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Signatures of charge noise and its impact on exciton qubits

Vishal Purohit

Published Jun 1, 2016• 0 citations
OpenAlex

Abstract

The research contained within this thesis concerns the detection, identification and \neffect of charge noise on quantum dot systems. \nIn the first research chapter we study the cross correlation between pairs of exciton \nqubits subject to a common fluctuating charge environment, whose dynamics \nare solved using a transfer matrix approach. Our results show that we are able to \ndiscern features showing whether or not the charges interact with both quantum \ndots simultaneously i.e., form a correlated noise source. We find that qubits in a \ncommon charge environment display photon bunching, if both dots are driven on \nresonance or if the laser detunings are equal in both qubits and anitibunching if the \nlaser detunings are in opposite directions. \nIn the second research chapter we study the auto-correlation function of a single \noptically driven exciton qubit interacting with an environment consisting of 1/f \nnoise and a fluctuating charge. We again use the transfer matrix method and a \nsum of Lorentzian distributions to approximate 1/f noise. Our simulations show \nthat signatures of 1/f noise do exist in photon correlation measurements. From such \nmeasurements we are also able to determine a minimum cut-off frequency of the 1/f \nnoise, in the case that there is such a cut-off. In addition we also show that a 1/f \nand a single fluctuator can be distinguished using the auto-correlation. \nIn the final research chapter we study a pair of quantum dots, each with a low \nlying electron spin qubit and one higher lying level that can be selectively optically \nexcited from one of the two spin states. Entanglement between the two spins can be \nachieved through path erasure. We look at the effect of a single fluctuating charge \nof the entanglement between these two `L' shaped electronic structures.

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arXiv:cond-mat/0207597•Applied Physics Letters•Jan 23, 2003•42 citations
The research contained within this thesis concerns the detection, identification and \neffect of charge noise on quantum dot systems. \nIn the first research chapter we study the cross correlation between pairs of exciton \nqubits subject to a common fluctuating charge environment, whose dynamics \nare solved using a transfer matrix approach. Our results show that we are able to \ndiscern features showing whether or not the charges interact with both quantum \ndots simultaneously i.e., form a correlated noise source. We find that qubits in a \ncommon charge environment display photon bunching, if both dots are driven on \nresonance or if the laser detunings are equal in both qubits and anitibunching if the \nlaser detunings are in opposite directions. \nIn the second research chapter we study the auto-correlation function of a single \noptically driven exciton qubit interacting with an environment consisting of 1/f \nnoise and a fluctuating charge. We again use the transfer matrix method and a \nsum of Lorentzian distributions to approximate 1/f noise. Our simulations show \nthat signatures of 1/f noise do exist in photon correlation measurements. From such \nmeasurements we are also able to determine a minimum cut-off frequency of the 1/f \nnoise, in the case that there is such a cut-off. In addition we also show that a 1/f \nand a single fluctuator can be distinguished using the auto-correlation. \nIn the final research chapter we study a pair of quantum dots, each with a low \nlying electron spin qubit and one higher lying level that can be selectively optically \nexcited from one of the two spin states. Entanglement between the two spins can be \nachieved through path erasure. We look at the effect of a single fluctuating charge \nof the entanglement between these two `L' shaped electronic structures.
We study a pair of quantum dot exciton qubits interacting with a number of fluctuating charges that can induce a Stark shift of both exciton transition energies. We do this by solving the optical master equation using a numerical transfer matrix method. We find that the collective influence of the charge environment on the dots can be detected by measuring the correlation between the photons emitted when each dot is driven independently. Qubits in a common charge environment display photon bunching, if both dots are driven on resonance or if the driving laser detunings have the same sense for both qubits, and antibunching if the laser detunings have opposite signs. We also show that it is possible to detect several charges fluctuating at different rates using this technique. Our findings expand the possibility of measuring qubit dynamics in order to investigate the fundamental physics of the environmental noise that causes decoherence.
The spin of an electron confined in semiconductor quantum dots is currently a promising candidate for quantum bit (qubit) implementations. Taking advantage of the existing CMOS integration technologies, such devices can offer a platform for large scale quantum computation. However, a quantum mechanical framework bridging a device's physical design and operational parameters to the qubit's energy space is lacking. Furthermore, the spin to charge coupling introduced by intrinsic or induced spin-orbit interaction (SOI) exposes the qubits to charge noise compromising their coherence properties and inducing quantum gate errors. We present here a comodeling framework for double quantum dot (DQD) devices and their charge noise environments. We use a combination of an electrostatic potential solver, full configuration interaction quantum mechanical methods, and two-level-fluctuator models to study the quantum gate performance in realistic device designs and operation conditions. We utilize the developed models together alongside the single electron solutions of the quantum dots to simulate one- and two-qubit gates in the presence of charge noise. We find an inverse correlation between quantum gate errors and quantum dot confinement frequencies. We calculate X-gate fidelities >97>97%>97 in the simulated Si-MOS devices at a typical TLF density of ${10}^{11}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. We also find that exchange driven two-qubit SWAP gates show higher sensitivity to charge noise with fidelities down to 9191%91 in the presence of the same density of TLFs. We further investigate the one- and two-qubit gate fidelities at different TLF densities. We find that given the small size of the quantum dots, sensitivity of a quantum gate to the distance between the noise sources and the quantum dot creates a strong variability in the quantum gate fidelities, which can compromise the device yields in scaled qubit technologies.
Cross-correlated charge noise in silicon quantum dots has emerged as an important issue for fault-tolerant quantum computation, as it has been suggested to both limit quantum error correction performance and reveal unique information about noise sources. We develop a method to characterize charge noise cross-correlations in a semiconductor qubit device based solely on quantum-dot transport current. By dynamically switching the site-selective sensitivity of transport current through a double quantum dot to charge noise, we extract potential fluctuations at individual dots without requiring additional sensing devices or spin operations. Based on the obtained power spectral densities, we discuss the coupling of a dominant two-level fluctuator to individual dots. We furthermore verify the consistency of the measurement protocol by applying artificial noise via gate electrodes and investigate the temperature dependence of charge noise correlations. The method offers a convenient and versatile alternative approach that substantially facilitates the assessment of charge noise correlations in diverse semiconductor qubit devices.
The work contained in this thesis deals with two closely related topics. Papers I-III deal with charge transport statistics in nanoscale devices to investigate the presence of entanglement in the constituent electrons. We identify the contributions of entangled electron pairs to the current cross-correlations as a signature of entanglement. Paper IV treats the electric control of a quantum dot based spin qubit via Electric Dipole Spin Resonance.In Paper I, we present a spin entanglement detection model in a hybrid nanoscale device, with a general entangler connected to a set of quantum dots and anti-collinear ferromagnetic contacts. Our model allows us to study the implications of a non-ideal detector in an entanglement detection scheme.In Paper II, we investigate a scheme for generating and detecting orbitally entangled states in a device consisting of six quantum dots. The underlying process which enables our proposed scheme is sufficiently fast, even for the rapidly decaying quantum coherence of orbital states defined in a solid.In Paper III, we investigate a minimal entanglement detection scheme based on measurements of current cross correlations. We find a significant reduction in the number of measurements required for an entanglement test.In Paper IV, we investigate the optimisation of electrically controlling an electron spin in a gate defined semiconductor quantum dot. Specifically, we study how the shape of the quantum dot together with the direction of an in-plane magnetic field affect the spin relaxation rate and frequency of the induced Rabi oscillations.
This thesis addresses two different topics related to the physics of nanoscale systems. The first topic concerns quantum correlations and entanglement between electrons in solid-state systems, with a focus on how to generate electronic orbital entanglement on a sub-decoherence time scale and how to achieve experimentally more feasible entanglement detection schemes. The second topic concerns heat transport and temperature fluctuations in nanoscale systems, with a focus on how to utilize temperature fluctuations for calorimetric detection of single particles. The thesis comprises five papers.In Paper I, we propose a quantum dot system to generate and detect, using cotunneling processes, orbitally entangled pairs of electrons on a sub-decoherence time scale.In Paper II, we investigate, by applying an entanglement witness, the minimal number of zero-frequency current cross-correlation measurements needed to detect bipartite entanglement between two flying qubits.In Paper III, we consider energy and temperature fluctuations, and the influence of charging effects, in a metallic island tunnel coupled to a normal metallic lead, the so-called single electron box.In Paper IV, we investigate nanoscale quantum calorimetry and propose a setup consisting of a metallic island and a superconducting lead to realize a nanoscale calorimeter that may probe the energies of tunneling electrons.In Paper V, we investigate photon transport statistics of a microwave cavity, including the short-time statistics of single photon emissions and the long-time statistics of heat transport through the cavity.
The single-electron transistor (SET) is a prime candidate for reading out the final state of a qubit in a solid-state quantum computer. Such a measurement requires the detection of subelectron charge motion in the presence of random charging events. We present a detection scheme where the signals from two SETs are cross-correlated to suppress unwanted artifacts due to charge noise. This technique is demonstrated by using the two SETs to detect the charge state of two coupled metal dots, thereby simulating charge transfer and readout in a two-qubit system. These measurements indicate that for comparable buried dopant semiconductor architectures, the minimum measurement time required to distinguish between the two charge states is of the order of 10 ns.