scholarly journals Fundamental limits of electron and nuclear spin qubit lifetimes in an isolated self-assembled quantum dot

2021 ◽  
Vol 7 (1) ◽  
Author(s):  
George Gillard ◽  
Ian M. Griffiths ◽  
Gautham Ragunathan ◽  
Ata Ulhaq ◽  
Callum McEwan ◽  
...  
Keyword(s):  
Quantum Dot ◽  
Spin Relaxation ◽  
Nuclear Spin ◽  
Operating Conditions ◽  
External Control ◽  
Spin Qubits ◽  
Fundamental Limits ◽  
Trade Offs ◽  
Wide Range ◽  
Spin Qubit

AbstractCombining external control with long spin lifetime and coherence is a key challenge for solid state spin qubits. Tunnel coupling with electron Fermi reservoir provides robust charge state control in semiconductor quantum dots, but results in undesired relaxation of electron and nuclear spins through mechanisms that lack complete understanding. Here, we unravel the contributions of tunnelling-assisted and phonon-assisted spin relaxation mechanisms by systematically adjusting the tunnelling coupling in a wide range, including the limit of an isolated quantum dot. These experiments reveal fundamental limits and trade-offs of quantum dot spin dynamics: while reduced tunnelling can be used to achieve electron spin qubit lifetimes exceeding 1 s, the optical spin initialisation fidelity is reduced below 80%, limited by Auger recombination. Comprehensive understanding of electron-nuclear spin relaxation attained here provides a roadmap for design of the optimal operating conditions in quantum dot spin qubits.

scholarly journals Quantum interface of an electron and a nuclear ensemble

Science ◽  
2019 ◽  
Vol 364 (6435) ◽  
pp. 62-66 ◽  
Author(s):  
D. A. Gangloff ◽  
G. Éthier-Majcher ◽  
C. Lang ◽  
E. V. Denning ◽  
J. H. Bodey ◽  
...  
Keyword(s):  
Quantum Dot ◽  
Nuclear Spin ◽  
Building Blocks ◽  
Spin Excitation ◽  
Many Body ◽  
Quantum Objects ◽  
All Optical ◽  
Spin Qubit ◽  
Many Body Systems ◽  
Optical Rotations

Coherent excitation of an ensemble of quantum objects underpins quantum many-body phenomena and offers the opportunity to realize a memory that stores quantum information. Thus far, a deterministic and coherent interface between a spin qubit and such an ensemble has remained elusive. In this study, we first used an electron to cool the mesoscopic nuclear spin ensemble of a semiconductor quantum dot to the nuclear sideband–resolved regime. We then implemented an all-optical approach to access individual quantized electronic-nuclear spin transitions. Lastly, we performed coherent optical rotations of a single collective nuclear spin excitation—a spin wave. These results constitute the building blocks of a dedicated local memory per quantum-dot spin qubit and promise a solid-state platform for quantum-state engineering of isolated many-body systems.

scholarly journals An Operation Guide of Si-MOS Quantum Dots for Spin Qubits

Nanomaterials ◽  
2021 ◽  
Vol 11 (10) ◽  
pp. 2486
Author(s):  
Rui-Zi Hu ◽  
Rong-Long Ma ◽  
Ming Ni ◽  
Xin Zhang ◽  
Yuan Zhou ◽  
...  
Keyword(s):  
Quantum Dots ◽  
Quantum Dot ◽  
Rabi Oscillation ◽  
Stability Diagram ◽  
Near Neighbor ◽  
Spin Qubits ◽  
Orbital State ◽  
Single Spin ◽  
Spin Qubit ◽  
Charge Stability

In the last 20 years, silicon quantum dots have received considerable attention from academic and industrial communities for research on readout, manipulation, storage, near-neighbor and long-range coupling of spin qubits. In this paper, we introduce how to realize a single spin qubit from Si-MOS quantum dots. First, we introduce the structure of a typical Si-MOS quantum dot and the experimental setup. Then, we show the basic properties of the quantum dot, including charge stability diagram, orbital state, valley state, lever arm, electron temperature, tunneling rate and spin lifetime. After that, we introduce the two most commonly used methods for spin-to-charge conversion, i.e., Elzerman readout and Pauli spin blockade readout. Finally, we discuss the details of how to find the resonance frequency of spin qubits and show the result of coherent manipulation, i.e., Rabi oscillation. The above processes constitute an operation guide for helping the followers enter the field of spin qubits in Si-MOS quantum dots.

scholarly journals Spin manipulation and decoherence in a quantum dot mediated by a synthetic spin-orbit coupling of broken $T$-symmetry

2021 ◽  
Author(s):  
Peihao Huang ◽  
Xuedong Hu
Keyword(s):  
Magnetic Field ◽  
Quantum Dot ◽  
Spin Relaxation ◽  
Orbit Coupling ◽  
Spin Orbit Coupling ◽  
Spin Orbit ◽  
Field Orientation ◽  
Spin Manipulation ◽  
Pure Dephasing ◽  
Spin Qubit

Abstract The electrical control of a spin qubit in a quantum dot relies on spin-orbit coupling (SOC), which could be either intrinsic to the underlying crystal lattice or heterostructure, or extrinsic via, for example, a micro-magnet. In experiments, micromagnets have been used as a synthetic SOC to enable strong coupling of a spin qubit in quantum dots with electric fields. Here we study theoretically the spin relaxation, pure dephasing, spin manipulation, and spin-photon coupling of an electron in a quantum dot due to the synthetic SOC induced spin-orbit mixing. We find qualitative difference in the spin dynamics in the presence of a synthetic SOC compared with the case of the intrinsic SOC. Specifically, spin relaxation due to the synthetic SOC and deformation potential phonon emission (or Johnson noise) shows $B_0^5$ (or $B_0$) dependence with the magnetic field, which is in contrast with the $B_0^7$ (or $B_0^3$) dependence in the case of the intrinsic SOC. Moreover, charge noise induces fast spin dephasing to the first order of the synthetic SOC, which is in sharp contrast with the negligible spin pure dephasing in the case of the intrinsic SOC. These qualitative differences are attributed to the broken time-reversal symmetry ($T$-symmetry) of the synthetic SOC. An SOC with broken $T$-symmetry (such as the synthetic SOC from a micro-magnet) eliminates the ``Van Vleck cancellation'' and causes a finite longitudinal spin-electric coupling that allows the longitudinal coupling between spin and electric field, and in turn allows spin pure dephasing. Finally, through proper choice of magnetic field orientation, the electric-dipole spin resonance via the synthetic SOC can be improved with potential applications in spin-based quantum computing.

scholarly journals A silicon quantum-dot-coupled nuclear spin qubit

2019 ◽  
Vol 15 (1) ◽  
pp. 13-17 ◽  
Author(s):  
Bas Hensen ◽  
Wister Wei Huang ◽  
Chih-Hwan Yang ◽  
Kok Wai Chan ◽  
Jun Yoneda ◽  
...  
Keyword(s):  
Quantum Dot ◽  
Nuclear Spin ◽  
Silicon Quantum Dot ◽  
Spin Qubit

scholarly journals Opinion: Democratizing Spin Qubits

Quantum ◽  
2021 ◽  
Vol 5 ◽  
pp. 584
Author(s):  
Charles Tahan
Keyword(s):  
Quantum Dot ◽  
Materials Science ◽  
Real Life ◽  
Quantum Computers ◽  
Spin Qubits ◽  
Small Quantum ◽  
Chip Fabrication ◽  
Spin Qubit ◽  
On Chip ◽  
And Control

I've been building Powerpoint-based quantum computers with electron spins in silicon for 20 years. Unfortunately, real-life-based quantum dot quantum computers are harder to implement. Materials, fabrication, and control challenges still impede progress. The way to accelerate discovery is to make and measure more qubits. Here I discuss separating the qubit realization and testing circuitry from the materials science and on-chip fabrication that will ultimately be necessary. This approach should allow us, in the shorter term, to characterize wafers non-invasively for their qubit-relevant properties, to make small qubit systems on various different materials with little extra cost, and even to test spin-qubit to superconducting cavity entanglement protocols where the best possible cavity quality is preserved. Such a testbed can advance the materials science of semiconductor quantum information devices and enable small quantum computers. This article may also be useful as a light and light-hearted introduction to quantum dot spin qubits.

scholarly journals Engineering coherent interactions in molecular nanomagnet dimers

2015 ◽  
Vol 1 (1) ◽  
Author(s):  
Arzhang Ardavan ◽  
Alice M Bowen ◽  
Antonio Fernandez ◽  
Alistair J Fielding ◽  
Danielle Kaminski ◽  
...  
Keyword(s):  
Self Assembly ◽  
Quantum Coherence ◽  
Quantum Information Processing ◽  
Building Blocks ◽  
Molecular Structures ◽  
Spin Qubits ◽  
Large Numbers ◽  
Wide Range ◽  
Spin Qubit ◽  
Molecular Components

AbstractProposals for systems embodying condensed matter spin qubits cover a very wide range of length scales, from atomic defects in semiconductors all the way to micron-sized lithographically defined structures. Intermediate scale molecular components exhibit advantages of both limits: like atomic defects, large numbers of identical components can be fabricated; as for lithographically defined structures, each component can be tailored to optimise properties such as quantum coherence. Here we demonstrate what is perhaps the most potent advantage of molecular spin qubits, the scalability of quantum information processing structures using bottom-up chemical self-assembly. Using Cr7Ni spin qubit building blocks, we have constructed several families of two-qubit molecular structures with a range of linking strategies. For each family, long coherence times are preserved, and we demonstrate control over the inter-qubit quantum interactions that can be used to mediate two-qubit quantum gates.

Functional geometry of ciliated tentacular arrays in active suspension feeders

1998 ◽  
Vol 201 (18) ◽  
pp. 2575-2589 ◽  
Author(s):  
D Grünbaum ◽  
D Eyre ◽  
A Fogelson
Keyword(s):  
Flow Rate ◽  
Blue Mussel ◽  
Performance Measure ◽  
Operating Conditions ◽  
Hydrodynamic Characteristics ◽  
Flow Rates ◽  
Pressure Drops ◽  
Trade Offs ◽  
Wide Range ◽  
Piping Systems

Parallel tentacular structures with lateral cilia that produce suspension-feeding and respiratory flows occur repeatedly in many diverse taxonomic groups. We use a computational hydrodynamic model of flow through ciliated tentacles to simulate flow rates through ciliated tentacle arrays. We examine the functional relationship of one performance measure, flow rate per unit length of array, to geometrical variables, such as cilia length, cilia tip speed and the gap between adjacent tentacles, and to hydrodynamic operating conditions, such as adverse pressure drops across the array. We present a scaling and interpolation scheme to estimate flow rates for a wide range of geometries that span many taxa. Our estimates of flow rate can be coupled with the hydrodynamic characteristics of biological piping systems to understand design trade-offs between components of these systems. As a case study, we apply the model to the blue mussel Mytilus edulis by investigating the effect on performance of changes in the gap between neighboring tentacles. Our model suggests that the observed gaps between tentacles in M. edulis reflect flow-maximizing geometries. Even relatively weak adverse pressure drops have strong effects on flow-maximizing geometries and flow rates. One consequence is that an intermediate range of pressure drops may be unfavorable, suggesting that animals may specialize into high-pressure and low-pressure piping systems associated with differences in organism size and with their strategy for eliminating depleted water.

scholarly journals Ultrafast coherent control of a hole spin qubit in a germanium quantum dot

2022 ◽  
Vol 13 (1) ◽  
Author(s):  
Ke Wang ◽  
Gang Xu ◽  
Fei Gao ◽  
He Liu ◽  
Rong-Long Ma ◽  
...  
Keyword(s):  
Quantum Dot ◽  
Coherent Control ◽  
Coherence Time ◽  
Double Quantum ◽  
Spin Orbit ◽  
Spin Qubits ◽  
Single Spin ◽  
Operation Speed ◽  
Spin Qubit ◽  
Core Measures

AbstractOperation speed and coherence time are two core measures for the viability of a qubit. Strong spin-orbit interaction (SOI) and relatively weak hyperfine interaction make holes in germanium (Ge) intriguing candidates for spin qubits with rapid, all-electrical coherent control. Here we report ultrafast single-spin manipulation in a hole-based double quantum dot in a germanium hut wire (GHW). Mediated by the strong SOI, a Rabi frequency exceeding 540 MHz is observed at a magnetic field of 100 mT, setting a record for ultrafast spin qubit control in semiconductor systems. We demonstrate that the strong SOI of heavy holes (HHs) in our GHW, characterized by a very short spin-orbit length of 1.5 nm, enables the rapid gate operations we accomplish. Our results demonstrate the potential of ultrafast coherent control of hole spin qubits to meet the requirement of DiVincenzo’s criteria for a scalable quantum information processor.

Does this list contain what you were searching for? Learning adaptive dialogue strategies for interactive question answering

2009 ◽  
Vol 15 (1) ◽  
pp. 55-72 ◽  
Author(s):  
V. RIESER ◽  
O. LEMON
Keyword(s):  
Objective Function ◽  
Question Answering ◽  
Wide Spectrum ◽  
Operating Conditions ◽  
Policy Learning ◽  
Systems Research ◽  
Trade Offs ◽  
Wide Range ◽  
Interactive Question ◽  
Better Than

AbstractPolicy learning is an active topic in dialogue systems research, but it has not been explored in relation to interactive question answering (IQA). We take a first step in learning adaptive interaction policies for question answering : we address the question of how to acquire enough reliable query constraints, how many database results to present to the user and when to present them, given the competing trade-offs between the length of the answer list, the length of the interaction, the type of database and the noise in the communication channel. The operating conditions are reflected in an objective function which we use to derive a hand-coded threshold-based policy and rewards to train a reinforcement learning policy. The same objective function is used for evaluation. We show that we can learn strategies for this complex trade-off problem which perform significantly better than a variety of hand-coded policies, for a wide range of noise conditions, user types, types of DB and turn-penalties. Our policy learning framework thus covers a wide spectrum of operating conditions. The learned policies produce an averagerelativeincrease in reward of 86.78% over the hand-coded policies. In 93% of the cases the learned policies perform significantly better than the hand-coded ones (p< .001). Furthermore we show that the type of database has a significant effect on learning and we give qualitative descriptions of the learned IQA policies.

scholarly journals Controllable freezing of the nuclear spin bath in a single-atom spin qubit

2020 ◽  
Vol 6 (27) ◽  
pp. eaba3442 ◽  
Author(s):  
Mateusz T. Mądzik ◽  
Thaddeus D. Ladd ◽  
Fay E. Hudson ◽  
Kohei M. Itoh ◽  
Alexander M. Jakob ◽  
...  
Keyword(s):  
Electron Spin ◽  
Nuclear Spin ◽  
Quantum Coherence ◽  
Spin Dynamics ◽  
Spin Fluctuations ◽  
Single Atom ◽  
Spin Qubits ◽  
Nuclear Dynamics ◽  
Spin Bath ◽  
Spin Qubit

The quantum coherence and gate fidelity of electron spin qubits in semiconductors are often limited by nuclear spin fluctuations. Enrichment of spin-zero isotopes in silicon markedly improves the dephasing time T2*, which, unexpectedly, can extend two orders of magnitude beyond theoretical expectations. Using a single-atom 31P qubit in enriched 28Si, we show that the abnormally long T2* is due to the freezing of the dynamics of the residual 29Si nuclei, caused by the electron-nuclear hyperfine interaction. Inserting a waiting period when the electron is controllably removed unfreezes the nuclear dynamics and restores the ergodic T2* value. Our conclusions are supported by a nearly parameter-free modeling of the 29Si nuclear spin dynamics, which reveals the degree of backaction provided by the electron spin. This study clarifies the limits of ergodic assumptions in nuclear bath dynamics and provides previously unidentified strategies for maximizing coherence and gate fidelity of spin qubits in semiconductors.

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