arXiv subjects
Publications and source records attributed to Alexander N. Korotkov.
We develop a theory of quadratic quantum measurements by a mesoscopic detector. It is shown that quadratic measurements should have non-trivial quantum information properties, providing, for instance, a simple way of entangling two non-interacting qubits. We also calculate output spectrum of a quantum detector with both linear and quadratic response continuously monitoring coherent oscillations in two qubits.
We have analyzed numerically the response and noise-limited charge sensitivity of a radio-frequency single-electron transistor (RF-SET) in a non-superconducting state using the orthodox theory. In particular, we have studied the performance dependence on the quality factor Q of the tank circuit for Q both below and above the value corresponding to the impedance matching between the coaxial cable and SET.
We have analyzed the response and noise-limited sensitivity of the radio-frequency single-electron transistor (RF-SET), extending the previously developed theory to the case of arbitrary large quality factor Q of the RF-SET tank circuit. It is shown that while the RF-SET response reaches the maximum at Q roughly corresponding to the impedance matching condition, the RF-SET sensitivity monotonically worsens with the increase of Q. Also, we propose a novel operation mode of the RF-SET, in which an overtone of the incident rf wave is in resonance with the tank circuit.
The Bayesian formalism for a continuous measurement of solid-state qubits is derived for a model which takes into account several factors of the detector nonideality. In particular, we consider additional classical output and backaction noises (with finite correlation), together with quantum-limited output and backaction noises, and take into account possible asymmetry of the detector coupling. The formalism is first derived for a single qubit and then generalized to the measurement of entangled qubits.
We discuss a recently developed formalism which describes the quantum evolution of a solid-state qubit due to its continuous measurement. In contrast to the conventional ensemble-averaged formalism, it takes into account the measurement record and therefore is able to consider individual realizations of the measurement process. The formalism provides testable experimental predictions and can be used for the analysis of a quantum feedback control of solid-state qubits. We also discuss generalization of the Bayesian formalism to the continuous measurement of entangled qubits.
We show that two identical solid-state qubits can be made fully entangled (starting from completely mixed state) with probability 1/4 just measuring them by a detector, equally coupled to the qubits. This happens in the case of repeated strong (projective) measurements as well as in a more realistic case of weak continuous measurement. In the latter case the entangled state can be identified by a flat spectrum of the detector shot noise, while the non-entangled state (probability 3/4) leads to a spectral peak at the Rabi frequency with the maximum peak-to-pedestal ratio of 32/3.
Various aspects of single-electron memory are discussed. In particular, we analyze the single-electron charging by Fowler-Nordheim tunneling, propose the idea of background charge compensation, and discuss the defect-tolerant architecture based on nanofuses.
We have studied theoretically the basic operation of a quantum feedback loop designed to maintain a desired phase of quantum coherent oscillations in a single solid-state qubit. The degree of oscillations synchronization with external harmonic signal is calculated as a function of feedback strength, taking into account available bandwidth and coupling to environment. The feedback can efficiently suppress the dephasing of oscillations if the qubit coupling to the detector is stronger than coupling to environment.
We have developed a formalism suitable for calculation of the output spectrum of a detector continuously measuring quantum coherent oscillations in a solid-state qubit, starting from microscopic Bloch equations. The results coincide with that obtained using Bayesian and master equation approaches. The previous results are generalized to the cases of arbitrary detector response and finite detector temperature.
We have developed Bayesian formalism to describe the process of continuous measurement of entangled qubits. We start with the case of two qubits and then generalize it to an arbitrary number of qubits.
We have studied theoretically the basic operation of a quantum feedback loop designed to maintain the desired phase of quantum coherent oscillations in a two-level system. Such feedback can suppress the dephasing of oscillations due to interaction with environment. Prospective experiments can be realized using metallic single-electron devices or GaAs technology.
We have used Monte Carlo simulation to calculate the shot noise intensity $S_I(ω)$ at 2D hopping using two models: a slanted lattice of localized sites with equal energies and a set of localized sites with random positions and energies. For wide samples we have found a similar dependence of the Fano factor $F \equiv S_I(0)/2eI$ on the sample length $L$: $F \propto L^{-α}$ where $α=0.85 \pm 0.02$ and $α= 0.85\pm 0.07$ in uniform and random models, respectively. Moreover, at least for the uniform model, all the data for $F$ as the function of sample length $L$ and width $W$ may be presented via a single function of the ratio $W/L^β$, with $β= 2α-1 \approx 0.7$. This relation has been interpreted using a simple scaling theory.
Numerical modeling of correlated single-electron tunneling in uniform 2D arrays of small conducting islands separated by tunnel junctions shows the possiblility of soliton-antisoliton avalanches. Though the time duration of any avalanche, and the total charge $ΔQ = ne$ transferred across the array during the avalanche, are always finite, in arrays with length $N$ larger than certain critical value $N_c$ and large width $M \gg f(N)$, the avalanche magnitude $n$ may be exponentially large, resulting in particular in a giant increase of shot noise. Thermal fluctuations and disorder gradually suppress the avalanche effect.
We consider a two-level quantum system (qubit) which is continuously measured by a detector. The information provided by the detector is taken into account to describe the evolution during a particular realization of measurement process. We discuss the Bayesian formalism for such ``selective'' evolution of an individual qubit and apply it to several solid-state setups. In particular, we show how to suppress the qubit decoherence using continuous measurement and the feedback loop.
We propose a solid-state experiment to study the process of continuous quantum measurement of a qubit state. The experiment would verify that an individual qubit stays coherent during the process of measurement (in contrast to the gradual decoherence of the ensemble-averaged density matrix) thus confirming the possibility of the qubit purification by continuous measurement. The experiment can be realized using quantum dots, single-electron transistors, or SQUIDs.
The average current and the shot noise at correlated sequential tunneling via two localized sites are studied. At zero temperature the Fano factor averaged over the positions and energies of sites is shown to be 0.707. The noise dependence on temperature and frequency is analyzed numerically.
We consider a two-level quantum system (qubit) which is continuously measured by a detector and calculate the spectral density of the detector output. In the weakly coupled case the spectrum exhibits a moderate peak at the frequency of quantum oscillations and a Lorentzian-shape increase of the detector noise at low frequency. With increasing coupling the spectrum transforms into a single Lorentzian corresponding to random jumps between two states. We prove that the Bayesian formalism for the selective evolution of the density matrix gives the same spectrum as the conventional master equation approach, despite the significant difference in interpretation. The effects of the detector nonideality and the finite-temperature environment are also discussed.
We have carried out a preliminary analysis of shot noise at hopping, focusing on uniform 1D arrays of sites separated by N tunnel barriers. The results show that at low temperatures the low-frequency density of the shot noise varies from 1/N to 1 of the Schottky value, depending on the geometry, electron density, and Coulomb interaction strength. An interesting feature is w^{-1/3} dependence of the current spectral density at intermediate frequencies, which reflects self-similarity of the fluctuations at different size scales.