Quantum mechanics---the theory describing the fundamental workings of nature---is famously counterintuitive: it predicts that a particle can be in two places at the same time, and that two remote particles can be inextricably and instantaneously linked These predictions have been the topic of intense metaphysical debate ever since the theory's inception early last century However, supreme predictive power combined with direct experimental observation of some of these unusual phenomena leave little doubt as to its fundamental correctness In fact, without quantum mechanics we could not explain the workings of a laser, nor indeed how a fridge magnet operates Over the last several decades quantum information science has emerged to seek answers to the question: can we gain some advantage by storing, transmitting and processing information encoded in systems that exhibit these unique quantum properties? Today it is understood that the answer is yes Many research groups around the world are working towards one of the most ambitious goals humankind has ever embarked upon: a quantum computer that promises to exponentially improve computational power for particular tasks A number of physical systems, spanning much of modern physics, are being developed for this task---ranging from single particles of light to superconducting circuits---and it is not yet clear which, if any, will ultimately prove successful Here we describe the latest developments for each of the leading approaches and explain what the major challenges are for the future

Quantum Computing

In the past decade, a two-dimensional matter-light system called the microcavity exciton-polariton has emerged as a new promising candidate of Bose-Einstein condensation BEC in solids. Many pieces of important evidence of polariton BEC have been established recently in GaAs and CdTe microcavities at the liquid helium temperature, opening a door to rich many-body physics inaccessible in experiments before. Technological progress also made polariton BEC at room temperatures promising. In parallel with experimental progresses, theoretical frameworks and numerical simulations are developed, and our understanding of the system has greatly advanced. In this article, recent experiments and corresponding theoretical pictures based on the Gross-Pitaevskii equations and the Boltzmann kinetic simulations for a finite-size BEC of polaritons are reviewed.

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Exciton-polariton Bose-Einstein condensation

Single photons are a fundamental element of most quantum optical technologies. The ideal single-photon source is an on-demand, deterministic, single-photon source delivering light pulses in a well-defined polarization and spatiotemporal mode, and containing exactly one photon. In addition, for many applications, there is a quantum advantage if the single photons are indistinguishable in all their degrees of freedom. Single-photon sources based on parametric down-conversion are currently used, and while excellent in many ways, scaling to large quantum optical systems remains challenging. In 2000, semiconductor quantum dots were shown to emit single photons, opening a path towards integrated single-photon sources. Here, we review the progress achieved in the past few years, and discuss remaining challenges. The latest quantum dot-based single-photon sources are edging closer to the ideal single-photon source, and have opened new possibilities for quantum technologies.

High-performance semiconductor quantum-dot single-photon sources.

We have realized a fiber-based Fabry-Perot cavity with CO2 laser-machined mirrors. It combines very small size, high finesse F>=130000, small waist and mode volume, and good mode matching between the fiber and cavity modes. This combination of features is a major advance for cavity quantum electrodynamics (CQED), as shown in recent CQED experiments with Bose-Einstein condensates enabled by this cavity [Y. Colombe et al., Nature 450, 272 (2007)]. It should also be suitable for a wide range of other applications, including coupling to solid-state emitters, gas detection at the single-particle level, fiber-coupled single-photon sources and high-resolution optical filters with large stopband.

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Fiber Fabry-Perot cavity with high finesse

Photonic crystal nanobeam cavities are versatile platforms of interest for optical communications, optomechanics, optofluidics, cavity QED, etc. In a previous work [Appl. Phys. Lett. 96, 203102 (2010)], we proposed a deterministic method to achieve ultrahigh Q cavities. This follow-up work provides systematic analysis and verifications of the deterministic design recipe and further extends the discussion to air-mode cavities. We demonstrate designs of dielectric-mode and air-mode cavities with Q > 10⁹, as well as dielectric-mode nanobeam cavities with both ultrahigh-Q (> 10⁷) and ultrahigh on-resonance transmissions (T > 95%).

Deterministic design of wavelength scale, ultra-high Q photonic crystal nanobeam cavities

The authors report on AlAs∕GaAs micropillar cavities with unprecedented quality factors based on high reflectivity distributed Bragg reflectors (DBRs). Due to an increased number of mirror pairs in the DBRs and an optimized etching process record quality (Q) factors up to 165.000 are observed for micropillars with diameters of 4μm. Optical studies reveal a very small ellipticity of 5×10−4 of the pillar cross section. Because of the high Q factors, strong coupling with a vacuum Rabi splitting of 23μeV is observed for micropillars with a diameter of 3μm.

AlAs∕GaAs micropillar cavities with quality factors exceeding 150.000

We report on a scalable fabrication technology for devices based on single quantum dots (QDs) which combines site-controlled growth of QDs with an accurate alignment procedure. Placement of individual QDs and corresponding device structures with a standard deviation of around 50nm from the target position was achieved. The potential of the technology is demonstrated by fabricating arrays of mesas, each containing one QD at a defined position. The presence of single, optically active QDs in the mesas was probed by scanning microphotoluminescence of the mesa arrays.

Lithographic alignment to site-controlled quantum dots for device integration

Results obtained by an advanced growth of site-controlled quantum dots (SCQDs) on pre-patterned nanoholes and their integration into both photonic resonators and nanoelectronic memories are summarized. A specific technique has been pursued to improve the optical quality of single SCQDs. Quantum dot (QD) layers have been vertically stacked but spectrally detuned for single SCQD studies. Thereby, the average emission linewidth of single QDs could be reduced from 2.3 meV for SCQDs in a first QD layer close to the etched nanoholes down to 600 µeV in the third InAs QD layer. Accurate SCQD nucleation on large QD distances is maintained by vertical strain induced QD coupling throughout the QD stacks. Record narrow linewidths of individual SCQDs down to ~110 µeV have been obtained. Experiments performed on coupled photonic SCQD–resonator devices show an enhancement of spontaneous emission. SCQDs have also been integrated deterministically in high electron mobility heterostructures and flash memory operation at room temperature has been observed.

https://iopscience.iop.org/article/10.1088/0957-4484/20/43/434012/pdf

Single site-controlled In(Ga)As/GaAs quantum dots: growth, properties and device integration.

Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits—like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots—is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton–photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample’s coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling.

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Microcavity controlled coupling of excitonic qubits

We present a scalable method for the fabrication of site-controlled quantum dots (QDs) embedded in optical resonators. The position of the quantum dots is determined by nucleating their growth in an array of nanoholes that is aligned to a set of markers, allowing the precise overlay of the resonator geometry over the QD array. Coupling of the QDs to the resonators is demonstrated by the resonant enhancement of the spontaneous emission observed in photoluminescence experiments.

M. Strauß

Papers

AlAs∕GaAs micropillar cavities with quality factors exceeding 150.000

Lithographic alignment to site-controlled quantum dots for device integration

Single site-controlled In(Ga)As/GaAs quantum dots: growth, properties and device integration.

Microcavity controlled coupling of excitonic qubits

Scalable fabrication of optical resonators with embedded site-controlled quantum dots