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.
- Publications (on spin qubits only)
The spin qubit team
- Pasquale Scarlino (MSc student)
- Scarlett Hendrichs (MSc student)
- Mohammad Shafiei (PhD student)
- Floris Braakman (PhD student)
- Pierre Barthelemy (Postdoc)
- Katja Nowack (Postdoc)
- Lars Schreiber (Postdoc)
- Lieven Vandersypen (PI)
Our present research builds on earlier work with Leo Kouwenhoven, and by PhD students Ivo Vink, Frank Koppens (now in Barcelona), Laurens Willems van Beveren (now at UNSW), Ronald Hanson (Currently working on Quantum information in Diamond in the Quantum Transport group) and Jeroen Elzerman (now at ETH), postdocs Tristan Meunier (now at CNRS) 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.
Schematic diagram of the Loss & DiVincenzo computer.
AFM Image of a double quantum dot, integrated with quantum point contacts on both sides.
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An electron microscope image of a triple quantum dot. The light gray tones indicate the metal structure (in this case gold) which is used to create the quantum dots (dotted circles).
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Recent achievements
Control of the nuclear field via electron-nuclear feedback
Accurate coherent control of spin qubits in GaAs quantum dots has been hampered by their coupling to the uncontrolled bath of nuclear spins. We now achieved partial control over the nuclear field experienced by the electron spins by exploiting electron-nuclear feedback. This feedback arises from a combination of driving electron spin resonance and the hyperfine interaction between nuclear spins and the electron spin. In particular, we observe that the electron spin resonance frequency remains locked to the frequency of an applied microwave magnetic field, even when the external magnetic field or the excitation frequency are changed. The nuclear field thereby adjusts itself such that the electron spin resonance condition remains satisfied. General theoretical arguments indicate that this spin resonance locking is accompanied by a a narrowing of the nuclear field distribution, in the present experiment by more than a factor of 10. See also the press release and the article.
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 thepress 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.
Image gallery
Illustration Gemma Plum.
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An artist’s impression of the electron spin in the quantum dot formed by the metallic gates. This type of single quantum dot device with an adjacent charge detector is used for single-shot read-out of a single electron spin. Illustration Kavli Institute of Nanoscience Delft.
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PhD theses
Electrical Manipulation and Detection of Single Electron Spins in Quantum Dots
Katja Nowack
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Manipulation and Read-out of Spins in Quantum Dots
Ivo Vink
pdf
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
Support
This research is supported by the Dutch Organization for Fundamental Research on Matter (FOM), by the Netherlands Organization for Scientific Research (NWO) and by the European Research Council (ERC).
