Quantum Circuits and Correlations (NANO)

Group leader

Professor Pertti Hakonen

We have developed, among others, record-sensitive single electron transistor (SET) components made out of carbon nanotubes and nearly back-action-free, reactively read superconducting electrometers. In addition, we have developed a novel, low-noise current amplifier, Bloch oscillating transistor, which lies between the superconducting quantum interferometer (SQUID) and the SET according to its characteristics. We also work on graphene field effect devices.

The Nano group is involved in several European projects such as Graphene Flagship and Interfacing Quantum Optical, Electrical, and Mechanical Systems (iQUOEMS), both are sponsored by the European Commission. We also benefit from several national and international bilateral collaborations.

Physics of Graphene

Electronic properties in graphene are being intensively studied since the discovery of the anomalous quantum Hall effect in this purely two-dimensional system. Owing to its unique band structure, graphene conduction occurs via massless Dirac fermions. Graphene is a gapless semiconductor: the conduction and the valence band are touching in two inequivalent points (K and K', usually called Dirac points) where the density of state is vanished. However, the conductivity at the Dirac point remains finite. Indeed, at the Dirac point, the conduction occurs only via evanescent waves, i.e. via tunnelling between the leads. The first evidence of such mechanism has been recently given by studying the minimum conductivity in short and wide strips. Read more about Graphene research.

Physics and applications of mesoscopic Josephson junctions

Mesoscopic Josephson junctions provide a unique opportunity to construct ultra sensitive quantum detectors and amplifiers. These devices are important when performing single-shot read-out of quantum bits (qubits) or making quantum measurements a la quantum optics style. The ultimate goal is to develop phase sensitive quantum amplifiers, parametric amplifiers that would allow for quantum nondemolition measurements.

Noise and high-frequency measurement techniques

The dominating noise mechanism in mesoscopic samples at low temperatures is shot noise. In some cases, it is the limiting factor for the measurement sensitivity, but shot noise itself may be the actual quantity of interest as it, contrary to the thermal noise, contains information about the sample, complement the average current. Many of the interesting predictions for noise have been obtained for nonlinear elements (with voltage-dependent response) whose resistance is typically in the range of kΩ or more. However, measurement of shot noise in such samples is not always straightforward as the excess noise added by the amplifiers depends on the sample impedance, and thus on the applied voltage.

Electronic transport in carbon nanotubes

Carbon nanotubes, found in 1991 by Sumio Iijima, represent extraordinary building blocks for nanotechnology and nanoelectronics. They may be considered as graphite sheets wrapped into seamless cylinders. The two types of nanotubes are multiwalled carbon nanotube (MWNT), where many tubes are arranged in a coaxial fashion, and a single-walled nanotube (SWNT), consisting of only a single layer. The tubes are either metallic, semi-metallic or semiconducting depending on how the graphite sheets are wrapped around.

Facilities

Dry pulse-tube based dilution refrigerators 

Two dry pulse-tube based dilution refrigerators: One BlueFors BF-LD250 cryostats, one BF-SD250. Cryostats are configured so that they can be employed from simple DC electrical measurements to involved microwave measurements for noise, cross-correlations, and vibrations in NEMS resonators. One cryostat is equipped with 9T superconducting magnet. Specifications: - Base temperature: 8-10 mK, - Cooling power mixing chamber @ 100 mK: 250 uW, - Cool-down time to base temperature SD: 18 hrs, - Cool-down time to base temperature LD 24 hrs, - Mixing chamber diameter: SD Series 150mm LD Series 290mm.

Dry nuclear demagnetization cryostat

Based on BlueFors model BF-LD400 dilution refrigerator with 9T superconducting magnet. Characteristics of dilution unit: - Base temperature: 7 mK, - Cooling power mixing chamber @ 100 mK: 550 μW. The experimental cell for liquid 3He (0.6 mol) is embedded inside the top part of the nuclear demagnetization stage. The refrigerator cools 3He in the experimental cell down to 160 μK.

Plastic Dilution Refrigerator (PDR) 

One small 3He/4He dilution refrigerator down to 30-50 mK with approximately 4-hour cool down time. These plastic dilution refrigerators, which are built in-house, provide us with fast cool down times and thus high throughput in sample characterization.

Room temperature measurement and characterization equipment 

A wide range of equipment for sample characterisation, including Micro-Raman, scanning electron microscope, atomic force microscope, RF testing equipment, etc.

Nano- and Micro-fabrication facilities 

The following facilities are available in the Low Temperature Labs semi-clean room: electron beam lithography, field emission electron microscope JEOL7100F, DCA UHV e-beam evaporator system with sample stage having angle control and rotation, film deposition system, reactive ion etching, critical point dryer, etc.