arXiv subjects

Leonid Fedichkin

Publications and source records attributed to Leonid Fedichkin.

12 recordsLinked to original sources

arXiv2026

Quantum register based on double quantum dots in semiconductor nanowires

An implementation of a universal solid-state quantum register based on electron space states in field-defined double quantum dots (a DQD possesses one electron in two adjacent tunnel bound dots) in an ultrathin semiconductor wire is discussed. To some extent, the structure resembles that of a field-effect transistor with multiple controlling electrodes (gates). Scalability is audible and it opens up a possibility of large-scale universal quantum computer fabricated by advanced silicon technology. Moreover, the structure could be developed into an ensemble quantum register where an array of nanowires with common controlling electrodes and contacts is fabricated. That register is much more resistant against environment noise. It is crucial that an individual qubit consists of two DQDs. The quantum information is encoded and processed inside the Hilbert subspace without charge transfer between dots. The filling factor of each quantum dot is permanently equal to 0.5. This guarantees a linear dynamics of qubits necessary for now existing quantum algorithms. Worth noting, the dynamics of qubits with altering charge state is more or less nonlinear due to interaction with surrounding dielectrics and metals (polaron effect). The basic two-qubit operations in the system are SWAP and sqrtSWAP. The latter operation is universal as well as CNOT. The two-qubit operations are performed by Coulomb interaction. Although that kind of interaction is incessant, the strength of its action depends on mutual states of interacting DQDs (in-resonance or off-resonance). In the proposed register any quantum algorithm could be effectuated via manipulation solely with digital voltage pulses on controlling electrodes that reminds a functioning of an integrated circuit. The final read-out of the register is performed after decoding into charge states of DQDs and a transmission of current through the wire.

cond-mat.mes-hall↗

arXiv2025

Dynamics of quantum measurement via electron transport in quantum dot systems: many-particle wavefunction approach

Measurement of a charge qubit via point contacts with complex internal structures is considered. In this context, a fully formalized derivation of the many-body wave function method is presented, together with the corresponding master equations for point contacts possessing an arbitrary number of internal states. The focus is placed on the current noise power spectrum and its dependence on the qubit dynamics and the point contact parameters.

cond-mat.mes-hall↗

arXiv2025

High-dimensional graphs convolution for quantum walks photonic applications

Quantum random walks represent a powerful tool for the implementation of various quantum algorithms. We consider a convolution problem for the graphs which provide quantum and classical random walks. We suggest a new method for lattices and hypercycle convolution that preserves quantum walk dynamics. Our method is based on the fact that some graphs represent a result of Kronecker's product of line graphs. We support our methods by means of various numerical experiments that check quantum and classical random walks on hypercycles and their convolutions. Our findings may be useful for saving a significant number of qubits required for algorithms that use quantum walk simulation on quantum devices.

quant-ph↗

arXiv2025

Stabilization of industrial processes with time series machine learning

The stabilization of time series processes is a crucial problem that is ubiquitous in various industrial fields. The application of machine learning to its solution can have a decisive impact, improving both the quality of the resulting stabilization with less computational resources required. In this work, we present a simple pipeline consisting of two neural networks: the oracle predictor and the optimizer, proposing a substitution of the point-wise values optimization to the problem of the neural network training, which successfully improves stability in terms of the temperature control by about 3 times compared to ordinary solvers.

cs.LG↗

arXiv2023

An exponentially-growing family of universal quantum circuits

Quantum machine learning has become an area of growing interest but has certain theoretical and hardware-specific limitations. Notably, the problem of vanishing gradients, or barren plateaus, renders the training impossible for circuits with high qubit counts, imposing a limit on the number of qubits that data scientists can use for solving problems. Independently, angle-embedded supervised quantum neural networks were shown to produce truncated Fourier series with a degree directly dependent on two factors: the depth of the encoding and the number of parallel qubits the encoding applied to. The degree of the Fourier series limits the model expressivity. This work introduces two new architectures whose Fourier degrees grow exponentially: the sequential and parallel exponential quantum machine learning architectures. This is done by efficiently using the available Hilbert space when encoding, increasing the expressivity of the quantum encoding. Therefore, the exponential growth allows staying at the low-qubit limit to create highly expressive circuits avoiding barren plateaus. Practically, parallel exponential architecture was shown to outperform the existing linear architectures by reducing their final mean square error value by up to 44.7% in a one-dimensional test problem. Furthermore, the feasibility of this technique was also shown on a trapped ion quantum processing unit.

quant-ph↗

arXiv2005

Non-Unitary Quantum Walks on Hyper-Cycles

We present analytical treatment of quantum walks on multidimensional hyper-cycle graphs. We derive the analytical expression of the probability distribution for strong and weak decoherence regimes. Upper bound to mixing time is obtained.

quant-ph↗

arXiv2005

Mixing and Decoherence in Continuous-Time Quantum Walks on Cycles

We prove analytical results showing that decoherence can be useful for mixing time in a continuous-time quantum walk on finite cycles. This complements the numerical observations by Kendon and Tregenna (Physical Review A 67 (2003), 042315) of a similar phenomenon for discrete-time quantum walks. Our analytical treatment of continuous-time quantum walks includes a continuous monitoring of all vertices that induces the decoherence process. We identify the dynamics of the probability distribution and observe how mixing times undergo the transition from quantum to classical behavior as our decoherence parameter grows from zero to infinity. Our results show that, for small rates of decoherence, the mixing time improves linearly with decoherence, whereas for large rates of decoherence, the mixing time deteriorates linearly towards the classical limit. In the middle region of decoherence rates, our numerical data confirms the existence of a unique optimal rate for which the mixing time is minimized.

quant-ph↗

arXiv2005

Continuous-time Quantum Walks on a Cycle Graph

We present analytical treatment of quantum walks on a cycle graph. The investigation is based on a realistic physical model of the graph in which decoherence is induced by continuous monitoring of each graph vertex with nearby quantum point contact. We derive the analytical expression of the probability distribution along the cycle. Upper bound estimate to mixing time is shown.

quant-ph↗

arXiv2004

Measures of Decoherence

Methods for quantifying environmentally induced decoherence in quantum systems are investigated. We formulate criteria for measuring the degree of decoherence and consider several representative examples, including a spin interacting with the modes of a bosonic, e.g., phonon, bath. We formulate an approach based on the operator norm measuring the deviation of the actual density matrix from the ideal one which would describe the system without environmental interactions.

cond-mat.mes-hall↗

arXiv2004

Evaluation of Decoherence for Quantum Control and Computing

Different approaches in quantifying environmentally-induced decoherence are considered. We identify a measure of decoherence, derived from the density matrix of the system of interest, that quantifies the environmentally induced error, i.e., deviation from the ideal isolated-system dynamics. This measure can be shown to have several useful features. Its behavior as a function of time has no dependence on the initial conditions, and is expected to be insensitive to the internal dynamical time scales of the system, thus only probing the decoherence-related time dependence. For a spin-boson model - a prototype of a qubit interacting with environment - we also demonstrate the property of additivity: in the regime of the onset of decoherence, the sum of the individual qubit error measures provides an estimate of the error for a several-qubit system, even if the qubits are entangled, as expected in quantum-computing applications. This makes it possible to estimate decoherence for several-qubits quantum computer gate designs for which explicit calculations are exceedingly difficult.

cond-mat.mes-hall↗

arXiv2003

Additivity of decoherence measures for multiqubit quantum systems

We introduce new measures of decoherence appropriate for evaluation of quantum computing designs. Environment-induced deviation of a quantum system's evolution from controlled dynamics is quantified by a single numerical measure. This measure is defined as a maximal norm of the density matrix deviation. We establish the property of additivity: in the regime of the onset of decoherence, the sum of the individual qubit error measures provides an estimate of the error for a several-qubit system. This property is illustrated by exact calculations for a spin-boson model.

cond-mat.mes-hall↗

arXiv2002

Quantum Computer with Fixed Interaction is Universal

It is proved that a quantum computer with fixed and permanent interaction of diagonal type between qubits proposed in the work quant-ph/0201132 is universal. Such computer is controlled only by one-qubit quick transformations, and this makes it feasible.

quant-ph↗