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AIP conference proceedings

Charge Readout of a Vertical Quantum Dot Using a Laterally Weakly Coupled Quantum Dot as a Detector

Tetsuo Kodera, Yohei Iwai, Wilfred G. van der Wiel, Takayuki Obata, T. Hatano, Seigo Tarucha

Published Jan 1, 2006• 1 citation
Publisher

Abstract

We have fabricated laterally coupled four‐vertical‐quantum‐dot devices that can be used for reading out electron charge and spin information of quantum dots. In these devices, the inter‐dot coupling is tunable with a coupling gate voltage and the number of electrons in each dot is tunable with a side gate voltage. We first use two of the four dots to confirm the effect of the inter‐dot coupling strength on the stability diagram. We then use one of the two dots as a single electron transistor (SET) charge detector to detect the charge information of the other dot.

DOI
10.1063/1.2355240

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We have fabricated laterally coupled four‐vertical‐quantum‐dot devices that can be used for reading out electron charge and spin information of quantum dots. In these devices, the inter‐dot coupling is tunable with a coupling gate voltage and the number of electrons in each dot is tunable with a side gate voltage. We first use two of the four dots to confirm the effect of the inter‐dot coupling strength on the stability diagram. We then use one of the two dots as a single electron transistor (SET) charge detector to detect the charge information of the other dot.
Single-spin detection will be crucial for solid-state quantum computer (QC) architectures in which information is encoded in the spin state of single nuclear or electron spins. The formidable problem of single-spin detection in a solid can be mapped to a more tractable problem of single-charge detection through spin-dependent electron transfer which may be observed using ultrasensitive solid-state nanostructure electrometers. Here we describe a read-out architecture using single-electron transistors (SETs) that can detect the charge state of two coupled metal dots, which simulates charge transfer in a two-quantum-bit (qubit) spin system. This twin-SET architecture allows significant reduction of random charge noise by correlating two detector outputs, reducing the probability of read-out errors in the QC.
PACS. 73.23.-b – Electronic transport in mesoscopic systems. Abstract. – We study entanglement of charge qubits in a vertical tunnel-coupled double quantum dot containing two interacting electrons. Exact diagonalization is used to compute the negativity characterizing entanglement. We find that entanglement can be efficiently generated and controlled by sidegate voltages, and describe how it can be detected. For large enough tunnel coupling, the negativity shows a pronounced maximum at an intermediate interaction strength within the Wigner molecule regime. Semiconductor few-electron quantum dots (QDs) continue to attract a lot of interest, as it has now become possible to experimentally control both the electronic spin and charge in a condensed-phase environment in an unprecedented manner. In particular, double quantum dots (DQDs) can be fabricated in a well-controlled fashion in high-quality semiconductor devices, and are currently under intense study [1–9]. Vertical or lateral tunnel-coupled DQDs are among the most promising candidates for realizing spin or charge qubits in a quantum information processor [10–12]. Their main advantages are scalability, good control of physical properties via tunable external (sidegate-) voltages or magnetic fields, and spatial separation
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 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.
We used a hybrid vertical-lateral double-dot device, consisting of laterally coupled vertical quantum dots, to measure the interdot tunnel coupling. By using nonlinear transport measurements of "Coulomb diamonds," we showed that an inherent asymmetry in the capacitances of the component dots influences the diamond slopes, thereby allowing for the determination of the dot through which the electron has passed. We used this technique to prepare a delocalized one-electron state and Heitler-London (HL) two-electron state, and we showed that the interdot tunnel coupling, which determines whether HL is the ground state, is tunable. This implies that our device may be useful for implementing two-electron spin entanglement.
Single-spin detection will be crucial for solid-state quantum computer architectures in which information is encoded in the spin-state of single nuclear or electron spins. The formidable problem of single-spin detection in a solid can be mapped to a more tractable problem of single-charge detection through spin-dependent electron transfer which may be observed using ultra-sensitive solid-state nanostructure electrometers. Here we describe a readout architecture using single electron transistors (SETs) that can detect the charge-state of two coupled metal dots, which simulates charge transfer in a two-quantum bit (qubit) spin system. This twin-SET architecture allows significant reduction of random charge noise by correlating two detector outputs, reducing the probability of readout errors in the quantum computer.