Nick Stoltz

Controlling cavity reflectivity with a single quantum dot

Solid-state cavity quantum electrodynamics (QED) systems offer a robust and scalable platform for quantum optics experiments and the development of quantum information processing devices. In particular, systems based on photonic crystal nanocavities and semiconductor quantum dots have seen rapid progress. Recent experiments have allowed the observation of weak and strong coupling regimes of interaction between the photonic crystal cavity and a single quantum dot in photoluminescence. In the weak coupling regime, the quantum dot radiative lifetime is modified; in the strong coupling regime, the coupled quantum dot also modifies the cavity spectrum. Several proposals for scalable quantum information networks and quantum computation rely on direct probing of the cavity–quantum dot coupling, by means of resonant light scattering from strongly or weakly coupled quantum dots. Such experiments have recently been performed in atomic systems and superconducting circuit QED systems, but not in solid-state quantum dot–cavity QED systems. Here we present experimental evidence that this interaction can be probed in solid-state systems, and show that, as expected from theory, the quantum dot strongly modifies the cavity transmission and reflection spectra. We show that when the quantum dot is coupled to the cavity, photons that are resonant with its transition are prohibited from entering the cavity. We observe this effect as the quantum dot is tuned through the cavity and the coupling strength between them changes. At high intensity of the probe beam, we observe rapid saturation of the transmission dip. These measurements provide both a method for probing the cavity–quantum dot system and a step towards the realization of quantum devices based on coherent light scattering and large optical nonlinearities from quantum dots in photonic crystal cavities.

Controlled phase shifts with a single quantum dot.

Optical nonlinearities enable photon-photon interaction and lie at the heart of several proposals for quantum information processing, quantum nondemolition measurements of photons, and optical signal processing. To date, the largest nonlinearities have been realized with single atoms and atomic ensembles. We show that a single quantum dot coupled to a photonic crystal nanocavity can facilitate controlled phase and amplitude modulation between two modes of light at the single-photon level. At larger control powers, we observed phase shifts up to π/4 and amplitude modulation up to 50%. This was accomplished by varying the photon number in the control beam at a wavelength that was the same as that of the signal, or at a wavelength that was detuned by several quantum dot linewidths from the signal. Our results present a step toward quantum logic devices and quantum nondemolition measurements on a chip.

Coherent generation of non-classical light on a chip via photon-induced tunnelling and blockade

Quantum dots in photonic crystals are interesting because of their potential in quantum information processing and as a testbed for cavity quantum electrodynamics. Recent advances in controlling and coherent probing of such systems open the possibility of realizing quantum networks originally proposed for atomic systems. Here, we demonstrate that non-classical states of light can be coherently generated using a quantum dot strongly coupled to a photonic crystal resonator. We show that the capture of a single photon into the cavity affects the probability that a second photon is admitted. This probability drops when the probe is positioned at one of the two energy eigenstates corresponding to the vacuum Rabi splitting, a phenomenon known as photon blockade, the signature of which is photon antibunching. In addition, we show that when the probe is positioned between the two eigenstates, the probability of admitting subsequent photons increases, resulting in photon bunching. We call this process photon-induced tunnelling. This system represents an ultimate limit for solid-state nonlinear optics at the single-photon level. Along with demonstrating the generation of non-classical photon states, we propose an implementation of a single-photon transistor in this system.

Optical pumping of a single hole spin in a quantum dot

The spin of an electron is a natural two-level system for realizing a quantum bit in the solid state. For an electron trapped in a semiconductor quantum dot, strong quantum confinement highly suppresses the detrimental effect of phonon-related spin relaxation. However, this advantage is offset by the hyperfine interaction between the electron spin and the 104 to 106 spins of the host nuclei in the quantum dot. Random fluctuations in the nuclear spin ensemble lead to fast spin decoherence in about ten nanoseconds. Spin-echo techniques have been used to mitigate the hyperfine interaction, but completely cancelling the effect is more attractive. In principle, polarizing all the nuclear spins can achieve this but is very difficult to realize in practice. Exploring materials with zero-spin nuclei is another option, and carbon nanotubes, graphene quantum dots and silicon have been proposed. An alternative is to use a semiconductor hole. Unlike an electron, a valence hole in a quantum dot has an atomic p orbital which conveniently goes to zero at the location of all the nuclei, massively suppressing the interaction with the nuclear spins. Furthermore, in a quantum dot with strong strain and strong quantization, the heavy hole with spin-3/2 behaves as a spin-1/2 system and spin decoherence mechanisms are weak. We demonstrate here high fidelity (about 99 per cent) initialization of a single hole spin confined to a self-assembled quantum dot by optical pumping. Our scheme works even at zero magnetic field, demonstrating a negligible hole spin hyperfine interaction. We determine a hole spin relaxation time at low field of about one millisecond. These results suggest a route to the realization of solid-state quantum networks that can intra-convert the spin state with the polarization of a photon.

A Coherent Single-Hole Spin in a Semiconductor

A Hole New Approach Quantum dots can behave as artificial atoms, exhibiting a ladder of quantized energy levels with the number of electrons added to the dot being controllable. They are thus being extensively studied for application in the likes of quantum information processing strategies. However, the electrons interact with their environment and quickly lose their coherence properties. Brunner et al. (p. 70; see the Perspective by Kolodrubetz and Petta) now show that if the charge of the dot is manipulated so that it is positive; that is, populated with a single hole, then the coherence properties of the dot can be extended. The strategy of using holes instead of electrons may provide a solution to the decoherence problem. Manipulating holes instead of electrons results in the enhancement of the coherence properties of quantum dots. Semiconductors have uniquely attractive properties for electronics and photonics. However, it has been difficult to find a highly coherent quantum state in a semiconductor for applications in quantum sensing and quantum information processing. We report coherent population trapping, an optical quantum interference effect, on a single hole. The results demonstrate that a hole spin in a quantum dot is highly coherent.

Local quantum dot tuning on photonic crystal chips

Quantum networks based on InAs quantum dots embedded in photonic crystal devices rely on quantum dots being in resonance with each other and with the cavities they are embedded in. The authors developed a technique based on temperature tuning to spectrally align different quantum dots located on the same chip. The technique allows for up to 1.8 nm reversible on-chip quantum dot tuning.

Local tuning of photonic crystal cavities using chalcogenide glasses

We developed a method to locally tune refractive index in photonic crystals. The technique, based on photodarkening of chalcogenide glasses, enables 3 nm resonance tuning of GaAs photonic crystal cavities operating at 940 nm.

Ultrafast nonlinear optical tuning of photonic crystal cavities

We demonstrate fast (up to 20 GHz), low power (5 muW) modulation of photonic crystal cavities in GaAs containing InAs quantum dots. Modulation is achieved via free carrier injection by an above-band picosecond laser pulse.

Dipole induced transparency in waveguide coupled photonic crystal cavities.

We demonstrate dipole induced transparency in an integrated photonic crystal device. We show that a single weakly coupled quantum dot can control the transmission of photons through a photonic crystal cavity that is coupled to waveguides on the chip. Control over the quantum dot and cavity resonance via local temperature tuning, as well as efficient out-coupling with an integrated grating structure is demonstrated.

High-quality factor optical microcavities using oxide apertured micropillars

An oxide aperture is used to confine optical modes in a micropillar structure. This method overcomes the limitations due to sidewall scattering loss typical in semiconductor etched micropillars. High cavity quality factors (Q) up to 48 000 are determined by external Fabry–Perot cavity scanning measurements, a significantly higher value than prior work in III-V etched micropillars. Measured Q values and estimated mode volumes correspond to a maximum Purcell factor figure of merit value of 72.