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Spontaneous emission is a fundamental quantum phenomenon whereby an electron transitions to a lower energy state while emitting a photon, manifesting across a plethora of fields from atomic physics and solid-state physics to astrophysics. Despite its ubiquity, there remain fundamental unanswered questions about spontaneous emission from systems with quantum correlations. Quantum correlations have become a critical resource in all platforms of quantum information science, such as coupled quantum dots and atomic arrays, enabling observations of previously elusive effects like super- and subradiance. Despite its significance, many aspects of spontaneous emission from correlated emitters remain unresolved. Here, we find the quantum-optical state of light spontaneously emitted from systems with arbitrary quantum correlations. We show under what conditions the correlations are not lost during the spontaneous emission but instead, transfer to the output light. The process of spontaneous emission can then create desired photonic states such as squeezed and Schrodinger-cat states. Our work captures the multi-mode nature of super- and subradiance and shows the roles of emitter locations, losses, and beyond-Markov dynamics on the emitted quantum state of light. We present manifestations of these effects in both cavity- and waveguide-QED. Our findings suggest new paths for creating and manipulating multi-photon quantum light for bosonic codes in continuous-variable-based quantum computation, communications, and sensing.

We show that the coherent interaction between free electrons and photons can be used for universal control of continuous-variable photonic quantum states in the form of GottesmanKitaev-Preskill (GKP) qubits. Specifically, we find that electron energy combs enable nondestructive measurements of the photonic state and can induce arbitrary gates. Moreover, a single electron interacting with multiple photonic modes can create highly entangled states such as Greenberger–Horne–Zeilinger states and cluster states of GKPs.

In strong-field physics and attosecond science, intense light induces ultrafast electron dynamics. Such ultrafast dynamics of electrons in matter is at the core of phenomena such as high harmonic generation (HHG), where these dynamics lead to emission of extreme UV bursts with attosecond duration. So far, all ultrafast dynamics of matter were understood to originate purely from the classical vector potential of the driving light, disregarding the influence of the quantum nature of light. Here we show that dynamics of matter driven by bright (intense) light significantly depend on the quantum state of the driving light, which induces an effective photon-statistics force. To provide a unified framework for the analysis & control over such a force, we extend the strong-field approximation (SFA) theory to account for non-classical driving light. Our quantum SFA (qSFA) theory shows that in HHG, experimentally feasible squeezing of the driving light can shift & shape electronic trajectories and attosecond pulses at the scale of hundreds of attoseconds. Our work presents a new degree-of-freedom for attosecond spectroscopy, by relying on nonclassical electromagnetic fields, and more generally, introduces a direct connection between attosecond science and quantum optics.

High harmonic generation (HHG) is an extreme nonlinear process where intense pulses of light drive matter to emit high harmonics of the driving frequency, reaching the extreme ultraviolet (XUV) and x-ray spectral ranges. So far, the HHG process was always generated by intense laser pulses that are well described as a classical electromagnetic field. Advances in the generation of intense squeezed light motivate us to revisit the fundamentals of HHG and ask how the photon statistics of light may alter this process, and more generally alter the field of extreme nonlinear optics. The role of photon statistics in non-perturbative interactions of intense light with matter has remained unexplored in both experiments and theory. Here we show that the defining spectral characteristics of HHG, such as the plateau and cutoff, are sensitive to the photon statistics of the driving light. While coherent (classical) and Fock light states induce the established HHG cutoff law, thermal and squeezed states substantially surpass it, extending the cutoff compared to classical light of the same intensity. Hence, shaping the photon statistics of light enables producing far higher harmonics in HHG. We develop the theory of extreme nonlinear optics driven by squeezed light, and more generally by arbitrary quantum states of light. Our work introduces quantum optical concepts to strong-field physics as new degrees of freedom in the creation and control of HHG, and finally shows that experiments in this field are feasible. Looking forward, HHG driven by quantum light creates quantum states of XUV and X-rays, enabling applications of quantum optics in new spectral regimes.

High harmonic generation (HHG) is a highly nonlinear emission process in which systems driven by intense laser pulses emit integer multiples (harmonics) of the driving field. This feature is considered universal to all occurrences of HHG. Here we show that a strong nonlinear response of certain systems can split the HHG spectrum into non-harmonic Mollow-type triplets, imprinting the internal electronic characteristics and confining potential on the HHG spectrum. The spectral lines become tunable by the driver field. These phenomena are universal to any material system with isolated bound states. We identify the conditions under which our predictions become accessible and propose experimental systems to observe them., 13 pages, 4 figures

Cat states and Gottesman-Kitaev-Preskill (GKP) states play a key role in quantum computation and communication with continuous variables. The creation of such states relies on strong nonlinear light-matter interactions, which are widely available in microwave frequencies as in circuit quantum electrodynamics platforms. However, strong nonlinearities are hard to come by in optical frequencies, severely limiting the use of continuous variable quantum information in the optical range. Here we propose using the strong interactions of free electrons with light as a source for optical cat and GKP states. The strong interactions can be realized by phase-matching of free electrons with photonic structures such as optical waveguides and photonic crystals. Our approach enables the generation of optical GKP states with above 10 dB squeezing and fidelities above 90% at post-selection probability of 10%, even reaching >30% using an initially squeezed vacuum state. Furthermore, the free-electron interaction allows for conditional rotations on the photonic state, enabling to entangle a pair of GKP states into a GKP Bell state. Since electrons can interact resonantly with light across the electromagnetic spectrum, our approach may be used for the generation of cat and GKP states over the entire electromagnetic spectrum, from radio-waves to X-rays., 26 pages, 4 figures, 1 table

There exist two families of superradiance phenomena: relying on correlated emission by an ensemble of atoms or by free electrons. Here we investigate emission from an ensemble of atoms driven by coherently shaped electrons. This interaction creates superradiance emerging from both the atoms' and the electrons' coherence - with emission intensity that scales quadratically in both the number of atoms and number of electrons. This phenomenon enables electrons to become novel probes of quantum correlations in matter, with high temporal and spatial resolution., Comment: 4 figures

Since the birth of quantum optics, the measurement of quantum states of nonclassical light has been of tremendous importance for advancement in the field. To date, conventional detectors such as photomultipliers, avalanche photodiodes, and superconducting nanowires, all rely at their core on linear excitation of bound electrons with light, posing fundamental restrictions on the detection. In contrast, the interaction of free electrons with light in the context of quantum optics is highly nonlinear and offers exciting possibilities. The first experiments that promoted this direction appeared over the past decade as part of photon-induced nearfield electron microscopy (PINEM), wherein free electrons are capable of high-order multi-photon absorption and emission. Here we propose using free electrons for quantum-optical detection of the complete quantum state of light. We show how the precise control of the electron before and after its interaction with quantum light enables to extract the photon statistics and implement full quantum state tomography using PINEM. This technique can reach sub-attosecond time resolutions, measure temporal coherence of any degree (e.g., g(1), g(2)), and simultaneously detect large numbers of photons with each electron. Importantly, the interaction of the electron with light is non-destructive, thereby leaving the photonic state (modified by the interaction) intact, which is conceptually different from conventional detectors. By using a pulse of multiple electrons, we envision how PINEM quantum detectors could achieve a single-shot measurement of the complete state of quantum light, even for non-reproducible emission events. Altogether, our work paves the way to novel kinds of photodetectors that utilize the ultrafast duration, high nonlinearity, and non-destructive nature of electron-light interactions., 5 figs, 20 pages + Supplementary Material