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 [5]. 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 [14]. 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.

Nanoscale resonators that oscillate at high frequencies are useful in many measurement applications.We studied a high-quality mechanical resonator made from a suspended carbon nanotube driven into motion by applying a periodic radio frequency potential using a nearby antenna. Single-electron charge fluctuations created periodic modulations of the mechanical resonance frequency. A quality factor exceeding 10 5 allows the detection of a shift in resonance frequency caused by the addition of a single-electron charge on the nanotube. Additional evidence for the strong coupling of mechanical motion and electron tunneling is provided by an energy transfer to the electrons causing mechanical damping and unusual nonlinear behavior. We also discovered that a direct current through the nanotube spontaneously drives the mechanical resonator, exerting a force that is coherent with the high-frequency resonant mechanical The combination of a high resonance frequency and a small mass also makes nanomechanical resonators attractive for a fundamental study of mechanical motion in the quantum limit [6, 7, 8, 9]. For a successful observation of quantum motion of a macroscopic object, a high-frequency nanoscale resonator must have low dissipation (which implies a high quality-factor Q), and a sensitive detector with minimum back-action (i.e. quantum limited) [10, 11]. Here, we demonstrate a dramatic backaction that strongly couples a quantum dot detector to the resonator dynamics of a carbon nanotube, and which, in the limit of strong feedback, spontaneously excites large amplitude resonant mechanical motion.Nanomechanical resonators have been realized by etching down larger structures. In small devices, however, surfaces effects impose a limit on the quality-factor [2]. Alternatively, suspended carbon nanotubes can be used to avoid surface damage from the (etching) fabrication process. We recently developed a mechanical resonator based on an ultra-clean carbon nanotube with high resonance frequencies of several 100 MHz and a Q exceeding 10 5 [12]. Here, we exploit this resonator to explore a strong coupling regime between single electron tunneling and nanomechanical motion. We followed the pioneering approaches in which aluminium single electron transistors were used as position detectors [6, 7, 8] and AFM cantilevers as resonators [13,14,15]; however, our experiment is in the limit of much stronger electro-mechanical coupling, achieved by embedding a quantum dot detector in the nanomechanical resonator itself.Our device consists of a nanotube suspended across a trench that makes electrical contact to two metal electrodes ( Fig. 1). Electrons are confined in the nanotube by Schottky barriers at the Pt metal contacts, forming a quantum dot in the suspended segment. The nanotube growth is the last step in the fabrication process, yielding ultra-clean devices [16], as demonstrated by the four-fold shell-filling of the Coulomb peaks (Fig. 1C). All measurements were performed at a temperature of 20 mK with an electron temperat...

We have observed the transversal vibration mode of suspended carbon nanotubes at millikelvin temperatures by measuring the single-electron tunneling current. The suspended nanotubes are actuated contact-free by the radio frequency electric field of a nearby antenna; the mechanical resonance is detected in the time-averaged current through the nanotube. Sharp, gate-tunable resonances due to the bending mode of the nanotube are observed, combining resonance frequencies of up to nu(0) = 350 MHz with quality factors above Q = 10(5), much higher than previously reported results on suspended carbon nanotube resonators. The measured magnitude and temperature dependence of the Q factor shows a remarkable agreement with the intrinsic damping predicted for a suspended carbon nanotube. By adjusting the radio frequency power on the antenna, we find that the nanotube resonator can easily be driven into the nonlinear regime.

We have measured the relaxation time, T1, of the spin of a single electron confined in a semiconductor quantum dot (a proposed quantum bit). In a magnetic field, applied parallel to the two-dimensional electron gas in which the quantum dot is defined, Zeeman splitting of the orbital states is directly observed by measurements of electron transport through the dot. By applying short voltage pulses, we can populate the excited spin state with one electron and monitor relaxation of the spin. We find a lower bound on T1 of 50 µs at 7.5 T, only limited by our signal-to-noise ratio. A continuous measurement of the charge on the dot has no observable effect on the spin relaxation.PACS numbers: 73.63. Kv, 03.67.Lx, 73.23.Hk The spin of an electron confined in a semiconductor quantum dot (QD) is a promising candidate for a scalable quantum bit [1,2]. The electron spin states in QDs are expected to be very stable, because the zerodimensionality of the electron states in QDs leads to a significant suppression of the most effective 2D spin-flip mechanisms [3]. Recent electrical transport measurements of relaxation between spin triplet and singlet states of two electrons, confined in a pillar etched from a GaAs double-barrier heterostructure ("vertical" QD), support this prediction (relaxation time > 200 µs at T ≤ 0.5 K) [4]. However, the triplet-to-singlet transition, in which the total spin quantum number S is changed from 1 to 0, is forbidden by a selection rule (∆S=0) that does not hinder relaxation between Zeeman sublevels (which conserves S ). Therefore, measurements on a single electron spin are needed in order to determine the relaxation time of the proposed qubit.Relaxation between Zeeman sublevels in closed GaAs QDs is expected to be dominated by hyperfine interaction with the nuclei at magnetic fields below 0.5 T [5] and by spin-orbit interaction at higher fields [6]. At 1 T, theory predicts a T 1 of 1 ms in GaAs [6]; at fields above a few Tesla, needed to resolve the Zeeman splitting in transport measurements, no quantitative estimates for T 1 exist.For comparison, in n-doped self-assembled InAs QDs containing one resident electron, pump-probe photoluminescence measurements gave a single-electron spin relaxation time of 15 ns (at B=0 T, T = 10 K) [7]. In undoped self-assembled InAs QDs, the exciton polarization is frozen throughout the exciton lifetime, giving a relaxation time >20 ns [8].Electrical measurements of the single-electron spin relaxation time have up to now remained elusive. In vertical QDs, where electrical measurements on a single electron were reported almost a decade ago [9], it has been difficult to directly resolve the Zeeman splitting of orbitals [10]. Recently, the one-electron regime was also reached in single [11] and double lateral GaAs QDs [12], which are formed electrostatically within a twodimensional electron gas (2DEG) by means of surface gates.In this Letter we study the spin states of a one-electron lateral QD directly, by performing energy spectroscopy and relaxation measurement...

We have used a suspended carbon nanotube as a frequency mixer to detect its own mechanical motion. A single gate-dependent resonance is observed, which we attribute to the fundamental bending mode vibration of the suspended carbon nanotubes. A continuum model is used to fit the gate dependence of the resonance frequency, from which we obtain values for the fundamental frequency, the residual and gate-induced tension in the nanotube. This analysis shows that the nanotubes in our devices have no slack and that, by applying a gate voltage, the nanotube can be tuned from a regime without strain to a regime where it behaves as a vibrating string under tension.