuits' energy levels and fabricate them in a cleanroom. This allows for arbitrary spaced energy levels but in contrast to real atoms, prevents two superconducting qubits from being alike. Still, this qubit platform is one of the frontrunners for future quantum computing technology and testing fundamental physics due to their scalability.

We interface superconducting qubits, which operate in the GHz regime, with microwave photons. We use 3D aluminum cavities as mediators between qubits and photons. The cavities allow for non-destructive readout of the qubit state, they shield the qubits from noise at the qubit frequency and they give us an easy way to frequency-tune these joint systems.

We need to operate superconducting qubits and their cavities at millikelvin temperatures in dilution refrigerators. At higher temperatures, superconductivity suffers and even worse, the environment is filled with thermal noise photons. This poses a fundamental limitation on the scalability of superconducting qubit devices. Also connecting multiple devices in different fridges does not work over room temperature links because the microwave photons used for this purpose will be covered in noise and the quantum information they carry, will be unusable.

Infrared photons do not suffer from this noise problem since there are close to zero thermal noise photons at their frequencies at room temperature. We cannot simply interface superconducting devices with optical photons due their frequency mismatch and the destructive effect of optical photons on superconductors. Therefore, we use microwave-to-optics transducers that allow to convert microwave photons into optical ones and vice-versa. The transducers that we use are macroscopic electro-optic transducers using the Pockels effect in a disk-shaped Lithium Niobate whispering gallery mode resonator. By using a strong optical pump, photons from the two frequency domains experience a beam-splitter interaction and get converted from one to the other.

We measure the generated optical photons using elaborate optical setups, optical heterodyning and single photon detectors to gain knowledge about the qubit state or the converted microwave photons. Bridging the microwave and the optical world allows us to take advantage of both of their strengths but it also requires deep knowledge about both of their working principles.

In this work, we describe two experiments that our group conducted to showcase the opportunities that arise from interfacing superconducting qubits with optical photons but also the pitfalls, one may encounter on the way.

In the first experiment, we managed to all-optically read out a superconducting qubit. We show that the assignment fidelity, the probability that a measurement of the qubit state matches the prepared state, is close to equal for all-optical, microwave-to-optics and conventional microwave readout. We show T1 and T2 measurements for all three readout types and give an analysis of the noise caused by the optics. Finally, we show that the infrared light does not affect the qubit performance in a negative way but that the heating it causes does. This is an important insight that we used in the next experiment.

The second experiment is the upconversion of itinerant single microwave photons to the optical domain. We show that we can generate single microwave photons from a qubit-cavity system. We upconvert these single photons, measure them with a single photon detector and reconstruct their shape. By conducting a single photon Rabi measurement, we show correlations between the microwave and the optical domain. And by thorough signal-to-noise measurements and noise analysis, we find that we can generate single infrared photons with high signal-to-noise ratio 5.1 and low transducer added noise (<0.012 quanta). We show that this measurement creates a path towards entanglement of a superconducting qubit and an optical photon and what parameters need to be improved to achieve it. Additionally, this experiment is a proof of principle for an on-demand infrared single photon source. More generally, it allows to link microwave quantum technology in general to the optical domain.},
  author       = {Werner, Thomas},
  issn         = {2663-337X},
  keywords     = {Superconducting qubits, Quantum optics, Single photons and quantum effects, Nonlinear optics},
  pages        = {97},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Interfacing superconducting qubits with optical photons}},
  doi          = {10.15479/AT-ISTA-21863},
  year         = {2026},
}

@phdthesis{20798,
  abstract     = {Atom interferometers measure the relative phase shifts between coherent matter-wave paths
that arise from interactions with external fields or inertial forces. Due to their exceptional
phase sensitivity, atom interferometers became an essential tool for precision measurements
and fundamental physics experiments, finding applications in geodesy, gravimetry, and inertial
navigation. However, their measurement precision is limited by quantum projection noise,
which arises from the Heisenberg uncertainty principle, preventing the measurement of atomic
states with absolute precision. The generation of entanglement between the atoms offers a
path to surpass this so-called standard quantum limit, thereby enhancing the interferometer’s
phase sensitivity beyond classical measurement bounds.
This thesis reports on the development of an atom interferometer experiment designed to
realize cavity-mediated, squeezed Mach-Zehnder-type interferometry with ultra-cold 87Rb atoms.
The experiment combines cavity-aided spin-squeezing with cavity-mediated Mach-Zehnder
interferometry to demonstrate entanglement-enhanced phase sensitivity. The experiment is
centered on a triangular optical cavity that mediates all relevant atom-light interactions. The
cavity provides optical trapping, spin-squeezing, and Raman beam-splitter operations, enabling
to perform interferometry on a continuously trapped atomic ensemble.
The thesis elaborates on the fundamental theoretical framework, the cavity design, and the full
optical setup, including the detailed configuration of the developed laser stabilization methods.
Experimentally, continuous loading methods were explored, resulting in an accumulation of
up to 4 × 106
atoms in the dipole trap within a cycle time of 500 ms. The AC Stark shift
compensation method developed for continuous loading was further applied for in-trap cooling
to 10 µK, and optical pumping for efficient atomic state preparation. Coherent state control
was verified via observation of microwave-driven Rabi oscillations, and used to characterize
atom-cavity coupling.
These presented results establish the experimental groundwork for the future development of
cavity-mediated, entanglement-enhanced Mach-Zehnder-type atom interferometry.},
  author       = {Wald, Sebastian},
  isbn         = {978-3-99078-075-6},
  issn         = {2663-337X},
  keywords     = {entanglement-enhanced atom interferometry, cavity QED, spin-squeezing, dipole trap, quantum optics},
  pages        = {152},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Atoms in a propagating-wave cavity for squeezed Mach-Zehnder atom interferometry}},
  doi          = {10.15479/AT-ISTA-20798},
  year         = {2025},
}

@phdthesis{19533,
  abstract     = {This thesis explores advancements in quantum remote sensing and non-equilibrium phase
transitions in the microwave regime, with a focus on dissipative phase transitions and quantumenhanced sensing.
In the first project, I experimentally studied photon blockade breakdown as a dissipative phase
transition in a zero-dimensional cavity-qubit system. By defining an appropriate thermodynamic
limit, we demonstrated that the observed bistability is a genuine signature of a first-order
phase transition in this system. This work provides insight into non-equilibrium quantum
dynamics and phase transitions in driven-dissipative open quantum systems.
The second project focuses on the experimental realization of a phase-conjugate receiver for
quantum illumination (QI), a quantum sensing protocol that enhances target detection in noisy
environments using entangled light. While an ideal spontaneous parametric down-conversion
(SPDC) source and receiver could, in theory, provide up to a 6 dB advantage over classical
illumination, no such ideal receiver exists. Instead, we explore an experimental realization of a
phase-conjugate receiver for QI in the microwave regime at millikelvin temperatures using a
Josephson parametric converter (JPC) as a source of continuous-variable Gaussian entangled
signal-idler pairs, where a maximum 3 dB advantage is theoretically achievable. We investigate
key experimental limitations that constrain practical QI performance, contributing to the
development of quantum-enhanced sensing.
Additionally, this thesis presents efficient digital signal processing (DSP) techniques implemented in C++ and Python in collaboration with Przemysław Zieliński and Luka Drmić. These
methods, optimized using the Intel Integrated Performance Primitives (IPP) library, have been
essential in data acquisition, noise filtering, and correlation analysis across multiple research
projects. Although not real-time, these DSP techniques significantly enhance the accuracy of
quantum measurements.
Overall, this thesis advances quantum-enhanced sensing by establishing the thermodynamic
limit in a single transmon-cavity system and experimentally exploring a phase-conjugate receiver
for QI. These findings contribute to quantum metrology, particularly for weak signal detection
and remote sensing in noisy environments.
},
  author       = {Sett, Riya},
  issn         = {2663-337X},
  keywords     = {phase transition, open quantum system, phase diagram, cavity quantum electrodynamics, superconducting qubits, semiclassical physics, quantum optics, josephson junction, parametric converter, phase conjugation, quantum radar, quantum entanglement, correlation, quantum sensing},
  pages        = {109},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Quantum remote sensing and non-equilibrium phase transitions in the microwave regime}},
  doi          = {10.15479/AT-ISTA-19533},
  year         = {2025},
}

@article{21529,
  abstract     = {A central challenge in the emerging field of free-electron quantum optics is to achieve strong quantum interaction and single-photon nonlinearity between a flying free electron and a photonic mode. Existing schemes are intrinsically limited by electron diffraction, which puts an upper bound on the interaction length and, therefore, on the strength of quantum coupling and nonlinearity. Here, we propose “free-electron fibers”: effectively one-dimensional photonic systems where free electrons copropagate with two guided modes. The first mode applies a ponderomotive trap to the free electron, removing the limitations due to electron diffraction. The second mode strongly couples to the guided free electron with an enhanced coupling that is orders of magnitude larger than previous designs. The extended interaction lengths enabled by our scheme allow for strong single-photon nonlinearities mediated by free electrons. We predict novel quantum effects in our system such as deterministic single-photon emission and nonlinear multimode dynamics. Our proposal paves the way toward the realization of heralded macroscopic nonclassical light generation, deterministic single-photon sources, and quantum gates controlled by free-electron–photon interactions.},
  author       = {Karnieli, Aviv and Roques-Carmes, Charles and Rivera, Nicholas and Fan, Shanhui},
  issn         = {2330-4022},
  journal      = {ACS Photonics},
  keywords     = {quantum optics, free electrons, single photon nonlinearity, electron-photon interaction},
  number       = {8},
  pages        = {3401--3411},
  publisher    = {American Chemical Society},
  title        = {{Strong coupling and single-photon nonlinearity in free-electron quantum optics}},
  doi          = {10.1021/acsphotonics.4c00908},
  volume       = {11},
  year         = {2024},
}

@phdthesis{12900,
  abstract     = {About a 100 years ago, we discovered that our universe is inherently noisy, that is, measuring any physical quantity with a precision beyond a certain point is not possible because of an omnipresent inherent noise. We call this - the quantum noise. Certain physical processes allow this quantum noise to get correlated in conjugate physical variables. These quantum correlations can be used to go beyond the potential of our inherently noisy universe and obtain a quantum advantage over the classical applications. 

Quantum noise being inherent also means that, at the fundamental level, the physical quantities are not well defined and therefore, objects can stay in multiple states at the same time. For example, the position of a particle not being well defined means that the particle is in multiple positions at the same time. About 4 decades ago, we started exploring the possibility of using objects which can be in multiple states at the same time to increase the dimensionality in computation. Thus, the field of quantum computing was born. We discovered that using quantum entanglement, a property closely related to quantum correlations, can be used to speed up computation of certain problems, such as factorisation of large numbers, faster than any known classical algorithm. Thus began the pursuit to make quantum computers a reality. 

Till date, we have explored quantum control over many physical systems including photons, spins, atoms, ions and even simple circuits made up of superconducting material. However, there persists one ubiquitous theme. The more readily a system interacts with an external field or matter, the more easily we can control it. But this also means that such a system can easily interact with a noisy environment and quickly lose its coherence. Consequently, such systems like electron spins need to be protected from the environment to ensure the longevity of their coherence. Other systems like nuclear spins are naturally protected as they do not interact easily with the environment. But, due to the same reason, it is harder to interact with such systems. 

After decades of experimentation with various systems, we are convinced that no one type of quantum system would be the best for all the quantum applications. We would need hybrid systems which are all interconnected - much like the current internet where all sorts of devices can all talk to each other - but now for quantum devices. A quantum internet. 

Optical photons are the best contenders to carry information for the quantum internet. They can carry quantum information cheaply and without much loss - the same reasons which has made them the backbone of our current internet. Following this direction, many systems, like trapped ions, have already demonstrated successful quantum links over a large distances using optical photons. However, some of the most promising contenders for quantum computing which are based on microwave frequencies have been left behind. This is because high energy optical photons can adversely affect fragile low-energy microwave systems. 

In this thesis, we present substantial progress on this missing quantum link between microwave and optics using electrooptical nonlinearities in lithium niobate. The nonlinearities are enhanced by using resonant cavities for all the involved modes leading to observation of strong direct coupling between optical and microwave frequencies. With this strong coupling we are not only able to achieve almost 100\% internal conversion efficiency with low added noise, thus presenting a quantum-enabled transducer, but also we are able to observe novel effects such as cooling of a microwave mode using optics. The strong coupling regime also leads to direct observation of dynamical backaction effect between microwave and optical frequencies which are studied in detail here. Finally, we also report first observation of microwave-optics entanglement in form of two-mode squeezed vacuum squeezed 0.7dB below vacuum level. 
With this new bridge between microwave and optics, the microwave-based quantum technologies can finally be a part of a quantum network which is based on optical photons - putting us one step closer to a future with quantum internet. },
  author       = {Sahu, Rishabh},
  isbn         = {978-3-99078-030-5},
  issn         = {2663-337X},
  keywords     = {quantum optics, electrooptics, quantum networks, quantum communication, transduction},
  pages        = {190},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Cavity quantum electrooptics}},
  doi          = {10.15479/at:ista:12900},
  year         = {2023},
}

@phdthesis{13175,
  abstract     = {About a 100 years ago, we discovered that our universe is inherently noisy, that is, measuring any physical quantity with a precision beyond a certain point is not possible because of an omnipresent inherent noise. We call this - the quantum noise. Certain physical processes allow this quantum noise to get correlated in conjugate physical variables. These quantum correlations can be used to go beyond the potential of our inherently noisy universe and obtain a quantum advantage over the classical applications. 

Quantum noise being inherent also means that, at the fundamental level, the physical quantities are not well defined and therefore, objects can stay in multiple states at the same time. For example, the position of a particle not being well defined means that the particle is in multiple positions at the same time. About 4 decades ago, we started exploring the possibility of using objects which can be in multiple states at the same time to increase the dimensionality in computation. Thus, the field of quantum computing was born. We discovered that using quantum entanglement, a property closely related to quantum correlations, can be used to speed up computation of certain problems, such as factorisation of large numbers, faster than any known classical algorithm. Thus began the pursuit to make quantum computers a reality. 

Till date, we have explored quantum control over many physical systems including photons, spins, atoms, ions and even simple circuits made up of superconducting material. However, there persists one ubiquitous theme. The more readily a system interacts with an external field or matter, the more easily we can control it. But this also means that such a system can easily interact with a noisy environment and quickly lose its coherence. Consequently, such systems like electron spins need to be protected from the environment to ensure the longevity of their coherence. Other systems like nuclear spins are naturally protected as they do not interact easily with the environment. But, due to the same reason, it is harder to interact with such systems. 

After decades of experimentation with various systems, we are convinced that no one type of quantum system would be the best for all the quantum applications. We would need hybrid systems which are all interconnected - much like the current internet where all sorts of devices can all talk to each other - but now for quantum devices. A quantum internet. 

Optical photons are the best contenders to carry information for the quantum internet. They can carry quantum information cheaply and without much loss - the same reasons which has made them the backbone of our current internet. Following this direction, many systems, like trapped ions, have already demonstrated successful quantum links over a large distances using optical photons. However, some of the most promising contenders for quantum computing which are based on microwave frequencies have been left behind. This is because high energy optical photons can adversely affect fragile low-energy microwave systems. 

In this thesis, we present substantial progress on this missing quantum link between microwave and optics using electrooptical nonlinearities in lithium niobate. The nonlinearities are enhanced by using resonant cavities for all the involved modes leading to observation of strong direct coupling between optical and microwave frequencies. With this strong coupling we are not only able to achieve almost 100\% internal conversion efficiency with low added noise, thus presenting a quantum-enabled transducer, but also we are able to observe novel effects such as cooling of a microwave mode using optics. The strong coupling regime also leads to direct observation of dynamical backaction effect between microwave and optical frequencies which are studied in detail here. Finally, we also report first observation of microwave-optics entanglement in form of two-mode squeezed vacuum squeezed 0.7dB below vacuum level. 
With this new bridge between microwave and optics, the microwave-based quantum technologies can finally be a part of a quantum network which is based on optical photons - putting us one step closer to a future with quantum internet. },
  author       = {Sahu, Rishabh},
  isbn         = {978-3-99078-030-5},
  issn         = {2663-337X},
  keywords     = {quantum optics, electrooptics, quantum networks, quantum communication, transduction},
  pages        = {202},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Cavity quantum electrooptics}},
  doi          = {10.15479/at:ista:13175},
  year         = {2023},
}