The dynamic response of InAs/GaAs self-assembled quantum dots (QDs) to strain is studied experimentally by periodically modulating the QDs with a surface acoustic wave and measuring the QD fluorescence with photoluminescence and resonant spectroscopy. When the acoustic frequency is larger than the QD linewidth, we resolve phonon sidebands in the QD fluorescence spectrum. Using a resonant pump laser, we have demonstrated optical frequency conversion via the dynamically modulated QD, which is the physical mechanism underlying laser sideband cooling a nanomechanical resonator by means of an embedded QD.

We explore the quantum aspects of an elastic bar supported at both ends and subject to compression. If strain rather than stress is held fixed, the system remains stable beyond the buckling instability, supporting two potential minima. The classical equilibrium transverse displacement is analogous to a Ginsburg-Landau order parameter, with strain playing the role of temperature. We calculate the quantum fluctuations about the classical value as a function of strain. Excitation energies and quantum fluctuation amplitudes are compared for silicon beams and carbon nanotubes.PACS numbers: 03.65.-w, 62.25.+g, 46.32.+x, 05.40.-a The continuing drive towards semiconductor device miniaturization and integration has resulted in fabrication and micromachining technologies that are capable of producing artificial structures with features approaching the ten nanometer length scale. To go beyond this scale, naturally occurring and chemically organized structures are receiving much attention. The availability of these top-down and bottom-up nanofabrication capabilities has initiated the new area of nanomechanics [1,2,3,4,5] in which ultra small mechanical systems are used to explore both fundamental and applied phenomena. Recently, two reports have appeared on two-state nanomechanical systems. In one [6], crossed carbon nanotubes were suspended between supports and the suspended element was electrostatically flexed between two states. In the second [7], it was proposed to use an electrostatically flexed cantilever to explore the possibility of tunneling in a nanomechanical system.In this Letter we discuss quantum effects in a two-state mechanical system that has a tunable, symmetric potential function. This mechanical system has analogies to the superconducting interference device in which the first observation of a coherent superposition of macroscopically distinct states was recently reported [8]. Specifically, we consider a suspended elastic bar under longitudinal compression. The compression is used to adjust the potential energy for transverse displacements from the harmonic to the double-well regime, as shown in Fig. 1, with strain playing a role analogous to temperature in a Ginzburg-Landau system. As the compressional strain is increased to the buckling instability [9], the frequency of the fundamental vibrational mode drops continuously to zero. By controlling the separation between the ends of the bar, i.e. fixing the strain, the system remains stable beyond the instability and develops a double well potential for the transverse motion. Since both the well depth and asymmetry are tunable, a variety of quantum phenomena may be explored, including zero-point fluctuations, tunneling, and coherent superpositions of macroscopically distinct states. In the latter two cases, the system may provide a mechanical realization (at least in theory) of models studied in Refs.[10] and [11], respectively. We have applied the model to suspended silicon beams and carbon nanotubes, and show that in both cases the quantum fluctuatio...

We report on elastic instability of nanomechanical SiO2 beams with widths 20 nm<d<110 nm and lengths 5 μm<L<10 μm. The beams are fabricated from a silicon substrate with a 500 nm thermal oxide layer. After release from the silicon substrate by reactive ion etching the beams buckle due to the residual Si/SiO2 strain. The measured buckling displacements of the beams are compared with the predictions of nonlinear continuum elasticity theory. We observe a continuous buckling transition, qualitatively different than the critical transition predicted by Euler buckling theory, which we attribute to system asymmetry. Finally, we determine the effective potential energy of the fundamental buckling mode.

We use a cryogenic high-electron-mobility transistor circuit to amplify the current from a single electron transistor, allowing for demonstration of single shot readout of an electron spin on a single P donor in Si with 100 kHz bandwidth and a signal to noise ratio of ∼9. In order to reduce the impact of cable capacitance, the amplifier is located adjacent to the Si sample, at the mixing chamber stage of a dilution refrigerator. For a current gain of ∼2.7×103, the power dissipation of the amplifier is 13 μW, the bandwidth is ∼1.3 MHz, and for frequencies above 300 kHz the current noise referred to input is ≤70 fA/Hz. With this amplification scheme, we are able to observe coherent oscillations of a P donor electron spin in isotopically enriched 28Si with 96% visibility.

We examine a silicon-germanium heterojunction bipolar transistor (HBT) for cryogenic pre-amplification of a single electron transistor (SET). The SET current modulates the base current of the HBT directly. The HBT-SET circuit is immersed in liquid helium, and its frequency response from low frequency to several MHz is measured. The current gain and the noise spectrum with the HBT result in a signal-to-noise-ratio (SNR) that is a factor of 10−100 larger than without the HBT at lower frequencies. The transition frequency defined by SNR = 1 has been extended by as much as a factor of 10 compared to without the HBT amplification. The power dissipated by the HBT cryogenic pre-amplifier is approximately 5 nW to 5 μW for the investigated range of operation. The circuit is also operated in a single electron charge read-out configuration in the time-domain as a proof-of-principle demonstration of the amplification approach for single spin read-out. Donor spin qubits have recently received increased interest because of the demonstration of high fidelity coherent control of phosphorus donors using a local electron spin resonance technique. 1,2 This approach is of interest both for quantum information 3-5 as well as representing a new experimental platform to investigate the behavior of single impurities in semiconductors using electron and nuclear magnetic resonance. Single-shot readout 6-8 of the spin polarization is an important component of the measurement. It may be accomplished using a wide-band measurement of the single electron transistor 9 (SET) conductance, which is sensitive to the ionization condition of any nearby donors. 10,11 The technique relies on alignment of the neighboring SET chemical potential between discrete Zeeman energy levels. The donor spinup electron ionizes into the SET, leading to a detectable transient change in the local electrostatic potential, while a SET electron waits to reload into the donor as a spindown. The temporary ionization of the donor changes the conductance of the SET, which is measured as a current pulse corresponding to a spin-up electron or no pulse if the electron was spin-down.Read-out fidelity can be no better than what the signal-to-noise-ratio (SNR) provides for a particular bandwidth, although other factors can introduce errors that degrade the fidelity, such as rapid tunneling events that are faster than the bandwidth of the read-out. The donor read-out technique is performed at cryogenic temperatures less than 4 K, which is typically necessary to observe the spin read-out of the donor state at reasonably low magnetic fields. The SET current is subsequently amplified at room-temperature (RT) using one or several amplification stages, typically including a transconductance amplifier. The line capacitance between the transconductance amplifier and the SET typically sets the limits of performance of the circuit. Increased readout bandwidth can improve fidelity, for example, by detecting faster tunnel events, however, the increased bandwidth reduces SNR. The SNR can be i...