Light-matter interfaces are pivotal for quantum computation and communication. While typically analyzed using single-mode or open-quantum-system approximations, these models often neglect multi-mode field states and light-matter entanglement, hindering exact protocol modeling. Here, we solve the full Hamiltonian dynamics of a solid-state spin-photon interface for three key protocols: the generation of photon-number superpositions, a controlled photon-photon gate, and the production of photonic cluster states. By deriving exact fidelities, we identify fundamental performance limits. Our results reveal that while realistic imperfections severely limit photon-photon gates, they only slightly affect linear photonic clusters and are nearly harmless for photon-number state superpositions.
Light-matter interfaces are pivotal for quantum computation and communication. While typically analyzed using single-mode or open-quantum-system approximations, these models often neglect multi-mode field states and light-matter entanglement, hindering exact protocol modeling. Here, we solve the full Hamiltonian dynamics of a solid-state spin-photon interface for three key protocols: the generation of photon-number superpositions, a controlled photon-photon gate, and the production of photonic cluster states. By deriving exact fidelities, we identify fundamental performance limits. Our results reveal that while realistic imperfections severely limit photon-photon gates, they only slightly affect linear photonic clusters and are nearly harmless for photon-number state superpositions.
The desire to understand the interaction between light and matter has stimulated centuries of research, leading to technological achievements that have shaped our world. One contemporary frontier of research into light-matter interaction considers regimes where quantum effects dominate. By understanding and manipulating these quantum effects, a vast array of new quantum-enhanced technologies become accessible. In this thesis, I explore and analyze fundamental components and processes for quantum optical devices with a focus on solid-state quantum systems. This includes indistinguishable single-photon sources, deterministic sources of entangled photonic states, photon-heralded entanglement generation between remote quantum systems, and deterministic optically-mediated entangling gates between local quantum systems. For this analysis, I make heavy use of an analytic quantum trajectories approach applied to a general Markovian master equation of an optically-active quantum system, which I introduce as a photon-number decomposition. This approach allows for many realistic system imperfections, such as emitter pure dephasing, spin decoherence, and measurement imperfections, to be taken into account in a straightforward and comprehensive way.
Quantum interfaces are ubiquitous in quantum technologies as they allow the implementation of several functionalities including quantum memories, quantum repeaters, photonic gates, and highly entangled photonic state generation. Based on the coherent coupling between quantum emitters and propagating pulses of light, they can be realized on a large variety of experimental platforms where quantum non-demolition measurements, entanglement generation, and efficient production of non-classical resources have been reported.This thesis explores the potential of so-called collision models to model exactly the light-matter dynamics in various kinds of quantum interfaces, providing direct access to their joint entangled states. It allows us to derive analytic expressions of the device performance, whether operated as a measuring apparatus or a source of cluster states. Since our model captures light and matter as a closed system, it verifies global energy conservation, providing access to the energy budget associated with the execution of the quantum task.Initially, we review the theoretical foundation for quantum measurements and introduce figures of merit used throughout the thesis. Subsequently, we review the collision model method, which is based on waveguide quantum electrodynamics (WGQED), setting the stage for subsequent analyses. Then, we address fundamental inquiries. First, we investigate the emergence of non-classical behavior of the field scattered by a quantum emitter that interacts with an intense resonant coherent field. It reveals signatures of quantum contextuality—negativity in the Wigner function and anomalous weak values. Second, we investigate the possibility of non-destructive spin state measurement by limiting the energy budget to at most one photon. We compare the performance of two different fields: coherent field and number state superposition. We demonstrate better performance of the latter in entanglement generation, thus providing a quantum advantage. Lastly, our analysis extends to technological applications. We propose a photon-photon gate, conduct a thorough error analysis, and we perform the modeling of cutting-edge experiments envisioning the deterministic generation of highly entangled states of light.This theoretical capacity opens the way to directly optimizing the device performance and efficiency as a function of the parameters accessible to experimentalists. It also allows us to fit experimental data with high accuracy.
Abstract By precisely timed optical excitation of their spin, optical emitters such as semiconductor quantum dots or atoms can be harnessed as sources of linear photonic cluster states. This significantly reduces the required resource overhead to reach fault-tolerant optical quantum computing. Here, we develop an algorithm that analytically tracks the global density matrix through the process of the protocol for generating linear-cluster states by Lindner and Rudolph. From this we derive a model to calculate the entangling gate fidelity and the state fidelity of the generated linear optical cluster states. Our model factors in various sources of error, such as spin decoherence and the finite excited state lifetime. Additionally, we highlight the presence of partial reinitialization of spin coherence with each photon emission, eliminating the hard limitation of coherence time. Our framework provides valuable insight into the cost-to-improvement trade-offs for device design parameters as well as the identification of optimal working points. For a combined state-of-the-art quantum dot with a spin coherence time of T2∗=535 T 2 ∗ = 535 ns and an excited state lifetime of τ=23 τ = 23 ps, we show that a near-unity entangling gate fidelity as well as near-unity state fidelity for 3-photon and 7-photon linear cluster states can be reached. Graphical Abstract
Self-assembled quantum dots can act as an interface between single spins and photons. They combine the flexibility and scalability of semiconductor systems with near-perfect two level systems, emitting highly coherent single photons. In potential quantum information processing architectures based upon optical networks, quantum dots permit light-matter coupling between matter qubits and photonic links. Single trapped charge carriers in quantum dots can be addressed optically, generating spin-photon entanglement between the matter qubits and \lq flying' photon qubits, a key first step in creation of a network. Such an optically linked network of spins in quantum dots has yet to be realized. The semiconductor environment presents a key challenge; it broadens the spectrum of the emitted light, which reduces the fidelity of schemes based upon photon exchange or interference. In addition, these quantum dots are subject to hyperfine coupling to a bath of > 105 nuclear spins. While this presents an opportunity to study the physics of a central spin in a spin bath, it degrades the spin coherence of a trapped electron or hole. The suggested gate operations to generate entanglement, and the spin measurements to verify a created state, rely upon photon detection. Thus a second limitation occurs due to low efficiency of detectors and photon collection probability. Recent developments in detector technology have enabled us to bring generation and verification of entangled states with quantum dots into the realm of possibility. This dissertation begins with a detailed study of the interaction of a single electron spin with the solid state environment of the quantum dot, analysing the intensity noise of scattered light under resonant excitation to uncover the timescales and relative amplitudes of identified noise sources. A technique for rapid characterization of samples is presented, focusing in particular upon comparison of spectral broadening of the optical transitions relevant to a spin-photon interface. Having identified the extent of environmental factors acting upon the quantum dots, we turn to a demonstration of photon-mediated entanglement between two distant electron spins using a probabilistic scheme. We are able to generate entangled states at 7 kHz, the fastest yet reported for photon-mediated protocols. The average state fidelity of the created Bell states is 63.5 ± 3.0 \%., violating the classical limit by over 4 standard deviations of the mean. In addition, we demonstrate full optical phase tuning of the Bell state ∣Ψ=21(∣↑∣↓+eiϕ∣↓∣↑). This is a proof of principle experiment, and sets a fidelity benchmark upon which gains can be made by mitigation of environmental dynamics.
Generating strong interactions between single quanta of light and matter is central to quantum information science, and a key component of quantum computers and long-distance quantum networks. In quantum information processing, these interactions are required to create elementary logic operations (quantum gates) between stationary matter quantum bits (qubits) and photonic qubits that can be transmitted over long distances. Efficient quantum gates between photonic and matter qubits are a crucial enabler for a broad range of applications that include robust quantum networks, nondestructive quantum measurements, and strong photon-photon interactions. So far these qubit-photon gates have been achieved using single atoms and at microwave frequencies in circuit QED systems. Their implementation with solidstate quantum emitters, however, has remained a difficult challenge. We demonstrate that the qubit state of a photon can be controlled by a single solid-state qubit composed of a quantum dot (QD) strongly coupled to an optical nanocavity. We show that the QD acts as a coherently controllable qubit system that conditionally flips the polarization of a photon reflected from the cavity mode on picosecond timescales. This operation implements a controlled NOT (cNOT) logic gate between the QD and the incident photon, which is a universal quantum operation that can serve as a general light-matter interface for remote entanglements and quantum computations. Our results represent an important step towards an all solid-state implementation of quantum networks and quantum computers, and provide a versatile approach for controlling and probing interactions between a photon and a single quantum emitter on ultra-fast timescales.
We propose a complete architecture for deterministic generation of entangled multiphoton states. Our approach utilizes periodic driving of a quantum-dot emitter and an efficient light-matter interface enabled by a photonic crystal waveguide. We assess the quality of the photonic states produced from a real system by including all intrinsic experimental imperfections. Importantly, the protocol is robust against the nuclear spin bath dynamics due to a naturally built-in refocusing method reminiscent to spin echo. We demonstrate the feasibility of producing Greenberger-Horne-Zeilinger and one-dimensional cluster states with fidelities and generation rates exceeding those achieved with conventional ``fusion'' methods in current state-of-the-art experiments. The proposed hardware constitutes a scalable and resource-efficient approach towards implementation of measurement-based quantum computing and quantum communication.