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Coherent excitation of an ensemble of quantum objects underpins quantum many-body phenomena and offers the opportunity to realize a memory that stores quantum information. Thus far, a deterministic and coherent interface between a spin qubit and such an ensemble has remained elusive. In this study, we first used an electron to cool the mesoscopic nuclear spin ensemble of a semiconductor quantum dot to the nuclear sideband–resolved regime.We then implemented an all-optical approach to access individual quantized electronic-nuclear spin transitions. Lastly, we performed coherent optical rotations of a single collective nuclear spin excitation—a spin wave. 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.

Accessing an ensemble of coherently interacting objects at the level of single quanta via a proxy qubit is transformative in the investigations of emergent quantum phenomena. An isolated nuclear spin ensemble is a remarkable platform owing to its coherence, but sensing its excitations with single spin precision has remained elusive. Here we achieve quantum sensing of a single nuclear-spin excitation (a nuclear magnon) in a dense ensemble of approximately 80,000 nuclei. A Ramsey measurement on the electron proxy qubit enables us to sense the hyperfine shift induced by a single nuclear magnon. We resolve multiple magnon modes distinguished by atomic species and spin polarity via the spectral dependence of this hyperfine shift. Finally, we observe the time-dependent shift induced by collective Rabi oscillations, revealing the competition between the buildup of quantum correlations and decoherence in the ensemble. These techniques could be extended to probe the engineered quantum states of the ensemble such as long-lived memory states.A single excitation in a semiconductor nuclear spin ensemble is detected with parts-per-million accuracy using the coupling between the ensemble and an electron-spin quantum dot.

The interaction between a confined electron and the nuclei of an optically active quantum dot provides a uniquely rich manifestation of the central spin problem. Coherent qubit control combines with an ultrafast spin–photon interface to make these confined spins attractive candidates for quantum optical networks. Reaching the full potential of spin coherence has been hindered by the lack of knowledge of the key irreversible environment dynamics. Through all-optical Hahn echo decoupling we now recover the intrinsic coherence time set by the interaction with the inhomogeneously strained nuclear bath. The high-frequency nuclear dynamics are directly imprinted on the electron spin coherence, resulting in a dramatic jump of coherence times from few tens of nanoseconds to the microsecond regime between 2 and 3 T magnetic field and an exponential decay of coherence at high fields. These results reveal spin coherence can be improved by applying large magnetic fields and reducing strain inhomogeneity. Spins confined to quantum dots are a possible qubit, but the mechanism that limits their coherence is unclear. Here, the authors use an all-optical Hahn-echo technique to determine the intrinsic coherence time of such spins set by its interaction with the inhomogeneously strained nuclear bath.

On quantum nature Cavity quantum electrodynamics (QED) studies the interaction between a quantum emitter (for example an atom or a quantum dot) and a single mode from a radiation field. When the two are strongly coupled it is possible to realize key quantum information processing tasks. In the solid state this could be achieved by coupling semiconductor quantum dots to optical microcavities. However, validating the efficacy of quantum dots in quantum information applications requires confirmation of the quantum nature of the quantum-dot–cavity system in the strong coupling regime. A collaboration between labs at ETH Zurich and the University of California, Santa Barbara, now provides this sought-after confirmation. The experiments involve a photonic crystal nanocavity in which one, and only one, quantum dot is located precisely at the cavity electric field maximum. A series of experiments that provide confirmation of the quantum nature of the quantum–dot–cavity system in the strong coupling regime by studying a photonic crystal nanocavity in which one, and only one, quantum dot is located precisely at the cavity electric field maximum. Cavity quantum electrodynamics (QED) studies the interaction between a quantum emitter and a single radiation-field mode. When an atom is strongly coupled to a cavity mode 1 , 2 , it is possible to realize important quantum information processing tasks, such as controlled coherent coupling and entanglement of distinguishable quantum systems. Realizing these tasks in the solid state is clearly desirable, and coupling semiconductor self-assembled quantum dots to monolithic optical cavities is a promising route to this end. However, validating the efficacy of quantum dots in quantum information applications requires confirmation of the quantum nature of the quantum-dot–cavity system in the strong-coupling regime. Here we find such confirmation by observing quantum correlations in photoluminescence from a photonic crystal nanocavity 3 , 4 , 5 interacting with one, and only one, quantum dot located precisely at the cavity electric field maximum. When off-resonance, photon emission from the cavity mode and quantum-dot excitons is anticorrelated at the level of single quanta, proving that the mode is driven solely by the quantum dot despite an energy mismatch between cavity and excitons. When tuned to resonance, the exciton and cavity enter the strong-coupling regime of cavity QED and the quantum-dot exciton lifetime reduces by a factor of 145. The generated photon stream becomes antibunched, proving that the strongly coupled exciton/photon system is in the quantum regime. Our observations unequivocally show that quantum information tasks are achievable in solid-state cavity QED.

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 \\%\\]\\[\\pi\\]-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.

Spin quantum jumps in real time A promising approach to realizing a practical qubit scheme for quantum computation involves the optical control of single electron spins in semiconductor quantum dots. Rapid progress towards the reliable preparation and manipulation of the quantum states of such spins has been achieved in recent years. The final challenge is to carry out 'single shot' measurements of the electron spin without interfering with it. Vamivakas et al . have now developed a technique that enables such a measurement through coupling of one quantum dot to another. This quantum dot 'molecule', unlike its single quantum dot counterpart, allows separate and independent optical transitions for state preparation, manipulation and measurement, avoiding the dilemma of relying on the same transition to address the spin state of an electron. As a result, the authors show, it is possible to observe spin quantum jumps in real time. A promising approach to realizing a practical quantum bit scheme is the optical control of single electron spins in quantum dots. The reliable preparation and manipulation of the quantum states of such spins have been demonstrated recently. The final challenge is to carry out single-shot measurements of the electron spin without interfering with it. A technique has now been developed that enables such measurement, by coupling one quantum dot to another to produce a quantum dot molecule. Reliable preparation, manipulation and measurement protocols are necessary to exploit a physical system as a quantum bit 1 . Spins in optically active quantum dots offer one potential realization 2 , 3 and recent demonstrations have shown high-fidelity preparation 4 , 5 and ultrafast coherent manipulation 6 , 7 , 8 . The final challenge—that is, single-shot measurement of the electron spin—has proved to be the most difficult of the three and so far only time-averaged optical measurements have been reported 9 , 10 , 11 , 12 . The main obstacle to optical spin readout in single quantum dots is that the same laser that probes the spin state also flips the spin being measured. Here, by using a gate-controlled quantum dot molecule 13 , 14 , 15 , we present the ability to measure the spin state of a single electron in real time via the intermittency of quantum dot resonance fluorescence 12 , 16 . The quantum dot molecule, unlike its single quantum dot counterpart, allows separate and independent optical transitions for state preparation, manipulation and measurement, avoiding the dilemma of relying on the same transition to address the spin state of an electron.

We have demonstrated laser cooling of a single electron spin trapped in a semiconductor quantum dot. Optical coupling of electronic spin states was achieved using resonant excitation of the charged quantum dot (trion) transitions along with the heavy-light hole mixing, which leads to weak yet finite rates for spin-flip Raman scattering. With this mechanism, the electron spin can be cooled from 4.2 to 0.020 kelvin, as confirmed by the strength of the induced Pauli blockade of the trion absorption. Within the framework of quantum information processing, this corresponds to a spin-state preparation with a fidelity exceeding 99.8%.

Resonant laser scattering along with photon correlation measurements established the atom-like character of quantum dots. Here, we show that for a wide range of experimental parameters it is impossible to isolate elementary quantum-dot excitations from a strong influence of nuclear spins; the absorption lineshapes at magnetic fields exceeding 1 T indicate that the nuclear spins get polarized by an amount that ensures locking of the quantum-dot resonance to the incident laser frequency. In stark contrast to earlier experiments, this nuclear-spin polarization is bidirectional, allowing the combined electron–nuclear-spin system to track the changes in laser frequency dynamically on both sides of the resonance. This unexpected feature stems from a competition between two spin-pumping processes that attempt to polarize nuclear spins in opposite directions. We find that the confluence of laser excitation and nuclear-spin polarization suppresses the fluctuations in resonant absorption. A master-equation analysis suggests narrowing of the nuclear-spin distribution, pointing to applications in quantum information processing. In semiconductor quantum dots, interactions between the confined electrons and the surrounding reservoir of nuclear spins limit the attainable electron-spin coherence. But the nuclear-spin reservoir can also take a constructive role, as it facilitates the locking of the optical quantum-dot resonance to the changing frequency of an external driving laser, as an experiment now demonstrates.

We demonstrate a deterministic approach to the implementation of solid-state cavity quantum electrodynamics (QED) systems based on a precise spatial and spectral overlap between a single self-assembled quantum dot and a photonic crystal membrane nanocavity. By fine-tuning nanocavity modes with a high quality factor into resonance with any given quantum dot exciton, we observed clear signatures of cavity QED (such as the Purcell effect) in all fabricated structures. This approach removes the major hindrances that had limited the application of solid-state cavity QED and enables the realization of experiments previously proposed in the context of quantum information processing.

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 control and probe, we prepare the quantum dot nuclei under the feedback of coherent population trapping and measure the 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_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.