Cavendish

Laboratory

Cavendish

Laboratory

Semiconductor Physics GroupCavendish Laboratory, University of Cambridge

http://www.sp.phy.cam.ac.uk/

Electrically Contacting

Nanocrystals Using GrapheneThe semiconductor industry is in search of new devices

the size of molecules (<10 nm) to replace traditional

transistors in integrated circuits. Suitable conducting

molecules or nanocrystals can be made, but they must be

connected to an external circuit. The aim of this project is

to fabricate and measure samples in which a layer of

nanocrystals has been deposited on a gold electrode with

a graphene electrode lowered on top, forming a sandwich

structure in which the electrodes are kept apart by the

nanocrystals. This will enable studies of the quantum

properties of nanocrystals for potential applications.

This project is a collaboration with the Optoelectronics

Group (Dept. of Physics) and the Electrical Engineering

Division of the Dept. of Engineering.

Contact: Prof Chris Ford (cjbf@cam.ac.uk)

Optical Single-Electron

Spin Detector

For the use of electron spin in quantum information

processing (QIP), we need to detect the spin state

(up/down). This project will develop a spin detection

technique for travelling electron wave packets, which

would enable the use of electron wave packets as flying

qubits interconnecting stationary qubit elements, as well

generation of entangled pairs. Spin measurements will

use using a lateral p-n junction LED technology. Electrons

from a single-electron source will be injected into a p-type

region, and the polarisation of emitted photons will be

measured. Experiments will be performed both in

Cambridge and NPL.

This project is a collaboration with the

National Physical Laboratory and is

supported by an industrial CASE award.

Photon

released by

electron-hole

recombination

Electron trap Hot electron

Valence band

Conduction band

Contact: Prof Chris Ford (cjbf@cam.ac.uk)

The Quantum Multiplexer

The quantum multiplexer is a technology for measuring

large arrays (>1000) of quantum devices. We have

demonstrated studies of quantum effects in 256 1D wires,

and are now extending the multiplexer technology to

other types of devices with important applications. Large

arrays of quantum dots can be used as an artificial solid

state system for quantum simulations and studies of

condensed matter phenomena. Large scale

parallelisation of single-electron pumps is a promising

candidate for a quantum standard of electrical current,

analogous to the Josephson voltage standard.

H. Al-Taie et al., Appl. Phys. Lett. 102, 243102 (2013)

R.K. Puddy et al., Appl. Phys. Lett. 107, 143501 (2015)

This work is a collaboration with the University of

Cambridge Department of Engineering.

Contact: Prof Charles Smith (cgs4@cam.ac.uk)

Topological Materials

Nanowire of topological insulator material

Sb2Te3, fabricated by collaborators in the

Dept. of Materials, Univ. of Cambridge.

States of matter have traditionally been classified by the

symmetries they break, e.g., broken translational

symmetry in crystals. Recently, states of matter have

emerged whose properties are governed by the topology

of their band structure, rather than symmetry alone. For

example in topological insulators (TIs), the conduction and

valence bands are 'inverted'. Remarkably, his leads to

conducting states on the surface of the material that

cannot be removed by local perturbations. Due to

symmetry restrictions these surface states are 'protected'

against backscattering. There is great interest in using

such topologically protected states towards low-power

electronics, spin-based logic, and fault-tolerant quantum

communication. Several projects in the SP group are

probing both the fundamental physics and potential

applications of these topological materials.

Contacts: Dr Vijay Narayan

(vn237@cam.ac.uk), Prof

Chris Ford (cjbf@cam.ac.uk)Contact: Dr Malcolm Connolly (mrc61@cam.ac.uk)

http://connollylab.weebly.com/

Topological Phases in

Superconductor-Semiconductor

Hybrids

In this project you will develop devices and techniques for

detecting topological phases of matter that are predicted

to emerge from the collective motion of electrons in 2D

materials in contact with superconductors. Confirming the

existence of these phases would not only dramatically

underscore quantum theory, but could also have massive

implications for topological qubits for quantum computing.

Working with state-of-the-art superconductor deposition

systems and semiconductor device fabrication, you will

develop devices and use a combination of low-

temperature scanning probe techniques, magnetometry,

and radiofrequency transport measurements to

investigate their properties.

THz Excitation of

Quantum DevicesThe SP group has a strong track record of developing

both quantum devices (e.g. quantum wires, dots and

charge pumps) and sources of THz-frequency radiation.

Studies of the interplay between THz EM waves and the

quantum behaviour of electrons are difficult because of

the lack of suitable THz lasers and the attenuation of THz

electrical signals by metallic conductors. This project will

couple the SP group’s quantum cascade lasers (QCLs)

with quantum devices at sub-kelvin temperatures, using

custom-built metallic waveguides (MWGs).

Flexible waveguideDetector

QCL

MWG

Contact: Prof. David Ritchie (dar11@cam.ac.uk)

Molecular-Beam

Epitaxy• Growth of layered

semiconductor structures,

one atomic layer at a time

• GaAs, AlGaAs, InAs, GaN

etc.

Electron Beam Lithograpy• Patterning of nanoscale devices

• Length scales less than de Broglie

wavelength → quantum behaviour

• Quantum dots, photonic crystals

etc.

Semiconductor Cleanroom• Extensive facilities for making

semiconductor devices

• Lithography, metallisation, insulators

etc.

Low-T Measurements• Transport measurements at

T down to 30 mK

• When kBT < ΔE, quantum

effects become apparent

• Magnetic fields up to 18 T

• Cryogenic scanning probe

microscopy

Group Facilities

THz Optics Laboratory• Development of THz devices and systems

Cavendish

Laboratory

Cavendish

Laboratory

Semiconductor Physics GroupCavendish Laboratory, University of Cambridge

http://www.sp.phy.cam.ac.uk/

Quantum Optics and Quantum InformationThe SP group works closely with the Quantum Information Group of Toshiba Research Europe Limited, developing

devices for quantum computing and quantum communication using the quantum states of single-photon

polarisation and quantum dot electron spin. Contact: Prof David Ritchie (dar11@cam.ac.uk)

1.00 µm

0.00 µm

200nm

Image size 2 x 2 µm

Growth and positioning of InAs quantum dots:

Recombination of electron-hole pairs in the dot is

a source of single photons

Nanotechnology, 22, 065302 (2011)

www.quantum.toshiba.co.uk

n-type DBR

p-type DBR

i-type

InAs

quantum dot

InP

substrate

(a) (b)

(c)

Quantum teleportation

using an entangled LED

Nature Commun., 4,

2859 (2013)

Photonic crystal waveguide:

The waveguide directs single

photons emitted from a quantum dot

so they can be used efficiently for

information transfer

Appl. Phys. Lett., 99, 261108 (2011)

Controlled-NOT gate with single photons

A key building block

of quantum logic.

Appl. Phys. Lett.,

100, 211103 (2012)

Terahertz PhysicsTechnology for generation, manipulation and detection of THz radiation is underdeveloped.

But THz has many potential applications, in medical imaging, security screening,

spectroscopy and communications. The THz research group studies many aspects of THz

technology, in collaboration with Teraview UK Ltd and major QCL research groups across

Europe. Contact: Prof D.A. Ritchie (dar11@cam.ac.uk), Dr H.E. Beere (heb1000@cam.ac.uk)

THz sources: Quantum

Cascade Lasers (QCLs)

Graphene-coupled

THz optics

Integrated copper

QCL waveguides

Other active research areas:

• QCL active-region design for improved divergence, mode selection, output power, and

maximum operating temperature.

• THz spectroscopy and microscopy

• Interaction of THz radiation with quantum condensed matter systems

• Integration of graphene into novel THz components e.g. modulators and detectors

Spin-Orbit and Superconducting Hybrids

In materials with strong spin-orbit interactions (SOI), the electron spin can be controlled

using gate voltages, without the need for magnetic fields, giving the possibility of a spin

transistor and other spintronic components for quantum computing. We are studying the

effects of the SOI on the quantum behaviour of electrons and holes in 2D, 1D and 0D

(quantum dot) systems, using materials such as InGaAs and Ge. We are now extending

this research to hybrid superconductor-semiconductor devices, which are predicted to

show many novel quantum phenomena. Contact: Prof Charles Smith (cgs4@cam.ac.uk)

Controlled splitting of superconductor Cooper

pairs: Entangled pairs of electron spins in a Cooper

pair could be split and manipulated using spin-orbit

interactions in a semiconductor, for use in spin-based

quantum computing.

Majorana fermions in a quantum-dot-superconductor

chain: Superconducting proximity effect is predicted to

give rise to Majorana fermions, which have neither

fermionic or bosonic exchange statistics and are important

candidates for topological quantum computing.

J.D. Sau + S. Das Sarma, Nature Comms. 3, 964 (2012)L. Hofstetter et al., Nature 461, 960 (2009)

Physics of Low-Dimensional Systems

Low-dimensional nanostructures made from high quality group III/group V compound

semiconductors, such as GaAs, AlGaAs, InAs etc., allow us to study the fundamental

quantum properties of electrons confined to two, one or even zero spatial dimensions.

Contact: Prof David Ritchie (dar11@cam.ac.uk), Prof Charles Smith (cgs4@cam.ac.uk)

High-frequency read out of electron charge and

spin states in quantum dots – a key quantum

information processing technique.

(K.D. Petersson et al, Nanoletters 2010, 2789)

Electron-hole interactions in closely-spaced 2D bilayers, which

are predicted to lead to quantum phases like a superfluid made of

electron-hole pairs (excitons). (K. Das Gupta et al, Advances in

Condensed Matter Physics 2011, 727958)

Electron pumps transmit an integer no. of electrons, with repeat

frequency f, giving quantised current I = nef. They can act as

quantum current standards and on-demand single-electron

sources. (S.P. Giblin et al, Nature Communications 3 930 (2012))

Low-Temperature Scanning Probe Microscopy

This project uses novel scanning probe techniques to visualise and directly influence the

wave-like properties of electrons in nanostructures made from a new breed of quasi-2D

Dirac materials such as graphene and the surface states of topological insulators. Future

directions for the project include controlling electron trajectories with patterned

electrostatic junctions, pinpointing topological currents, and reading out single-electron

effects using radiofrequency reflectometry. Contacts: Prof. Charles Smith

(cgs4@cam.ac.uk) and Dr Malcolm Connolly (mrc61@cam.ac.uk)

The spatial coherence of the

electron field can be visualized

directly by scanning a sharp metallic

tip over the surface while measuring

the conductance, in a technique

known as scanning probe

microscopy (SPM). (M.A. Topinka et

al. Physics Today 56, 47 (2003))

The biased tip of the SPM can be

used to deposit charge in a

dielectric layer electrostatically

coupled to a nanodevice, so new

features can be written into the

device in-situ. (M.R. Connolly et al.,

Applied Physics Letters 101,

023505 (2012))

SPM techniques allow us to

understand how the behaviour of

electrons in 2D materials arise from

the interplay between the

geometrical, crystallographic, and

electromagnetic environment seen

by electrons. (D. Herschleb et al.,

Phys. Rev. B 92, 125414 (2015))