Electric-field-induced coherent coupling of the exciton states in a single quantum dot

The signature of coherent coupling between two quantum states is an anticrossing in their energies as one is swept through the other. In single semiconductor quantum dots containing an electron-hole pair the eigenstates form a two-level system that can be used to demonstrate quantum effects in the solid state, but in all previous work these states were independent. Here we describe a technique to control the energetic splitting of these states using a vertical electric field, facilitating the observation of coherent coupling between them. Near the minimum splitting the eigenstates rotate in the plane of the sample, being orientated at 45{\deg} when the splitting is smallest. Using this system we show direct control over the exciton states in one quantum dot, leading to the generation of entangled photon pairs

Ultrafast optical control of entanglement between two quantum dot spins

The interaction between two quantum bits enables entanglement, the two-particle correlations that are at the heart of quantum information science. In semiconductor quantum dots much work has focused on demonstrating single spin qubit control using optical techniques. However, optical control of entanglement of two spin qubits remains a major challenge for scaling from a single qubit to a full-fledged quantum information platform. Here, we combine advances in vertically-stacked quantum dots with ultrafast laser techniques to achieve optical control of the entangled state of two electron spins. Each electron is in a separate InAs quantum dot, and the spins interact through tunneling, where the tunneling rate determines how rapidly entangling operations can be performed. The two-qubit gate speeds achieved here are over an order of magnitude faster than in other systems. These results demonstrate the viability and advantages of optically controlled quantum dot spins for multi-qubit systems.Comment: 24 pages, 5 figure

Growth of Low-Density Vertical Quantum Dot Molecules with Control in Energy Emission

In this work, we present results on the formation of vertical molecule structures formed by two vertically aligned InAs quantum dots (QD) in which a deliberate control of energy emission is achieved. The emission energy of the first layer of QD forming the molecule can be tuned by the deposition of controlled amounts of InAs at a nanohole template formed by GaAs droplet epitaxy. The QD of the second layer are formed directly on top of the buried ones by a strain-driven process. In this way, either symmetric or asymmetric vertically coupled structures can be obtained. As a characteristic when using a droplet epitaxy patterning process, the density of quantum dot molecules finally obtained is low enough (2 × 108 cm−2) to permit their integration as active elements in advanced photonic devices where spectroscopic studies at the single nanostructure level are required

Optical control of one and two hole spins in interacting quantum dots

A single hole spin in a semiconductor quantum dot has emerged as a quantum bit that is potentially superior to an electron spin. A key feature of holes is that they have a greatly reduced hyperfine interaction with nuclear spins, which is one of the biggest difficulties in working with an electron spin. It is now essential to show that holes are viable for quantum information processing by demonstrating fast quantum gates and scalability. To this end we have developed InAs/GaAs quantum dots coupled through coherent tunneling and charged with controlled numbers of holes. We report fast, single qubit gates using a sequence of short laser pulses. We then take the important next step toward scalability of quantum information by optically controlling two interacting hole spins in separate dots.Comment: 5 figure

Quantum dots conjugated E. coli living cells as fluorescent reporters to detect cytotoxicity of chemicals

Quantum dots (QDs) have attracted a lot of interest for imaging, diagnostics and therapy due to their bright, stable fluorescence. Surface activated QDs can be conjugated to a variety of bio-active molecules for binding to bacteria or mammalian cells for various applications. In the current work we report on; (a) controlled bioconjugation of carboxylic-CdTe QDs with gram negative E. coli and (b) determining the effect of fluorescent properties of QD-E.colil bioconjugates upon exposure to toxic chemicals. For this, first the cells were bioconjugated with non-toxic, capped and water-soluble CdSe quantum dots (QD). Model toxic chemicals, such as pesticide (paraquat) and oxidative stress inducing chemical (H2O2) at concentrations ranging 0.5-5 mM were incubated with QD-E.coli bioconjugates. When these chemicals interacted with cells, the behavior of fluorescent QD-E.coli bioconjugates was altered by the stress imposed by chemicals and thus the fluorescent ability of QD-E.coli bioconjugate diminished with time and concentration of toxic chemicals. This stress is attributed to the damages occurred as a result of interaction of toxic chemicals to cell-wall or membrane of QD-E.coli bioconjugate and therefore, florescence signal was lost (Figs. 1 and 2.) The loss in fluorescence (signal off phenomena) of QD-E.coli bioconjugates can be used as probes to develop a variety of fluorescence based detection kits for the rapid determination of toxic drugs or food sample testing

Quantum dots conjugated E. coli living cells as fluorescent reporters to detect cytotoxicity of chemicals

Quantum dots (QDs) have attracted much of research interest in recent years for imaging, diagnostics, and therapy due to their unique optical properties, such as broad excitation spectra and long fluorescence stability. In this study, a controlled bioconjugation using CdTe QDs with gram negative E. coli cells was performed to develop QD-E. coli bioconjugates. These bioconjugates were used as whole cell living baits to determine cytotoxicity of model toxic chemicals, such as oxidative stress inducer (H2O2) and a pesticide (methyl viologen or paraquat). These chemicals over a wide concentration ranges were exposed to QD-E. coli bioconjugates that interacted with cells and the real time fluorescence responses, with QD-E. coli bioconjugates, were analyzed. The results showed that the fluorescent ability of QD-E. coli bioconjugates tend to diminish with increasing concentration of toxic chemicals. This stress is attributed to the damages occurred as a result of interaction of toxic chemicals to the cell-wall or membrane of cells that resulted in the loss of fluorescence signal. This loss in the fluorescence (signal off phenomena) of QD-E. coli bioconjugates can be used as probes to develop a variety of fluorescence-based detection kits for the rapid determination of toxic drugs or food sample testing

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 bit1. Spins in optically active quantum dots offer one potential realization2,3 and recent demonstrations have shown high-fidelity preparation4,5 and ultrafast coherent manipulation6,7,8. 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 reported9,10,11,12. 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 molecule13,14,15, we present the ability to measure the spin state of a single electron in real time via the intermittency of quantum dot resonance fluorescence12,16. 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