V. Antonov

A single-photon detector in the far-infrared range

The far-infrared region (wavelengths in the range 10 µm–1 mm) is one of the richest areas of spectroscopic research, encompassing the rotational spectra of molecules and vibrational spectra of solids, liquids and gases. But studies in this spectral region are hampered by the absence of sensitive detectors—despite recent efforts to improve superconducting bolometers, attainable sensitivities are currently far below the level of single-photon detection. This is in marked contrast to the visible and near-infrared regions (wavelengths shorter than about 1.5 µm), in which single-photon counting is possible using photomultiplier tubes. Here we report the detection of single far-infrared photons in the wavelength range 175–210 µm (6.0–7.1 meV), using a single-electron transistor consisting of a semiconductor quantum dot in high magnetic field. We detect, with a time resolution of a millisecond, an incident flux of 0.1 photons per second on an effective detector area of 0.1 mm2—a sensitivity that exceeds previously reported values by a factor of more than 104. The sensitivity is a consequence of the unconventional detection mechanism, in which one absorbed photon leads to a current of 106–1012 electrons through the quantum dot. By contrast, mechanisms of conventional detectors or photon assisted tunnelling in single-electron transistors produce only a few electrons per incident photon.

Single-photon detector in the microwave range

Single-photon counting at microwave frequencies around 500 GHz is demonstrated by using a single-electron transistor (SET) formed by two capacitively coupled GaAs/AlxGa1−xAs parallel quantum dots (QDs). A point contact separating the double QDs allows the prompt escape of an excited electron from one of the QDs to another. The resulting long-lived photoinduced ionization of the QD is detected as a change in the SET current.

Counting Individual Trapped Electrons on Liquid Helium

We show that small numbers of electrons, including a single isolated electron, can be held in an electrostatic trap above the surface of superfluid helium. A potential well is created using microfabricated electrodes in a 5 μm diameter pool of helium. Electrons are injected into the trap from an electron reservoir on a helium microchannel. They are individually detected using a superconducting single-electron transistor as an electrometer. A Coulomb staircase is observed as electrons leave the trap one–by–one until the trap is empty. A design for a scalable quantum information processor using an array of electron traps is presented.

Highly sensitive detector for submillimeter wavelength range

A highly sensitive detector of submillimeter wavelength radiation is reported. The detector consists of a semiconductor quantum dot (QD) and a metallic single-electron transistor (SET). The SET detects change in the potential distribution induced by photon absorption within the QD. We have fabricated and studied this detector at wavelengths longer than 200μm. High sensitivity, ∼10−20W∕Hz in terms of noise equivalent power, is found. Further optimization of the detector design is suggested.

Magnetic bead detection using domain wall-based nanosensor

We investigate the effect of a single magnetic bead (MB) on the domain wall (DW) pinning/depinning fields of a DW trapped at the corner of an L-shaped magnetic nanodevice. DW propagation across the device is investigated using magnetoresistance measurements. DW pinning/depinning fields are characterized in as-prepared devices and after placement of a 1 μm-sized MB (Dynabeads® MyOne™) at the corner. The effect of the MB on the DW dynamics is seen as an increase in the depinning field for specific orientations of the device with respect to the external magnetic field. The shift of the depinning field, ΔBdep = 4.5–27.0 mT, is highly stable and reproducible, being significantly above the stochastic deviation which is about 0.5 mT. The shift in the deppinning field is inversely proportional to the device width and larger for small negative angles between the device and the external magnetic field. Thus, we demonstrate that DW-based devices can be successfully used for detection of single micron size MB.

Point contact readout for a quantum dot terahertz sensor

We introduce a terahertz radiation sensor in which the photon-induced ionization state of a quantum dot is monitored by a point contact formed in the same semiconductor heterostructure. For comparison we used a readout based on a single electron transistor coupled to the same quantum dot. The experiments prove functionality of the point contact-based device with additional practical advantage of a higher operation temperature up to 1.5K and ease of nanofabrication.

Sensitive detector for a passive terahertz imager

We report progress in developing a sensitive detector for terahertz radiation, based on a semiconductor quantum dot (QD) capacitively coupled to a metallic single electron transistor (SET). A charge polarization of the QD induced by the absorption of individual photons is detected by the voltage-biased SET. We investigate the sensitivity of the detector to broadband radiation, over a range of QD barrier heights, and find that there is a measurable photo-signal over wide range of gate voltages defining the QD. This is an improvement on previous designs of terahertz detector based on the QD/SET principle, and makes the new detector a candidate for use in an imaging device.

Electric field effects and screening in mesoscopic bismuth wires

Large time-independent conduction fluctuations were observed as a function of transverse electric field in thin (25 nm) and narrow (60 nm) bismuth wires. The conduction of leads far away from a gate capacitor was influenced by changes in the gate voltage. The effects are interpreted as being due to a variation in the Fermi wavelength caused by gate-induced changes in the charge concentration of the leads rather than an electrostatic Aharonov-Bohm-type interference. The screening of charge is strongly reduced in narrow wires

Sensing individual terahertz photons.

One of the promising ways to perform single-photon counting of terahertz radiation consists in sensitive probing of plasma excitation in the electron gas upon photon absorption. We demonstrate the ultimate sensor operating on this principle. It is assembled from a GaAs/AlGaAs quantum dot, electron reservoir and superconducting single-electron transistor. The quantum dot is isolated from the surrounding electron reservoir in such a way that when the excited plasma wave decays, an electron could tunnel off the dot to the reservoir. The resulting charge polarization of the dot is detected with the single-electron transistor. Such a system forms an easy-to-use sensor enabling single-photon counting in a very obscure wavelength region.

A Highly Sensitive Detector for Radiation in the Terahertz Region

In this paper, we report progress in the development of a detector for photons in the terahertz region consisting of a lateral quantum dot (QD), defined in a semiconductor heterostructure by mesa patterning and three negatively biased metallic gates, and a single-electron transistor (SET) on top of the mesa and, hence, capacitively coupled to the QD. We study the behavior of the QD as a function of the potential applied to the gates using the SET as a sensitive charge detector and identify the bias region of the device, where it is sensitive to incident terahertz radiation. The QD converts incident photons into charge excitations, which can be detected by the SET, resulting in a signal of the order 10 8 electrons for each absorbed photon. Based on the dark count rate and an estimate of the quantum efficiency, the detector should enable low-power measurements in the terahertz region with noise-equivalent power ~10-19 W/Hz1/2 exceeding the sensitivity of commercially available bolometers by two orders of magnitude