Turnstile device for heralded single photons: Coulomb blockade of electron and hole tunneling in quantum confined p-i-n heterojunctions

1GHz clocked distribution of electrically generated entangled photon pairs

Quantum networks are essential for realising distributed quantum computation and quantum communication. Entangled photons are a key resource, with applications such as quantum key distribution, quantum relays, and quantum repeaters. All components integrated in a quantum network must be synchronised and therefore comply with a certain clock frequency. In quantum key distribution, the most mature technology, clock rates have reached and exceeded 1GHz. Here we show the first electrically pulsed sub-Poissonian entangled photon source compatible with existing fiber networks operating at this clock rate. The entangled LED is based on InAs/InP quantum dots emitting in the main telecom window, with a multi-photon probability of less than 10% per emission cycle and a maximum entanglement fidelity of 89%. We use this device to demonstrate GHz clocked distribution of entangled qubits over an installed fiber network between two points 4.6km apart.

Manifestation of the Purcell Effect in Current Transport through a Dot-Cavity-QED System

We study the transport properties of a wire-dot system coupled to a cavity and a photon reservoir. The system is considered to be microstructured from a two-dimensional electron gas in a GaAs heterostructure. The 3D photon cavity is active in the far-infrared or the terahertz regime. Tuning the photon energy, Rabi-resonant states emerge and in turn resonant current peaks are observed. We demonstrate the effects of the cavity-photon reservoir coupling, the mean photon number in the reservoir, the electron-photon coupling and the photon polarization on the intraband transitions occurring between the Rabi-resonant states, and on the corresponding resonant current peaks. The Rabi-splitting can be controlled by the photon polarization and the electron-photon coupling strength. In the selected range of the parameters, the electron-photon coupling and the cavity-environment coupling strengths, we observe the results of the Purcell effect enhancing the current peaks through the cavity by increasing the cavity-reservoir coupling, while they decrease with increasing electron-photon coupling. In addition, the resonant current peaks are also sensitive to the mean number of photons in the reservoir.

Surface effects on ionic Coulomb blockade in nanometer-size pores

Ionic Coulomb blockade in nanopores is a phenomenon that shares some similarities but also differences with its electronic counterpart. Here, we investigate this phenomenon extensively using all-atom molecular dynamics of ionic transport through nanopores of about one nanometer in diameter and up to several nanometers in length. Our goal is to better understand the role of atomic roughness and structure of the pore walls in the ionic Coulomb blockade. Our numerical results reveal the following general trends. First, the nanopore selectivity changes with its diameter, and the nanopore position in the membrane influences the current strength. Second, the ionic transport through the nanopore takes place in a hopping-like fashion over a set of discretized states caused by local electric fields due to membrane atoms. In some cases, this creates a slow-varying 'crystal-like' structure of ions inside the nanopore. Third, while at a given voltage, the resistance of the nanopore depends on its length, the slope of this dependence appears to be independent of the molarity of ions. An effective kinetic model that captures the ionic Coulomb blockade behavior observed in MD simulations is formulated.

Competition of static magnetic and dynamic photon forces in electronic transport through a quantum dot

We investigate theoretically the balance of the static magnetic and the dynamical photon forces in the electron transport through a quantum dot in a photon cavity with a single photon mode. The quantum dot system is connected to external leads and the total system is exposed to a static perpendicular magnetic field. We explore the transport characteristics through the system by tuning the ratio, [Formula: see text], between the photon energy, [Formula: see text], and the cyclotron energy, [Formula: see text]. Enhancement in the electron transport with increasing electron-photon coupling is observed when [Formula: see text]. In this case the photon field dominates and stretches the electron charge distribution in the quantum dot, extending it towards the contact area for the leads. Suppression in the electron transport is found when [Formula: see text], as the external magnetic field causes circular confinement of the charge density around the dot.

Electron transport through a quantum dot assisted by cavity photons

We investigate transient transport of electrons through a single quantum dot controlled by a plunger gate. The dot is embedded in a finite wire with length Lx assumed to lie along the x-direction with a parabolic confinement in the y-direction. The quantum wire, originally with hard-wall confinement at its ends, ±Lx/2, is weakly coupled at t = 0 to left and right leads acting as external electron reservoirs. The central system, the dot and the finite wire, is strongly coupled to a single cavity photon mode. A non-Markovian density-matrix formalism is employed to take into account the full electron-photon interaction in the transient regime. In the absence of a photon cavity, a resonant current peak can be found by tuning the plunger-gate voltage to lift a many-body state of the system into the source-drain bias window. In the presence of an x-polarized photon field, additional side peaks can be found due to photon-assisted transport. By appropriately tuning the plunger-gate voltage, the electrons in the left lead are allowed to undergo coherent inelastic scattering to a two-photon state above the bias window if initially one photon was present in the cavity. However, this photon-assisted feature is suppressed in the case of a y-polarized photon field due to the anisotropy of our system caused by its geometry.

Engineered quantum dot single-photon sources

Fast, high efficiency and low error single-photon sources are required for the implementation of a number of quantum information processing applications. The fastest triggered single-photon sources to date have been demonstrated using epitaxially grown semiconductor quantum dots (QDs), which can be conveniently integrated with optical microcavities. Recent advances in QD technology, including demonstrations of high temperature and telecommunications wavelength single-photon emission, have made QD single-photon sources more practical. Here we discuss the applications of single-photon sources and their various requirements, before reviewing the progress made on a QD platform in meeting these requirements.

Temperature dependent empirical pseudopotential theory for self-assembled quantum dots

We develop a temperature dependent empirical pseudopotential theory to study the electronic and optical properties of self-assembled quantum dots (QDs) at finite temperature. The theory takes the effects of both lattice expansion and lattice vibration into account. We apply the theory to InAs/GaAs QDs. For the unstrained InAs/GaAs heterostructure, the conduction band offset increases whereas the valence band offset decreases with increasing temperature, and there is a type-I to type-II transition at approximately 135 K. Yet, for InAs/GaAs QDs, the holes are still localized in the QDs even at room temperature, because the large lattice mismatch between InAs and GaAs greatly enhances the valence band offset. The single-particle energy levels in the QDs show a strong temperature dependence due to the change of confinement potentials. Because of the changes of the band offsets, the electron wavefunctions confined in QDs increase by about 1-5%, whereas the hole wavefunctions decrease by about 30-40% when the temperature increases from 0 to 300 K. The calculated recombination energies of excitons, biexcitons and charged excitons show red shifts with increasing temperature which are in excellent agreement with available experimental data.

Single-photon generation by electron beams

We propose a drastically new method for generating single photons in a deterministic way by interaction of electron beams with optical waveguides. We find a single swift electron to produce a guided photon with large probability. The change in energy and propagation direction of the electron reveals the creation of a photon, with the photon energy directly read from the energy-loss spectrum or the beam displacement. Our study demonstrates the viability of deterministically creating single guided photons using electron beams with better than picosecond time uncertainty, thus opening a new avenue for making room temperature, heralded frequency-tunable sources affordable for scientific and commercial developments.

Bright and grey states in CdSe-CdS nanocrystals exhibiting strongly reduced blinking

When compared to standard colloidal nanocrystals, individual CdSe-CdS core-shell nanocrystals with thick shells exhibit strongly reduced blinking. Analyzing the photon statistics and lifetime of the on state, we first demonstrate that bright periods correspond to single photon emission with a fluorescence quantum efficiency of the monoexcitonic state greater than 95%. We also show that low intensity emitting periods are not dark but correspond to a grey state, with a fluorescence quantum efficiency of 19%. From these measurements, we deduce the radiative lifetime (45 ns) and the Auger lifetime (10.5 ns) of the grey state.

Coherent particle transfer in an on-demand single-electron source

Electron transfer from a localized state in a quantum dot into a ballistic conductor generally results in particle-hole excitations. We study this effect, considering a resonance level with time-dependent energy coupled to particle states in the Fermi sea. We find that, as the resonance level is driven through the Fermi-level, particle-hole excitations can be suppressed for certain driving protocols. In particular, such noiseless transfer occurs if the level moves with constant rapidity, its energy changing linearly with time. A scheme to study the coherence of particle transfer is proposed.

An on-demand coherent single-electron source

We report on the electron analog of the single-photon gun. On-demand single-electron injection in a quantum conductor was obtained using a quantum dot connected to the conductor via a tunnel barrier. Electron emission was triggered by the application of a potential step that compensated for the dot-charging energy. Depending on the barrier transparency, the quantum emission time ranged from 0.1 to 10 nanoseconds. The single-electron source should prove useful for the use of quantum bits in ballistic conductors. Additionally, periodic sequences of single-electron emission and absorption generate a quantized alternating current.

Spin-photon entangling diode

We propose a semiconductor device that can electrically generate entangled electron spin-photon states, providing a building block for entanglement of distant spins. The device consists of a p-i-n diode structure that incorporates a coupled double quantum dot. We show that electronic control of the diode bias and local gating allow for the generation of single photons that are entangled with a robust quantum memory based on the electron spins. Practical performance of this approach to controlled spin-photon entanglement is analyzed.

Time-resolved double ionization with few cycle laser pulses

Ionization of D2 launches a vibrational wave packet on the ground state of D+2. Removal of the second electron places a pair of D+ ions onto a Coulombic potential. Measuring the D+ kinetic energy determines the time delay between the first and the second ionization. Caught between a falling ionization and a rapidly rising intensity, the typical lifetime of the D+2 intermediate is less than 5 fs when an intense 8.6 fs laser pulse is used. We simulate Coulomb explosion imaging of the ground state wave function of D2 by a 4 fs optical pulse and compare with our experimental observations.

Electrically driven single-photon source

Electroluminescence from a single quantum dot within the intrinsic region of a p-i-n junction is shown to act as an electrically driven single-photon source. At low injection currents, the dot electroluminescence spectrum reveals a single sharp line due to exciton recombination, while another line due to the biexciton emerges at higher currents. The second-order correlation function of the diode displays anti-bunching under a continuous drive current. Single-photon emission is stimulated by subnanosecond voltage pulses. These results suggest that semiconductor technology can be used to mass-produce a single-photon source for applications in quantum information technology.

Photon antibunching in the fluorescence of a single dye molecule embedded in a thin polymer film

We used scanning confocal microscopy to study the fluorescence from a single terrylene molecule embedded in a thin polymer film of polymethyl methacrylate, at room temperature, with a high signal-to-background ratio. The photon-pair correlation function g((2))(tau) exhibits perfect photon antibunching at tau = 0 and a limit of 1.3, compatible with bunching associated with the molecular triplet state. Application of this molecular system to a triggered single-photon source based on single-molecule fluorescence is investigated.

A quantum dot single-photon turnstile device

Quantum communication relies on the availability of light pulses with strong quantum correlations among photons. An example of such an optical source is a single-photon pulse with a vanishing probability for detecting two or more photons. Using pulsed laser excitation of a single quantum dot, a single-photon turnstile device that generates a train of single-photon pulses was demonstrated. For a spectrally isolated quantum dot, nearly 100% of the excitation pulses lead to emission of a single photon, yielding an ideal single-photon source.

Single photons on demand from a single molecule at room temperature

The generation of non-classical states of light is of fundamental scientific and technological interest. For example, 'squeezed' states enable measurements to be performed at lower noise levels than possible using classical light. Deterministic (or triggered) single-photon sources exhibit non-classical behaviour in that they emit, with a high degree of certainty, just one photon at a user-specified time. (In contrast, a classical source such as an attenuated pulsed laser emits photons according to Poisson statistics.) A deterministic source of single photons could find applications in quantum information processing, quantum cryptography and certain quantum computation problems. Here we realize a controllable source of single photons using optical pumping of a single molecule in a solid. Triggered single photons are produced at a high rate, whereas the probability of simultaneous emission of two photons is nearly zero--a useful property for secure quantum cryptography. Our approach is characterized by simplicity, room temperature operation and improved performance compared to other triggered sources of single photons.

Regulated and entangled photons from a single quantum Dot

We propose a new method of generating nonclassical optical field states. The method uses a semiconductor device, which consists of a single quantum dot as active medium embedded in a p- i- n junction and surrounded by a microcavity. Resonant tunneling of electrons and holes into the quantum dot ground states, together with the Pauli exclusion principle, produce regulated single photons or regulated pairs of photons. We propose that this device also has the unique potential to generate pairs of entangled photons at a well-defined repetition rate.