Semiconductor Few-Electron Quantum Dots as Spin Qubits (original) (raw)
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Experimental Methods for Implementing Electron Spin Qubits in Double Quantum Dots
To achieve universal quantum computation it is necessary to encode quantum information in a physical system, called a quantum bit or qubit. These can be realized in a multitude of ways, and the aim of this Master’s project has been to explore one possibility in which the spin states of entangled electrons encode quantum information, known as electron spin qubits. The thesis contains a description of the experimental setup and techniques used to confine electrons in lateral, gate-defined double quantum dot (DQD) systems for potential use as electron spin qubits, as well as experimental control routines used and/or developed during the project. Experimental results are presented to elucidate the possibilities and limitations of the measured devices for use as DQD systems, as well as quantifying the performance of the reflectometry circuit used for fast readout of charge sensing. The main focus of the thesis has been to provide new experimentalists with an overview of the experimental setups and techniques used for electron spin qubit experiments at Center for Quantum Devices.
2010
Single-shot measurement of the charge arrangement and spin state of a double quantum dot are reported, with times down to 100 ns. Sensing uses radio-frequency reflectometry of a proximal quantum dot in the Coulomb blockade regime. The sensor quantum dot is up to 30 times more sensitive than a comparable quantum point contact sensor, and yields three times greater signal to noise in rf single-shot measurements. Numerical modeling is qualitatively consistent with experiment and shows that the improved sensitivity of the sensor quantum dot results from reduced screening and smaller characteristic energy needed to change transmission.
Single-shot read-out of an individual electron spin in a quantum dot
Nature, 2004
Spin is a fundamental property of all elementary particles. Classically it can be viewed as a tiny magnetic moment, but a measurement of an electron spin along the direction of an external magnetic field can have only two outcomes: parallel or anti-parallel to the field [1]. This discreteness reflects the quantum mechanical nature of spin. Ensembles of many spins have found diverse applications ranging from magnetic resonance imaging [2] to magneto-electronic devices [3], while individual spins are considered as carriers for quantum information. Read-out of single spin states has been achieved using optical techniques [4], and is within reach of magnetic resonance force microscopy . However, electrical read-out of single spins [6-13] has so far remained elusive. Here, we demonstrate electrical single-shot measurement of the state of an individual electron spin in a semiconductor quantum dot . We use spinto-charge conversion of a single electron confined in the dot, and detect the single-electron charge using a quantum point contact; the spin measurement visibility is ∼ 65%. Furthermore, we observe very long single-spin energy relaxation times (up to ∼ 0.85 ms at a magnetic field of 8 Tesla), which are encouraging for the use of electron spins as carriers of quantum information.
Electron spin qubits in quantum dots
2004
We review our experimental progress on the spintronics proposal for quantum computing where the quantum bits (qubits) are implemented with electron spins confined in semiconductor quantum dots. Out of the five criteria for a scalable quantum computer, three have already been satisfied. We have fabricated and characterized a double quantum dot circuit with an integrated electrometer. The dots can be tuned to contain a single electron each. We have resolved the two basis states of the qubit by electron transport measurements. Furthermore, initialization and single-shot read-out of the spin state have been achieved. The single-spin relaxation time was found to be very long, but the decoherence time is still unknown. We present concrete ideas on how to proceed towards coherent spin operations and two-qubit operations.
Electron spins in quantum dots for spintronics and quantum computation
Solid State Communications, 2001
Coherent manipulation, ®ltering, and measurement of electronic spin in quantum dots and other nanostructures have promising applications in conventional and in quantum information processing and transmission. We present an overview of our theoretical proposal to implement a quantum computer using electron spins in quantum dots as qubits. We discuss all necessary requirements towards a scalable quantum computer including one-and two qubit gates and read in/out tasks. We then present some concepts for promising single quantum dot devices which eventually could be used as building blocks for sophisticated spintronic devices. We show how a single quantum dot can act as an ef®cient spin ®lter. Further, in combination with an ESR source, a quantum dot can be used as a single spin memory or as a spin pump. In addition, the sequential tunneling current through a quantum dot in the presence of an ESR ®eld can exhibit a resonance whose line width is determined by the decoherence time T 2 of a single dot-spin. Finally, we consider mobile non-local spin entangled electrons as needed for quantum communication. We propose how to create such EPR pairs by means of Andreev tunneling at a superconductor-normal junction and discuss experimental setups in which spin entanglement may be detected via transport measurements.
Control and measurement of electron spins in semiconductor quantum dots
2006
Abstract We present an overview of experimental steps taken towards using the spin of a single electron trapped in a semiconductor quantum dot as a spin qubit [Loss and DiVincenzo, Phys. Rev. A 57, 120 (1998)]. Fabrication and characterization of a double quantum dot containing two coupled spins has been achieved, as well as initialization and single-shot read-out of the spin state.
Charge and spin readout scheme for single self-assembled quantum dots
Physical Review B, 2008
We propose an all optical spin initialization and readout concept for single self assembled quantum dots and demonstrate its feasibility. Our approach is based on a gateable single dot photodiode structure that can be switched between charge and readout mode. After optical electron generation and storage, we propose to employ a spin-conditional absorption of a circularly polarized light pulse tuned to the single negatively charged exciton transition to convert the spin information of the resident electron to charge occupancy. Switching the device to the charge readout mode then allows us to probe the charge state of the quantum dot (1e, 2e) using non-resonant luminescence.
Semiconductor quantum dots for electron spin qubits
New Journal of Physics, 2006
We report on our recent progress in applying semiconductor quantum dots for spin-based quantum computation, as proposed by Loss and DiVincenzo (1998 Phys. Rev. A 57 120). For the purpose of single-electron spin resonance, we study different types of single quantum dot devices that are designed for the generation of a local ac magnetic field in the vicinity of the dot. We observe photon-assisted tunnelling as well as pumping due to the ac voltage induced by the ac current driven through a wire in the vicinity of the dot, but no evidence for ESR so far. Analogue concepts for a double quantum dot and the hydrogen molecule are discussed in detail. Our experimental results in laterally coupled vertical double quantum dot device show that the Heitler-London model forms a good approximation of the two-electron wavefunction. The exchange coupling constant J is estimated. The relevance of this system for two-qubit gates, in Institute of Physics ⌽ DEUTSCHE PHYSIKALISCHE GESELLSCHAFT particular the SWAP operation, is discussed. Density functional calculations reveal the importance of the gate electrode geometry in lateral quantum dots for the tunability of J in realistic two-qubit gates.
Quantum Computing with Electron Spins in Quantum Dots
2002
We present a set of concrete and realistic ideas for the implementation of a small-scale quantum computer using electron spins in lateral GaAs/AlGaAs quantum dots. Initialization is based on leads in the quantum Hall regime with tunable spin-polarization. Read-out hinges on spin-to-charge conversion via spin-selective tunneling to or from the leads, followed by measurement of the number of electron charges on the dot via a charge detector. Single-qubit manipulation relies on a microfabricated wire located close to the quantum dot, and two-qubit interactions are controlled via the tunnel barrier connecting the respective quantum dots. Based on these ideas, we have begun a series of experiments in order to demonstrate unitary control and to measure the coherence time of individual electron spins in quantum dots.