The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin-based quantum computer. In combination with the recently demonstrated exchange gate between two neighbouring spins, driven coherent single spin rotations would permit universal quantum operations. Here, we report the experimental realization of single electron spin rotations in a double quantum dot. First, we apply a continuouswave oscillating magnetic field, generated on-chip, and observe electron spin resonance in spin-dependent transport measurements through the two dots. Next, we coherently control the quantum state of the electron spin by applying short bursts of the oscillating magnetic field and observe about eight oscillations of the spin state (so-called Rabi oscillations) during a microsecond burst. These results demonstrate the feasibility of operating single-electron spins in a quantum dot as quantum bits.The use of quantum mechanical superposition states and entanglement in a computer can theoretically solve important mathematical and physical problems much faster than classical computers 1,2 . However, the realization of such a quantum computer represents a formidable challenge, because it requires fast and precise control of fragile quantum states. The prospects for accurate quantum control in a scalable system are thus being explored in a rich variety of physical systems, ranging from nuclear magnetic resonance and ion traps to superconducting devices 3 .Electron spin states were identified early on as an attractive realization of a quantum bit 4 , because they are relatively robust against decoherence (uncontrolled interactions with the environment). Advances in the field of semiconductor quantum dots have made this system very fruitful as a host for the electron spin. Since Loss and DiVincenzo's proposal 5 on electron spin qubits in quantum dots in 1998, many of the elements necessary for quantum computation have been realized experimentally. It is now routine to isolate with certainty a single electron in each of two coupled quantum dots [6][7][8][9] . The spin of this electron can be reliably initialized to the ground state, spin-up, via optical pumping 10 or by thermal equilibration at sufficiently low temperatures and strong static magnetic fields (for example, T=100 mK and B ext =1 T). The spin states are also very long-lived, with relaxation times of the order of milliseconds [11][12][13] .Furthermore, a lower bound on the spin coherence time exceeding 1 µs was established, using spin-echo techniques on a two-electron system 14 . These long relaxation and coherence times are possible in part because the magnetic moment of a single 1 electron spin is so weak. On the other hand, this property makes read-out and manipulation of single spins particularly challenging. By combining spin-to-charge conversion with real-time single-charge detection [15][16][17] , it has nevertheless been possible to accomplish single-shot read-out of spin states in a qu...

We study competition between the Kondo effect and superconductivity in a single self-assembled InAs quantum dot contacted with Al lateral electrodes. Due to Kondo enhancement of Andreev reflections the zero-bias anomaly develops sidepeaks, separated by the superconducting gap energy ∆. For ten valleys of different Kondo temperature TK we tune the gap ∆ with an external magnetic field. We find that the zero-bias conductance in each case collapses onto a single curve with ∆/kBTK as the only relevant energy scale, providing experimental evidence for universal scaling in this system.

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Spin-dependent transport measurements through a double quantum dot are a valuable tool for detecting both the coherent evolution of the spin state of a single electron as well as the hybridization of two-electron spin states. In this paper, we discuss a model that describes the transport cycle in this regime, including the effects of an oscillating magnetic field (causing electron spin resonance) and the effective nuclear fields on the spin states in the two dots. We numerically calculate the current flow due to the induced spin flips via electron spin resonance and we study the detector efficiency for a range of parameters. The experimental data are compared with the model and we find a reasonable agreement. A. IntroductionRecently, coherent spin rotations of a single electron were demonstrated in a double quantum dot device [1]. In this system, spin-flips of an electron in the dot were induced via an oscillating magnetic field (electron spin resonance or ESR) and detected through a spin-dependent transition of the electron to another dot, which already contained one additional electron. This detection scheme is an extension of the proposal for ESR detection in a single quantum dot by Engel and Loss [2]. Briefly, the device can be operated (in a spin blockade regime [3]) such that the electron in the left dot can only move to the right dot if a spin flip in one of the two dots is induced via ESR. From the right dot, the electron exits to the right reservoir and another electron enters the left dot from the left reservoir. A continuous repetition of this transition will result in a net current flow.Compared to the single dot detection scheme [2], using the double-dot as the detector has two major advantages. First, the experiment can be performed at a lower static magnetic field and consequently with lower, technically less demanding, excitation frequencies. Second, the spin detection is rather insensitive to unwanted oscillating electric fields, because the relevant dot levels can be positioned far from the Fermi energies of the leads. These electric fields are unavoidably generated together with the oscillating magnetic field as well.The drawback of the double-dot detector is that spin detection is based on the projection in the two-electron singlet-triplet basis, while the aim is to detect single spin rotations. However, this detection is still possible because the electrons in the two dots experience different effective nuclear fields. This is due to the hyperfine interaction of the electron spins with the (roughly 10 6 ) nuclear spins in the host semiconductor material of each quantum dot [4][5][6][7][8][9][10][11]. In order to provide more insight in this double-dot ESR detection scheme for single spin rotations, it is necessary to analyze the coherent evolution of the two-electron spin states together with the transitions * Electronic address: f.h.l.koppens@tudelft.nl; Kavli Institute of NanoScience Delft, P.O. Box 5046, 2600 GA Delft, The Netherlands in the transport cycle.In this paper, we discuss a mod...