The Delft Spin Qubit Project
Electron spin qubits in semiconductor quantum dots
Electron spins isolated in quantum dots placed in a magnetic field provide natural two-level quantum systems. In the Quantum Transport group, we study the coherent properties of single and coupled electron spins, with the goal of realizing an elementary quantum computer in the solid-state.
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People
Research plan
Recent achievements
Present goals
Image gallery
Publications
(of the entire Quantum Transport section)
PhD thesis
Spin qubits and quantum
transport in the news
Frank Koppens, Katja
Nowack, Ivo Vink
(PhD students)
Tristan Meunier (postdoc)
Lieven Vandersypen
(PI)
Our present research builds on earlier work with Leo Kouwenhoven, and by PhD students and postdocs Jeroen Elzerman (now at ETH), Ronald Hanson (now at UCSB), Laurens Willems van Beveren (now at UNSW) and Joshua Folk (now at UBC). In addition, a large number of Master students contribute(d) to our work.
The spin qubit research plan:
The starting point for our work is the proposal by Loss and DiVincenzo [PRA 57, 120, 1998]. They suggested that the spin of a single electron confined in a quantum dot could be used as a quantum bit (qubit), the building block of a quantum computer. Lateral quantum dots are best suited as all the tunnel barriers can be freely controlled using electrostatic gates. Based on this proposal, we have worked out a set of concrete and realistic ideas for initialization, manipulation and read-out of the spin states of the electron.
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Right: An electron microscope image of a double quantum dot. The light gray tones indicate the metal structure (in this case gold) which is used to create the quantum dots (dotted circles). |
Electrical control of a single electron spin
Recently we achieved coherent manipulation of a single electron spin in a double dot by applying
an ac magnetic field. Now we realized electron spin resonance also by means of an ac electric field.
In comparison to magnetic fields, electric fields can be generated much more easily, simply by exciting
a local gate electrode. In addition, this allows for greater spatial selectivity, which is important for
local addressing of individual spins. Here, the ac electric field is generated by applying
an ac voltage to one of the existing local gates. Our analysis indicates that the
electric field couples to the electron spin via spin-orbit interaction.
See also the press release,
QT newsitems
or the article.
Coherent control of a single electron spin
We realized magnetic resonance of a single electron spin in a quantum
dot, whereby spin flips are induced via an oscillating magnetic field, generated
on-chip. Electron spin resonance (ESR) in one quantum dot was detected with the
help of a second dot that contained a reference spin, via a spin-dependent
transport measurement through the two dots in series.
Coherent control of the quantum
state of the electron spin was achieved by applying short bursts of the
oscillating magnetic field. We observe about eight oscillations of the spin
state (so-called Rabi oscillations) during a microsecond burst. See also the press release,
QT newsitems
or the article.
Electron spin decoherence
We explored decoherence of electron spins in a quantum dot caused by interactions with
nuclear spins in the host semiconductor. From spin dependent transport measurements, we found
that the effect of the nuclear spins on the electron spins can be viewed as that of a
randomly oriented and slowly varying semiclassical magnetic field, with a magnitude
of about 1 mTesla. When electrons flow through the device, the electron spins in turn act back
on the nuclear spin bath, and cause dynamical nuclear polarization. This sometimes leads to a
striking bistable behavior.
See also the press release or
article
Electron spin relaxation
We have studied on what timescale an electron spin in a quantum dot can be flipped,
thereby transferring its energy to the environment. We found energy relaxation times of
the order of milliseconds, both for single electron spin states and
two-electron spin states.
Furthermore, we established that the dominant heat bath where spin-flip energy is
dissipated is the phonon bath (the phonons can couple to spin thanks to spin-orbit
interaction).
splitting have revealed the role of phonons in spin relaxation between
two-electron spin states. article;
Single-shot spin read-out
We have demonstrated single-shot read-out of single- and two-electron
spin states in a quantum dot. The spin is converted to charge, by allowing an
electron to escape from the dot or not depending on the spin state. The charge
state of the dot is then measured using a quantum point contact next to the
dot, so we also know what the spin state was. We have demonstrated two different ways
for the spin-dependent escape; one method exploits the energy difference between
the two spin states and the other method exploits a difference in tunnel rates.
Depending on the method, we achieved single-shot spin read-out fidelities from
82% to 97.5%.
See also the press release, article 1 or
article 2.
This work builds on the following earlier results:
• Isolation of a single electron in single and double lateral quantum dot structures. This was made possible by a special design of the dot and an integrated quantum point contact as a charge detector (see AFM image).
• Observation of the qubit levels in a direct electrical transport measurement of the Zeeman splitting of one electron in a quantum dot in a magnetic field. The splitting is about 20 μeV per Tesla. For comparison, the orbital level spacing is 1 meV and the charging energy a few meV.
• Spectroscopy of a nearly-isolated quantum dot, by modulating the potential of the dot while monitoring the conductance through a nearby quantum point contact.
• Real-time detection of single electron tunneling between the quantum dot and a reservoir, by monitoring the conductance fluctuations of a quantum point contact next to the dot. The shortest events we can see are about 8 μs.
- Demonstration and control of entanglement of two spins in neighbouring quantum dots.
- Suppressing decoherence by reducing the randomness in the normally uncontrolled nuclear field.
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Illustration Gemma Plum. |
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Coherence and control of a single electron spin in a quantum dot
Frank Koppens
pdf
Electron spins in few-electron lateral quantum dots
Laurens H. Willems van Beveren
pdf
Electron Spins in Semiconductor Quantum Dots
Ronald Hanson
pdf
Electron spin and charge in semiconductor quantum dots
Jeroen M. Elzerman
pdf
Page created by Lieven Vandersypen, Dec 2002. Last updated December 2007