Spin qubit projects

Back to spin qubits.

Control of the nuclear field via electron-nuclear feedback

The nuclear spins in the quantum dot electron spin’s environment collective produce an “Overhauser” effective magnetic field on the electron spin. This nuclear field is normally uncontrolled and its randomness quickly dephases the electron spin. We demonstrate partial control over the nuclear field by exploiting electron-nuclear feedback: when applying microwave magnetic fields, we observe that the electron spin resonance frequency can remain locked to the excitation frequency, even when this excitation frequency or the external magnetic field are changed. The nuclear field thereby adjusts itself such that the electron spin resonance condition remains satisfied. We argue that this feedback reduces the nuclear field randomness by more than a factor of 10. See also the press release and the article.

Electrical control of a single electron spin

Usually spins are manipulated using ac magnetic fields (see also below). Here we control an electron spin in a quantum dot by means of ac electric fields. The electric field couples to the spin due to the spin-orbit interaction. In comparison to magnetic fields, electric fields can be generated much more easily, simply by exciting one of the gates that forms the dot. In addition, this allows for greater spatial selectivity, which is important for local addressing of individual spins.  See also the press release and 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 studied 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).  article

Single-shot spin read-out of a single spin

We have demonstrated single-shot read-out of single- and two-electron spin states in a quantum dot. We allow an electron to escape from the dot or not depending on its spin state. The occupation of the dot is then measured using a quantum point contact next to the dot, so we can infer 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.

Earlier projects:

• 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.