Query: Si/SiGe quantum dots for highly coherent spin qubits

Candidate papers:

abstract: We demonstrated coherent control of a quantum two level system based on two electron spin states in a double quantum dot, allowing state preparation, coherent manipulation, and projective readout. These techniques are based on rapid electrical control of the exchange interaction. Separating and later recombining a singlet spin state provided a measurement of the spin dephasing time, T2* of ~10 nanoseconds, limited by hyperfine interactions with the gallium arsenide host nuclei. Rabi oscillations of two electron spin states were demonstrated, and spin echo pulse sequences were used to suppress hyperfine induced dephasing. Using these quantum control techniques, a coherence time for two electron spin states exceeding 1 microsecond was observed.
author_names: Jason R Petta

A C Johnson

J M Taylor

E A Laird

Amir Yacoby

Mikhail D Lukin

Charles M Marcus

Micah P Hanson

Arthur C Gossard
title: Coherent Manipulation of Coupled Electron Spins in Semiconductor Quantum Dots
venue: Science
year: 2005

abstract: We propose an implementation of a universal set of one and two quantum bit gates for quantum computation using the spin states of coupled single electron quantum dots. Desired operations are effected by the gating of the tunneling barrier between neighboring dots. Several measures of the gate quality are computed within a recently derived spin master equation incorporating decoherence caused by a prototypical magnetic environment. Dot array experiments that would provide an initial demonstration of the desired nonequilibrium spin dynamics are proposed.
author_names: Daniel Loss

David P DiVincenzo
title: Quantum computation with quantum dots
year: 1998

abstract: The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin based quantum computer. In combination with the recently demonstrated controlled exchange gate between two neighbouring spins, driven coherent single spin rotations would permit universal quantum operations. Here, we report the experimental realization of single electron spin rotations in a double quantum dot. First, we apply a continuous wave oscillating magnetic field, generated on chip, and observe electron spin resonance in spin dependent transport measurements through the two dots. Next, we coherently control the quantum state of the electron spin by applying short bursts of the oscillating magnetic field and observe about eight oscillations of the spin state (so called Rabi oscillations) during a microsecond burst. These results demonstrate the feasibility of operating single electron spins in a quantum dot as quantum bits.
author_names: Frank H L Koppens

Christo Buizert

Klaas-Jan Tielrooij

I T Vink

Katja C Nowack

Tristan Meunier

L P Kouwenhoven

Lieven M K Vandersypen
title: Driven coherent oscillations of a single electron spin in a quantum dot
venue: Nature
year: 2006

abstract: We investigate the effects of the orientation of the magnetic field and the orientation of a quantum dot, with respect to crystallographic coordinates, on the quality of an electrically controlled qubit realized in a gated semiconductor quantum dot. We find that, due to the anisotropy of the spin orbit interactions, by varying the two orientations it is possible to tune the qubit in the sense of optimizing the ratio of its couplings to phonons and to a control electric field. We find conditions under which such optimal setup can be reached by solely reorienting the magnetic field, and when a specific positioning of the dot is required. We also find that the knowledge of the relative sign of the spin orbit interaction strengths allows to choose a robust optimal dot geometry, with the dot main axis along [110] or [110] where the qubit can be always optimized by reorienting the magnetic field. (c) 2016 American Physical Society. (Less)
author_names: Ognjen Malkoc

Peter Stano

Daniel Loss
title: Optimal geometry of lateral GaAs and Si/SiGe quantum dots for electrical control of spin qubits
year: 2016

abstract: The canonical example of a quantum mechanical two level system is spin. The simplest picture of spin is a magnetic moment pointing up or down. The full quantum properties of spin become apparent in phenomena such as superpositions of spin states, entanglement among spins, and quantum measurements. Many of these phenomena have been observed in experiments performed on ensembles of particles with spin. Only in recent years have systems been realized in which individual electrons can be trapped and their quantum properties can be studied, thus avoiding unnecessary ensemble averaging. This review describes experiments performed with quantum dots, which are nanometer scale boxes defined in a semiconductor host material. Quantum dots can hold a precise but tunable number of electron spins starting with 0, 1, 2, etc. Electrical contacts can be made for charge transport measurements and electrostatic gates can be used for controlling the dot potential. This system provides virtually full control over individual electrons. This new, enabling technology is stimulating research on individual spins. This review describes the physics of spins in quantum dots containing one or two electrons, from an experimentalist's viewpoint. Various methods for extracting spin properties from experiment are presented, restricted exclusively to electrical measurements. Furthermore, experimental techniques are discussed that allow for 1 the rotation of an electron spin into a superposition of up and down, 2 the measurement of the quantum state of an individual spin, and 3 the control of the interaction between two neighboring spins by the Heisenberg exchange interaction. Finally, the physics of the relevant relaxation and dephasing mechanisms is reviewed and experimental results are compared with theories for spin orbit and hyperfine interactions. All these subjects are directly relevant for the fields of quantum information processing and spintronics with single spins i.e. single spintronics.
author_names: Ronald Hanson

L P Kouwenhoven

Jason R Petta

Seigo Tarucha

Lieven M K Vandersypen
title: Spins in few electron quantum dots
year: 2007

abstract: Nanofabricated quantum bits permit large scale integration but usually suffer from short coherence times due to interactions with their solid state environment. The outstanding challenge is to engineer the environment so that it minimally affects the qubit, but still allows qubit control and scalability. Here, we demonstrate a long lived single electron spin qubit in a Si/SiGe quantum dot with all electrical two axis control. The spin is driven by resonant microwave electric fields in a transverse magnetic field gradient from a local micromagnet, and the spin state is read out in the single shot mode. Electron spin resonance occurs at two closely spaced frequencies, which we attribute to two valley states. Thanks to the weak hyperfine coupling in silicon, a Ramsey decay timescale of 1 ms is observed, almost two orders of magnitude longer than the intrinsic timescales in GaAs quantum dots, whereas gate operation times are comparable to those reported in GaAs. The spin echo decay time is ~40 ms, both with one and four echo pulses, possibly limited by intervalley scattering. These advances strongly improve the prospects for quantum information processing based on quantum dots.
author_names: Erika Kawakami

P Scarlino

Daniel R Ward

Floris R Braakman

Donald E Savage

Max G Lagally

Mark Friesen

Susan N Coppersmith

M A Eriksson

Lieven M K Vandersypen
title: Electrical control of a long lived spin qubit in a Si/SiGe quantum dot.
venue: Nature nanotechnology
year: 2014

abstract: Recent theory predicted that the quantum spin Hall effect, a fundamentally new quantum state of matter that exists at zero external magnetic field, may be realized in HgTe/(Hg,Cd)Te quantum wells. We fabricated such sample structures with low density and high mobility in which we could tune, through an external gate voltage, the carrier conduction from n type to p type, passing through an insulating regime. For thin quantum wells with well width d 6.3 nanometers, the insulating regime showed the conventional behavior of vanishingly small conductance at low temperature. However, for thicker quantum wells (d 6.3 nanometers) the nominally insulating regime showed a plateau of residual conductance close to 2e2/h, where e is the electron charge and h is Planck's constant. The residual conductance was independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance was destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, d 6.3 nanometers, was also independently determined from the magnetic field induced insulator to metal transition. These observations provide experimental evidence of the quantum spin Hall effect.
author_names: Markus Konig

Steffen Wiedmann

Christoph Brune

Andreas Roth

Hartmut Buhmann

Laurens W Molenkamp

Xiao-liang Qi

Shou-Cheng Zhang
title: Quantum Spin Hall Insulator State in HgTe Quantum Wells
venue: Science
year: 2007

abstract: Quantum computers promise to exceed the computational efficiency of ordinary classical machines because quantum algorithms allow the execution of certain tasks in fewer steps. But practical implementation of these machines poses a formidable challenge. Here I present a scheme for implementing a quantum mechanical computer. Information is encoded onto the nuclear spins of donor atoms in doped silicon electronic devices. Logical operations on individual spins are performed using externally applied electric fields, and spin measurements are made using currents of spin polarized electrons. The realization of such a computer is dependent on future refinements of conventional silicon electronics.
author_names: Bruce E Kane
title: A silicon based nuclear spin quantum computer
venue: Nature
year: 1998

abstract: We demonstrate coherent driving of a single electron spin using second harmonic excitation in a Si/SiGe quantum dot. Our estimates suggest that the anharmonic dot confining potential combined with a gradient in the transverse magnetic field dominates the second harmonic response. As expected, the Rabi frequency depends quadratically on the driving amplitude, and the periodicity with respect to the phase of the drive is twice that of the fundamental harmonic. The maximum Rabi frequency observed for the second harmonic is just a factor of 2 lower than that achieved for the first harmonic when driving at the same power. Combined with the lower demands on microwave circuitry when operating at half the qubit frequency, these observations indicate that second harmonic driving can be a useful technique for future quantum computation architectures.
author_names: P Scarlino

Erika Kawakami

Daniel R Ward

Donald E Savage

Max G Lagally

Mark Friesen

Susan N Coppersmith

Mark A Eriksson

Lieven M K Vandersypen
title: Second Harmonic Coherent Driving of a Spin Qubit in a Si/SiGe Quantum Dot.
venue: Physical review letters
year: 2015

abstract: We implement a technique for measuring the singlet triplet energy splitting responsible for spin to charge conversion in semiconductor quantum dots. This method, which requires fast, single shot charge measurement, reliably extracts an energy in the limits of both large and small splittings. We perform this technique on an undoped, accumulation mode Si/SiGe triple quantum dot and find that the measured splitting varies smoothly as a function of confinement gate biases. Not only does this demonstration prove the value of having an $in~situ$ excited state measurement technique as part of a standard tune up procedure, it also suggests that in typical Si/SiGe quantum dot devices, spin blockade can be limited by lateral orbital excitation energy rather than valley splitting.
author_names: Emily J Pritchett

Tyler E Keating

R W Andrews

Jacob Z Blumoff

L A De Lorenzo

K Eng

Andrey A Kiselev

Se'an M Meenehan

Seth T Merkel

L F Edge

Matthew T Rakher

M G Borselli

Andrew T Hunter
title: Spin Blockade Spectroscopy of Si Si Ge Quantum Dots
venue: Physical Review Applied
year: 2019