Thermal stability of uni-size Pt cluster disk constructed on silicon substrate

We propose a scheme to implement controlled not gate for topological qubits in a quantum-dot and Majorana fermion hybrid system. Quantum information is encoded on pairs of Majorana fermions, which live on the the interface between topologically trivial and nontrivial sections of a quantum nanowire deposited on an s-wave superconductor. A measurement based two-qubit controlled not gate is produced with the help of parity measurements assisted by the quantum-dot and followed by prescribed single-qubit gates. The parity measurement, on the quantum-dot and a topological qubit, is achieved by the Aharonov-Casher effect.     

Effect of substrate misorientation on quantum-dot size distribution in the InAs/GaAs system

A study is reported of the effect of (001)GaAs substrate misorientation in the [010] direction on the distribution of MBE-grown self-assembled InAs/GaAs quantum dots in size and position in the GaAs matrix. Temperature-induced narrowing of the exciton photoluminescence line of a quantum-dot ensemble caused by redistribution of photoexcited carriers among dots of different size has been observed. Fiz. Tverd. Tela (St. Petersburg) 40, 855–857 (May 1998)     

Optical orientation and alignment of excitons in quantum dots

Optical orientation and alignment of excitons in InAlAs quantum dots in the AlGaAs matrix have been studied both theoretically and experimentally. Experiments performed in a longitudinal magnetic field (Faraday geometry) reveal transformation of optical orientation to alignment and alignment to orientation, which is caused by exchange splitting of the dipole-active exciton doublet and allowed by the quantum-dot low symmetry. A comparison of theory with experiment made with inclusion of the anisotropy of exciton generation and recombination along the and [110] axes permits one to determine the character of dipole distribution in direction for resonant optical transitions in the self-organized quantum-dot ensemble studied. Fiz. Tverd. Tela (St. Petersburg) 40, 858–861 (May 1998)     

Solitons in semiconductor microstructures with a two-dimensional electron gas

Nonlinear waves in a two-dimensional electronic plasma with metal screening gates are investigated. It is shown that solitons described by the KdV equation exist in such a system. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 7, 479–481 (10 October 1999)     

Spontaneous far-IR emission accompanying transitions of charge carriers between levels of quantum dots

The spontaneous emission of far-infrared radiation (λ≅10–20 μm) from diode structures with vertically coupled InGaAs/AlGaAs quantum dots is observed. This emission is due both to transitions of holes and electrons between size-quantization levels in quantum dots and to transitions from the continuum to a level in a quantum dot. It is observed only when accompanied by lasing at short wavelengths (λ≅0.94 μm) and, like the short-wavelength emission, it exhibits a current threshold. The spontaneous emission of long-wavelength radiation is also observed in InGaAs/GaAs quantum-well laser structures. This radiation is approximately an order of magnitude weaker than that from quantum-dot structures, and it has no current threshold. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 256–260 (25 February 1998)     

On the design of all-optical gates based on quantum-dot semiconductor optical amplifier with effect of amplified spontaneous emission

The performances of all-optical logic XOR, AND and OR gates have been simulated. These Boolean operations are realized by using a Mach–Zehnder interferometer utilizing semiconductor optical amplifier (SOA) with quantum-dot (QD) active region. The study is carried out when the effect of amplified spontaneous emission is taken into account in the simulation analysis. Results show that these logic gates are capable of operating at a data speed of 250 Gb/s with high output quality factor ( \(Q\) -factor). The dependence of the output \(Q\) -factor on signals and QD–SOA parameters is also investigated and discussed.     

Measurement of the state of an individual spin using a “turnstile”

A scheme is proposed for measuring the state of an individual spin (system of spins). The scheme is based on the idea of a single-electron “turnstile” and the injection of single spin-polarized electrons from the magnetic metallic borders. Applications to the recently proposed scheme of quantum spin gates based on a silicon matrix are discussed (B. E. Kane, Nature 393, 133 (1998)). Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 141–147 (25 July 1999)     

Electron spin beats in InGaAs/GaAs quantum dots

Time-resolved picosecond spectroscopy is used for the first time to study optical orientation and spin dynamics of carriers in self-organized In(Ga)As/GaAs quantum-dot (QD) arrays. Optical orientation of carriers created by 1.2 ps light pulses, both in the GaAs matrix and wetting layer, and captured by QDs is found to last a few hundreds of picosecond. The saturation of electron ground state at high-excitation-light intensity leads to electron polarization in excited states close to 100% and to its vanishing in ground state. Electron-spin quantum beats in a transverse magnetic field are observed for the first time in semiconductor QDs. We thus determine the quasi-zero-dimensional electron g factor in In_0.5Ga_0.5As/GaAs QDs to be: |g _⊥|=0.27±0.03. Fiz. Tverd. Tela (St. Petersburg) 41, 871–874 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Measurement and analysis of optical gain spectra in 1.6 to 1.8 μm InAs/InP (100) quantum-dot amplifiers

Small signal modal gain measurements have been performed on two-section ridge waveguide InAs/InP (100) quantum-dot amplifiers that we have fabricated with a peak gain wavelength around 1.70 μm. The amplifier structure is suitable for monolithic active-passive integration, and the wavelength region and wide gain bandwidth are of interest for integrated devices in biophotonic applications. A 65 nm blue shift of the peak wavelength in the gain spectrum has been observed with an increase in injection current density from 1,000 to 3,000 A/cm². The quantum-dot amplifier gain spectra have been analyzed using a quantum-dot rate-equation model that considers only the carrier dynamics. The comparison between measured and simulated spectra shows that two effects in the quantum-dot material introduce this large blue shift in the gain spectrum. The first effect is the carrier concentration dependent state filling with carriers of the bound excited and ground states in the dots. The second effect is the decrease in carrier escape time from the dots to the wetting layer with decreasing dot size.     

Two-Photon Absorption in Nanoheterostructures with <Emphasis Type="Italic">D</Emphasis><Superscript>(−)</Superscript>-Centers

A two-photon impurity absorption coefficient of the “quantum dot — D⁽⁻⁾-center” complexes synthesized in a transparent dielectric matrix is calculated within the model of zero-radius potential. The evolution of spectral dependence of the absorption coefficient of the nanoheterostructure based on semiconductor CdS _x Se _1−x glasses is studied versus the average quantum-dot radius. It is shown that the contribution of the two-photon impurity absorption to the exited two-photon luminescence is fairly significant at a reasonable quantum-dot concentration. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 46–50, July, 2005.     

Low-frequency plasmons in coupled electronic microstructures

The plasmon spectrum and the absorption of an electromagnetic wave in an electronic two-dimensional plasma with strongly modulated density are studied. The appearance of additional plasmon modes in a system of electronic wires and islands with weak current coupling is described in a model of an electronic system covered with metallic gates. Such plasmon modes appear in the low-frequency region of the spectrum, as compared with the conventional plasma oscillations, and have recently been observed experimentally in paired wires. Fiz. Tverd. Tela (St. Petersburg) 40, 542–545 (March 1998)     

Investigation of the effect of larger monolayer coverage in the active layer of bilayer InAs/GaAs quantum-dot structure and effects of post-growth annealing

Morphological and optical properties of MBE-grown single and bilayer InAs/GaAs quantum-dot heterostructure with comparatively larger monolayer coverage (∼3.3 ML) are investigated. As compared to the similar single-layer quantum-dot (SQD) structure, the bilayer quantum-dot (BQD) structure showed a more uniform spatial distribution and increased size homogeneity of the dots. It also exhibited longer wavelength photoluminescence (PL) emission at room temperature, with the peak at a wavelength (1.34 μm) in the infrared communication window. The shift seen in the low temperature PL emission peak, even after annealing the BQD samples at temperatures up to 700°C, is negligible. However, the BQD samples showed an activation energy lower than that of the SQD sample, attributed to the In/Ga interdiffusion with the thin (8.5 nm thick) GaAs spacer layer during the growth process. Despite that, the higher thermal stability demonstrated by the BQD structure, along with a lower linewidth, makes the bilayer quantum dots a desirable proposition for efficient lasing devices requiring strict tolerances on operating wavelength.     

Parameter-dependent optical nutation in PbSe/CdSe/ZnS quantum-dot

The eigenenergies and eigenfunctions of the spherical PbSe/CdSe/ZnS quantum-dot with the core-shell-shell structure and the electric dipole moment between the a _1s state (the lowest state of the system) and b _1p state (the lowest p state above the first potential) have been calculated by using the rotational wave approximation and the effective mass approximation. The optical nutation signal induced by the transition between these two energy levels has been calculated based on the optical Bloch equations. In particular, the influence of the core’s radius increasing and the CdSe shell’s thickness increasing has been investigated respectively. It is shown from the numerical calculation results that the optical nutation signals are dependent on the size and the structure of the quantum-dot. Moreover, the quantum size-dependent Rabi frequency has been analyzed.     

Quantum computation with quantum-dot spin qubits inside a cavity

Universal set of quantum gates are realized from quantum-dot spin qubits inside a cavity via two-channel Raman interactions. Individual addressing and effective switch of the cavity mediated interaction are directly possible here. This simple realization of all wanted interaction for selective qubits makes current scenario more suitable for scalable quantum computation.     

Towards optimization of quantum circuits

Any unitary operation in quantum information processing can be implemented via a sequence of simpler steps — quantum gates. However, actual implementation of a quantum gate is always imperfect and takes a finite time. Therefore, searching for a short sequence of gates — efficient quantum circuit for a given operation, is an important task. We contribute to this issue by proposing optimization of the well-known universal procedure proposed by Barenco et al. [Phys. Rev. A 52, 3457 (1995)]. We also created a computer program which realizes both Barenco’s decomposition and the proposed optimization. Furthermore, our optimization can be applied to any quantum circuit containing generalized Toffoli gates, including basic quantum gate circuits.      

Tunneling-induced π-phase shift of a quantum-dot molecule coupled to a single-sided cavity

We study optical dispersive properties of a combined system which is composed of a single-sided cavity with low quality factor Q and a quantum-dot molecule (QDM) embedded in cavity. It is shown that, with a voltage-controlled tunneling, an input signal after reflection from cavity experiences a π phase shift while not suffering from absorption in the experimentally available parameter range.     

Comment on “Random Quantum Circuits are Approximate 2-designs” by A.W. Harrow and R.A. Low (Commun. Math. Phys. 291, 257–302 (2009))

In [A.W. Harrow and R.A. Low, Commun. Math. Phys. 291(1):257–302 (2009)], it was shown that a quantum circuit composed of random 2-qubit gates converges to an approximate quantum 2-design in polynomial time. We point out and correct a flaw in one of the paper’s main arguments. Our alternative argument highlights the role played by transpositions induced by the random gates in achieving convergence.     

Realization of arbitrary two-qubit quantum gates based on chiral Majorana fermions

Quantum computers are in hot-spot with the potential to handle more complex problems than classical computers can.Realizing the quantum computation requires the universal quantum gate set {T,H,CNOT} so as to perform any unitary transformation with arbitrary accuracy.Here we first briefly review the Majorana fermions and then propose the realization of arbitrary two-qubit quantum gates based on chiral Majorana fermions.Elementary cells consist of a quantum anomalous Hall insulator surrounded by a topological superconductor with electric gates and quantum-dot structures,which enable the braiding operation and the partial exchange operation.After defining a qubit by four chiral Majorana fermions,the singlequbit T and H quantum gates are realized via one partial exchange operation and three braiding operations,respectively.The entangled CNOT quantum gate is performed by braiding six chiral Majorana fermions.Besides,we design a powerful device with which arbitrary two-qubit quantum gates can be realized and take the quantum Fourier transform as an example to show that several quantum operations can be performed with this space-limited device.Thus,our proposal could inspire further utilization of mobile chiral Majorana edge states for faster quantum computation.     

Remote Implementation of Multi-qubit Quantum Phase Gates

Based on Wu et al.’s original idea (Phys. Lett. A 372:2802, 2008), we propose a scheme to remotely implement multi-qubit quantum phase gates. With the assistance of entanglement swapping, classical communication and quantum repeater, multi-qubit quantum phase gates can be realized perfectly nearly. It is emphasized that our proposal can overcome the limitation that error probability scales exponentially with the length of the channel during the realization of the gates.