C. C. Lo

Photon counting system for subnanosecond fluorescence lifetime measurements

A new photon counting system has been developed for subnanosecond fluorescence lifetime measurements. The system incorporates a nanosecond light pulser, a dual counter unit, and a constant‐fraction discriminator. The operating conditions of the light pulser have been adjusted to minimize the spread of the light pulse waveshape. The discriminator has upper and lower level adjustments and a time walk of no more than ±35 psec over a 50‐mV to 5‐V input pulse amplitude variation. The measuring system has a total system time resolution, expressed as the FWHM of the light pulse, of 800 and 1480 psec using photomultipliers 8850 and 8852, respectively, with full photocathode illumination and optimized operating conditions. The system will measure both single and multiple decay components, and it is designed and optimized for experiments involving measurements of decay time constants as short as 90 psec.

Short-Channel Transistors Constructed with Solution-Processed Carbon Nanotubes

We develop short-channel transistors using solution-processed single-walled carbon nanotubes (SWNTs) to evaluate the feasibility of those SWNTs for high-performance applications. Our results show that even though the intrinsic field-effect mobility is lower than the mobility of CVD nanotubes, the electrical contact between the nanotube and metal electrodes is not significantly affected. It is this contact resistance which often limits the performance of ultrascaled transistors. Moreover, we found that the contact resistance is lowered by the introduction of oxygen treatment. Therefore, high-performance solution-processed nanotube transistors with a 15 nm channel length were obtained by combining a top-gate structure and gate insulators made of a high-dielectric-constant ZrO(2) film. The combination of these elements yields a performance comparable to that obtained with CVD nanotube transistors, which indicates the potential for using solution-processed SWNTs for future aggressively scaled transistor technology.

Reaching the quantum limit of sensitivity in electron spin resonance

The detection and characterization of paramagnetic species by electron spin resonance (ESR) spectroscopy is widely used throughout chemistry, biology and materials science, from in vivo imaging to distance measurements in spin-labelled proteins. ESR relies on the inductive detection of microwave signals emitted by the spins into a coupled microwave resonator during their Larmor precession. However, such signals can be very small, prohibiting the application of ESR at the nanoscale (for example, at the single-cell level or on individual nanoparticles). Here, using a Josephson parametric microwave amplifier combined with high-quality-factor superconducting microresonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly four orders of magnitude. We demonstrate the detection of 1,700 bismuth donor spins in silicon within a single Hahn echo with unit signal-to-noise ratio, reduced to 150 spins by averaging a single Carr-Purcell-Meiboom-Gill sequence. This unprecedented sensitivity reaches the limit set by quantum fluctuations of the electromagnetic field instead of thermal or technical noise, which constitutes a novel regime for magnetic resonance. The detection volume of our resonator is ∼ 0.02 nl, and our approach can be readily scaled down further to improve sensitivity, providing a new versatile toolbox for ESR at the nanoscale.

Measurement of very short fluorescence lifetimes by single-photon counting.

Measurement of very short fluorescence lifetimes by the single‐photon technique is made possible by an improved fluorescence lifetime system. Fluorescence lifetimes of 4.94±0.07 nsec for anthracene in cyclohexane, 640±30 psec for diphenyl butadiene in cyclohexane, and 90±30 psec for erythrosin in water were determined. The use of a small wavelength shift between excitation and emission minimizes the effect of the wavelength dependence of the photomultiplier response and light pulser emission. The effects of deaeration of solutions and time averaging of the excitation profile are presented. We investigated the origin of small‐amplitude early and late artifactual peaks in the light pulser and fluorescence profiles. Complications in the analysis of lifetime data introduced by intrinsic fluorescence and phosphorescence processes in commonly used absorption filters are discussed. Certain ’’blind spots’’ are found in the electronic pulse pileup rejection schemes most commonly used in photon counting.

Single atom doping for quantum device development in diamond and silicon

The ability to inject dopant atoms with high spatial resolution, flexibility in dopant species, and high single ion detection fidelity opens opportunities for the study of dopant fluctuation effects and the development of devices in which function is based on the manipulation of quantum states in single atoms, such as proposed quantum computers. The authors describe a single atom injector, in which the imaging and alignment capabilities of a scanning force microscope (SFM) are integrated with ion beams from a series of ion sources and with sensitive detection of current transients induced by incident ions. Ion beams are collimated by a small hole in the SFM tip and current changes induced by single ion impacts in transistor channels enable reliable detection of single ion hits. They discuss resolution limiting factors in ion placement and processing and paths to single atom (and color center) array formation for systematic testing of quantum computer architectures in silicon and diamond.

Controlling spin relaxation with a cavity

Spontaneous emission of radiation is one of the fundamental mechanisms by which an excited quantum system returns to equilibrium. For spins, however, spontaneous emission is generally negligible compared to other non-radiative relaxation processes because of the weak coupling between the magnetic dipole and the electromagnetic field. In 1946, Purcell realized that the rate of spontaneous emission can be greatly enhanced by placing the quantum system in a resonant cavity. This effect has since been used extensively to control the lifetime of atoms and semiconducting heterostructures coupled to microwave or optical cavities, and is essential for the realization of high-efficiency single-photon sources. Here we report the application of this idea to spins in solids. By coupling donor spins in silicon to a superconducting microwave cavity with a high quality factor and a small mode volume, we reach the regime in which spontaneous emission constitutes the dominant mechanism of spin relaxation. The relaxation rate is increased by three orders of magnitude as the spins are tuned to the cavity resonance, demonstrating that energy relaxation can be controlled on demand. Our results provide a general way to initialize spin systems into their ground state and therefore have applications in magnetic resonance and quantum information processing. They also demonstrate that the coupling between the magnetic dipole of a spin and the electromagnetic field can be enhanced up to the point at which quantum fluctuations have a marked effect on the spin dynamics; as such, they represent an important step towards the coherent magnetic coupling of individual spins to microwave photons.

Performance Studies of High Gain Photomultiplier Having Z-Configuration of Microchannel Plates

The characteristics of a high gain type ITT F4129 photomultiplier having three microchannel plates in cascade for electron multiplications have been investigated. These plates are in the Z-configuration. Measurements are given of the gain, dark current, cathode quantum efficiency, anode pulse linearity, electron transit time, single and multiphoton time spreads, fatigue, and pulse height resolution. The gain as a function of transverse magnetic field has been measured and is discussed. Photomultiplier characteristics as a function of the input pulse repetition frequency have also been investigated and discussed.

Performance Studies of Prototype Microchannel Plate Photomultipliers

The characteristics of prototype photomultipliers having high gain microchannel plates for electron multiplication have been investigated. Measurements are given of the dark current, quantum efficiency, anode pulse amplitude, electron transit time, single photoelectron time spread, and pulse height resolution of LEP HR 350 and HR 400 photomultipliers. The gain, the collection efficiency, and the single electron pulse amplitude as functions of the ambient axial and transverse magnetic fields have been measured and are discussed. Measurement techniques and descriptions of the measuring systems are given in detail.

Spin-dependent scattering off neutral antimony donors in Si28 field-effect transistors

We report measurements of spin-dependent scattering of conduction electrons by neutral donors in accumulation-mode field-effect transistors formed in isotopically enriched silicon. Spin-dependent scattering was detected using electrically detected magnetic resonance where spectra show resonant changes in the source-drain voltage for conduction electrons and electrons bound to donors. We discuss the utilization of spin-dependent scattering for the readout of donor spin states in silicon based quantum computers.

Electrical activation and electron spin resonance measurements of implanted bismuth in isotopically enriched silicon-28

We have performed continuous wave and pulsed electron spin resonance measurements of implanted bismuth donors in isotopically enriched silicon-28. Donors are electrically activated via thermal annealing with minimal diffusion. Damage from bismuth ion implantation is repaired during thermal annealing as evidenced by narrow spin resonance linewidths (Bpp=12μT) and long spin coherence times (T2=0.7 ms, at temperature T=8 K). The results qualify ion implanted bismuth as a promising candidate for spin qubit integration in silicon.