Single-photon sources that emit photons at the same energy play a key role in the emerging concepts of quantum information, such as entanglement swapping, quantum teleportation and quantum networks. They can be realized in a variety of systems, where semiconductor quantum dots, or 'artificial atoms', are arguably among the most attractive. However, unlike 'natural atoms', no two artificial atoms are alike. This peculiarity is a serious hurdle for quantum information applications that require photonic quantum states with identical energies. Here we demonstrate a single artificial atom that generates photons with an absolute energy that is locked to an optical transition in a natural atom. Furthermore, we show that our system is robust and immune to drifts and fluctuations in the environment of the emitter. Our demonstration is crucial for realization of a large number of universally-indistinguishable solid-state systems at arbitrary remote locations, where frequency-locked artificial atoms might become fundamental ingredients.
Single-photon sources that emit photons at the same energy play a key role in the emerging concepts of quantum information, such as entanglement swapping, quantum teleportation and quantum networks. They can be realized in a variety of systems, where semiconductor quantum dots, or 'artificial atoms', are arguably among the most attractive. However, unlike 'natural atoms', no two artificial atoms are alike. This peculiarity is a serious hurdle for quantum information applications that require photonic quantum states with identical energies. Here we demonstrate a single artificial atom that generates photons with an absolute energy that is locked to an optical transition in a natural atom. Furthermore, we show that our system is robust and immune to drifts and fluctuations in the environment of the emitter. Our demonstration is crucial for realization of a large number of universally-indistinguishable solid-state systems at arbitrary remote locations, where frequency-locked artificial atoms might become fundamental ingredients.
Quantum light is a key resource for the development of quantum-enhanced technologies such as secure communications, quantum networks, distributed quantum computing, metrology, etc. The development of these technologies requires light sources emitting single photons in a pure quantum state as well as efficient photon-photon gates. Such sources and gates can be obtained making use of the single-photon sensitivity of an atomic transition. We study artificial atoms in the form of semiconductor quantum dots to develop building blocks for optical quantum technologies. We use the tools of opto-electronics and nanotechnology to fabricate close to ideal atom-photon interfaces, where a single artificial atom interacts with a single mode of the optical field. We show that cavity quantum electrodynamics allows to largely isolate the artificial atoms from all sources of decoherence such as charge noise and crystal vibrations. We obtain bright solid-state sources of single photons with very high quantum purity, not only in the frequency basis, but for the first time also in the photon number basis. Finally, we have made progresses toward the development of efficient two-photon gates, with devices performing as nonlinear switches at the single-photon level. Some references: High-performances quantum light sources Somaschi et al., Nature Photonics 10, 340 (2016) Grange et al, Phys. Rev. Lett. 118, 253602 (2017) Senellart, Solomon and White, Nature Nanotechnology 12, 1026 (2017) J. C. Loredo, C. Anton, et. al, arXiv:1810.05170 Scaling-up optical quantum computing Loredo et al., Optica 3, 433 (2016) Loredo et al., Phys. Rev. Lett. 118, 130503 (2017) Toward efficient photon-photon gates Giesz et al., Nat. Comm 7, 11986 (2016) De Santis et al, Nature Nanotechnology 12, 663 (2017)
Solid-state platforms are particularly attractive for quantum optics because they facilitate on-chip integration and are compatible with established semiconductor and photonic technologies. However, a major challenge in solid-state quantum optics is the fabrication of arrays of identical emitters, such as quantum dots. In this work, we propose moire superlattices as a novel solid-state platform for manipulating light at the single-photon level. Moire superlattices form arrays of artificial-atom states characterized by nearly identical optical transition energies, tunable spacing, and highly adjustable electronic structures. They naturally operate as atomically thin, scalable, periodic emitters, making them ideal for quantum applications. Additionally, the extensive materials database of moire superlattices offers spectral coverage spanning a broad range of optical wavelengths.
Single spins confined in semiconductor quantum dots - artificial atoms in the solid-state - are attractive candidates for quantum mechanical bits, the fundamental units and building blocks of a quantum computer. The ability to address quantum dot spins optically allows us to initialise and manipulate the state of the quantum bit. Gaining information on the qubit, for example by reading out its state, not only requires state-selective optical excitation, but also access to the single photons scattered in response by the quantum dot. Further, for a distributed computer architecture where nodes of few quantum bits are interlinked via optical communication channels photonic quantum bits are required to faithfully transmit the quantum information. In this thesis we advocate resonant excitation of quantum dot transitions and collection of the resonance fluorescence to address two outstanding challenges: generating dephasing-free single photons for use as flying quantum bits and single-shot spin readout. To this end we investigate the spectral and first-order coherence properties of quantum dot resonance fluorescence. In particular, we directly observe highly coherent scattering in the low Rabi frequency limit which has remained unexplored for solid-state single photon emitters so far. At the same time, interactions with the semiconductor environment are revealed and quantified through their optical signatures: exciton-phonon coupling, nuclear spin dynamics and local electric field fluctuations signal a departure from the ideal atom-like behaviour. Taking advantage of the laser-like coherence of single phase-locked quantum dot photons in the Heitler regime, we demonstrate near-ideal two-photon quantum interference. This benchmark measurement is a precursor for the photonic entanglement of distant quantum dot spins in a quantum optical network, and the results here predict a high fidelity operation. Finally, moving to tunnel-coupled quantum dot molecules we show that the overlap of carrier wave functions in two closely spaced quantum dots forms new spin-selective optical transitions not available in single quantum dots. Then, the presence or absence of scattered photons reveals the electron spin. Intermittency in the quantum dot resonance fluorescence allowed us, for the first time, to observe spin quantum jumps in real-time. Both achievements - highly coherent photons and spin readout - provide the missing link to attempt creation of a small-scale quantum network now.
The prospect of using the quantum nature of light for secure communication keeps spurring the search and investigation of suitable sources of entangled-photons. Semiconductor quantum dots are arguably the most attractive. They can generate indistinguishable entangled-photons deterministically, and are compatible with current photonic-integration technologies, a set of properties not shared by any other entanglement resource. However, as no two quantum dots are identical, they emit entangled-photons with random energies. This hinders their exploitation in communication protocols requiring entangled-states with well-defined energies. Here, we introduce scalable quantum-dot-based sources of polarization-entangled-photons whose energy can be controlled via dynamic strain-engineering without degrading the degree of entanglement of the source. As a test-bench, we interface quantum dots with clouds of atomic vapours, and we demonstrate slow-entangled-photons from a single quantum emitter. These results pave the way towards the implementation of hybrid quantum networks where entanglement is distributed among distant parties using scalable optoelectronic devices.
For the past fifteen years, single semiconductor quantum dots, often referred to as solid-state artificial atoms, have been at the forefront of various research direction lines for experimental quantum information science, in particular in the development of practical sources of quantum states of light. Here we review the research to date, on the tailoring of the emission properties from single quantum dots producing single photons, indistinguishable single photons and entangled photon pairs. Finally, the progress and future prospects for applications of single dots in quantum information processing is considered.
In a quantum network based on atoms and photons, a single atom should control the photon state and, reciprocally, a single photon should allow the coherent manipulation of the atom. Both operations require controlling the atom environment and developing efficient atom-photon interfaces, for instance by coupling the natural or artificial atom to cavities. So far, much attention has been drown on manipulating the light field with atomic transitions, recently at the few-photon limit. Here we report on the reciprocal operation and demonstrate the coherent manipulation of an artificial atom by few photons. We study a quantum dot-cavity system with a record cooperativity of 13. Incident photons interact with the atom with probability 0.95, which radiates back in the cavity mode with probability 0.96. Inversion of the atomic transition is achieved for 3.8 photons on average, showing that our artificial atom performs as if fully isolated from the solid-state environment.