Coherent excitation of an ensemble of quantum objects underpins quantum many-body phenomena, and offers the opportunity to realize a quantum memory to store information from a qubit. Thus far, a deterministic and coherent interface between a single quantum system, e.g. a qubit, and such an ensemble has remained elusive. We first use an electron to cool the mesoscopic nuclear-spin ensemble of a semiconductor quantum dot to the nuclear sidebandresolved regime. We then implement an all-optical approach to access these individual quantized electronic-nuclear spin transitions. Finally, we perform coherent optical rotations of a single collective nuclear spin excitation corresponding to a spin wave called a nuclear magnon. These results constitute the building blocks of a dedicated local memory per quantum-dot spin qubit and promise a solid-state platform for quantum-state engineering of isolated many-body systems.

A controlled quantum system can alter its environment by feedback, leading to reduced-entropy states of the environment and to improved system coherence. Here, using a quantum-dot electron spin as a control and probe, we prepare the quantum-dot nuclei under the feedback of coherent population trapping and observe their evolution from a thermal to a reduced-entropy state, with the immediate consequence of extended qubit coherence. Via Ramsey interferometry on the electron spin, we directly access the nuclear distribution following its preparation and measure the emergence and decay of correlations within the nuclear ensemble. Under optimal feedback, the inhomogeneous dephasing time of the electron, T Ã 2 , is extended by an order of magnitude to 39 ns. Our results can be readily exploited in quantum information protocols utilizing spin-photon entanglement and represent a step towards creating quantum many-body states in a mesoscopic nuclear-spin ensemble. DOI: 10.1103/PhysRevLett.119.130503 The interaction between a qubit and its mesoscopic environment offers the opportunity to access and control the ensemble properties of this environment. In turn, tailoring the environment improves qubit performance and can lead to nontrivial collective states. Significant steps towards such control have been taken in systems including nitrogenvacancy centers coupled to 13 C spins in diamond [1], superconducting qubits coupled to a microwave reservoir [2], and spins in electrostatically defined [3][4][5] and selfassembled [6] quantum dots (QDs) coupled to the host nuclei. In InGaAs QDs, the hyperfine interaction permits spin-flip processes to occur between a confined electron and the QD nuclei. Optical pumping of the electron spin induces a directional flipping of nuclear spins leading to a net polarization buildup [7]. The resulting effective magnetic (Overhauser) field can be as strong as 7 T [8], leading to significant shifts of the electron-spin energy levels [8][9][10][11]. In contrast to other systems, the polarization of this isolated mesoscopic ensemble can persist for hours [12]. Coupling the electronic energy shifts to the optical pumping rate closes a feedback loop [13][14][15][16] that allows for the selection of the degree of nuclear-spin polarization.A spectrally sharp version of such stabilizing feedback is achieved through coherent population trapping (CPT), when driving the Λ system formed by the two electronspin states and an excited trion state of a negatively charged QD [6,[17][18][19][20], as depicted in Fig. 1(a). Deviations from the dark-state resonance lead to a preferential driving of one of the two optical transitions, inducing an electron-spin polarization that pulls the Overhauser field back towards a lock point set by the two-photon resonance [ Fig. 1(a), bottom panel]. The narrow spectral feature defined by the electronic dark-state coherence thereby carves out a reduced variance Overhauser-field distribution from the initial thermal state with the prospect of improved qubit coherence, as inferr...

Quantum control of solid-state spin qubits typically involves pulses in the microwave domain, drawing from the well-developed toolbox of magnetic resonance spectroscopy. Driving a solid-state spin by optical means offers a high-speed alternative, which in the presence of limited spin coherence makes it the preferred approach for high-fidelity quantum control. Bringing the full versatility of magnetic spin resonance to the optical domain requires full phase and amplitude control of the optical fields. Here, we imprint a programmable microwave sequence onto a laser field and perform electron spin resonance in a semiconductor quantum dot via a two-photon Raman process. We show that this approach yields full SU(2) spin control with over 98% π-rotation fidelity. We then demonstrate its versatility by implementing a particular multi-axis control sequence, known as spin locking. Combined with electron-nuclear Hartmann-Hahn resonances which we also report in this work, this sequence will enable efficient coherent transfer of a quantum state from the electron spin to the mesoscopic nuclear ensemble.

Single-photon sources are essential building blocks in quantum photonic networks, where quantum-mechanical properties of photons are utilised to achieve quantum technologies such as quantum cryptography and quantum computing. Most conventional solid-state single-photon sources are based on single emitters such as self-assembled quantum dots, which are created at random locations and require spectral filtering. These issues hinder the integration of a singlephoton source into a scaleable photonic quantum network for applications such as on-chip photonic quantum processors. In this work, using only regular lithography techniques on a conventional GaAs quantum well, we realise an electrically triggered single-photon source with a GHz repetition rate and without the need for spectral filtering. In this device, a single electron is carried in the potential minimum of a surface acoustic wave (SAW) and is transported to a region of holes to form an exciton. The exciton then decays and creates a single photon in a lifetime of ∼ 100 ps. This SAW-driven electroluminescence (EL) yields photon antibunching with g (2) (0) = 0.39 ± 0.05, which satisfies the common criterion for a single-photon source g (2) (0) < 0.5. Furthermore, we estimate that if a photon detector receives a SAW-driven EL signal within one SAW period, this signal has a 79%-90% chance of being a single photon. This work shows that a single-photon source can be made by combining single-electron transport and a lateral n-i-p junction. This approach makes it possible to create multiple synchronised single-photon sources at chosen positions with photon energy determined by quantum-well thickness. Compared with conventional quantum-dot-based single-photon sources, this device may be more suitable for an on-chip integrated photonic quantum network.The development of single-photon sources is important for many quantum information technologies [1][2][3], such as quantum cryptography [4][5][6], quantum communication [7][8][9], quantum metrology [10, 11], and quantum computation [12, 13]. Currently, most high-performance single-photon sources are self-assembled InGaAs-based quantum dots (QDs) [14][15][16]. However, there are several issues that may limit their integration into practical quantum photonic networks [17][18][19][20][21][22][23][24]. Firstly, in conventional growth of self-assembled QDs, the location and size of each QD are quite random. Therefore, one has to rely on statistics to create structures like optical cavities and gates around a quantum dot. This will be an issue for applications that require several deterministicallyfabricated single-photon sources on a compact chip. Secondly, it is hard to precisely control the size of a quantum dot, which will affect the single-photon energy. Hence, it is challenging to make identical QD single-photon sources, which is essential for applications like quantum computation and a quantum repeater [12,25]. Finally, in order to ensure that a neutral exciton is created in every optical or electrical excitation, the e...

A first generation of parallel scanning tunneling microscopy ͑STM͒ simulator has been developed to accelerate the production of high quality STM images. An efficient master-slave parallel scheme has been constructed specially suited for large scale problems in which the amount of data communications remains a small fraction of the entire calculation. We apply the new parallel scheme to two examples, benzene adsorption on a metal surface and standing wave patterns on the Cu͑111͒ surface, highlighting the efficiency of our approach.