Improving a Solid-State Qubit through an Engineered Mesoscopic Environment.

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_{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.

Phase-Tuned Entangled State Generation between Distant Spin Qubits.

Quantum entanglement between distant qubits is an important feature of quantum networks. Distribution of entanglement over long distances can be enabled through coherently interfacing qubit pairs via photonic channels. Here, we report the realization of optically generated quantum entanglement between electron spin qubits confined in two distant semiconductor quantum dots. The protocol relies on spin-photon entanglement in the trionic Λ system and quantum erasure of the Raman-photon path information. The measurement of a single Raman photon is used to project the spin qubits into a joint quantum state with an interferometrically stabilized and tunable relative phase. We report an average Bell-state fidelity for |ψ^{(+)}⟩ and |ψ^{(-)}⟩ states of 61.6±2.3% and a record-high entanglement generation rate of 7.3 kHz between distant qubits.

Observation of spin-dependent quantum jumps via quantum dot resonance fluorescence.

Reliable preparation, manipulation and measurement protocols are necessary to exploit a physical system as a quantum bit. Spins in optically active quantum dots offer one potential realization and recent demonstrations have shown high-fidelity preparation and ultrafast coherent manipulation. 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. 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, we present the ability to measure the spin state of a single electron in real time via the intermittency of quantum dot resonance fluorescence. 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.

Direct photonic coupling of a semiconductor quantum dot and a trapped ion.

Coupling individual quantum systems lies at the heart of building scalable quantum networks. Here, we report the first direct photonic coupling between a semiconductor quantum dot and a trapped ion and we demonstrate that single photons generated by a quantum dot controllably change the internal state of a Yb^{+} ion. We ameliorate the effect of the 60-fold mismatch of the radiative linewidths with coherent photon generation and a high-finesse fiber-based optical cavity enhancing the coupling between the single photon and the ion. The transfer of information presented here via the classical correlations between the σ_{z} projection of the quantum-dot spin and the internal state of the ion provides a promising step towards quantum-state transfer in a hybrid photonic network.

Dynamically controlled resonance fluorescence spectra from a doubly dressed single InGaAs quantum dot.

We report the first experimental demonstration of the interference-induced spectral line elimination predicted by Zhu and Scully [Phys. Rev. Lett. 76, 388 (1996)] and Ficek and Rudolph [Phys. Rev. A 60, R4245 (1999)]. We drive an exciton transition of a self-assembled quantum dot in order to realize a two-level system exposed to a bichromatic laser field and observe the nearly complete elimination of the resonance fluorescence spectral line at the driving laser frequency. This is caused by quantum interference between coupled transitions among the doubly dressed excitonic states, without population trapping. We also demonstrate a multiphoton ac Stark effect with shifted subharmonic resonances and dynamical modifications of resonance fluorescence spectra by using double dressing.

Quantum nature of a strongly coupled single quantum dot-cavity system.

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, 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 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.

Nanoscale optical electrometer.

We propose and demonstrate an all-optical approach to single-electron sensing using the optical transitions of a semiconductor quantum dot. The measured electric-field sensitivity of 5 (V/m)/√Hz corresponds to detecting a single electron located 5 µm from the quantum dot-nearly 10 times greater than the diffraction limited spot size of the excitation laser-in 1 s. The quantum-dot-based electrometer is more sensitive than other devices operating at a temperature of 4.2 K or higher and further offers suppressed backaction on the measured system.

Quantum interface of an electron and a nuclear ensemble.

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.

Quantum dot spin coherence governed by a strained nuclear environment.

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.

Dispersion-independent high-visibility quantum interference in ultrafast parametric down-conversion

We demonstrate the effective removal of intrinsic distinguishability between entangled-photon pairs in femtosecond spontaneous parametric down-conversion. High-visibility quantum interference is recovered (an increase to 96% from 17%) while preserving the high photon-flux density associated with the use of long nonlinear crystals. This new technique is expected to serve as a basic component in the preparation of multiphoton entangled states.

Experimental demonstration of three mutually orthogonal polarization states of entangled photons

Linearly polarized classical light can be expressed in a vertical and a horizontal component. Geometrically rotating vertically polarized light by 90 degrees will convert it to the orthogonal horizontal polarization. We have experimentally generated a two-photon state of light which evolves into an orthogonal state upon geometrical rotation by 60 degrees. Rotating this state by an additional 60 degrees will yield a state which is mutually orthogonal to the first two states. Generalizing this procedure, one can generate N+1 mutually orthogonal N-photon states that cyclicly evolve from one to another upon a geometric rotation by 180/(N+1) degrees.

Strong extinction of a far-field laser beam by a single quantum dot.

Through the utilization of index-matched GaAs immersion lens techniques, we demonstrate a record extinction (12%) of a far-field focused laser beam by a single InAs/GaAs quantum dot. This contrast level enables us to report for the first time resonant laser transmission spectroscopy on a single InAs/GaAs quantum dot without the need for phase-sensitive lock-in detection.

Entanglement in cascaded-crystal parametric down-conversion.

We use spontaneous parametric down-conversion in a cascade of crystals, driven by a single monochromatic cw pump laser, to study the interference of entangled photon pairs. By changing the distance between the crystals, the observed quantum interference pattern varies continuously from that associated with a longer single crystal to that associated with independent emissions from two distinct crystals. Postselection via spectral filtering suppresses this phenomenon. These findings are expected to advance the field of quantum-state engineering.