Semiconductor Physics Group

SAW Quantum Computation

• Ideas:
  • SAW quantum computation
  • Quantum information transfer
• Recent results:
  • Playing ping-pong with single electrons
  • Interaction of SAWs with a static quantum dot
  • Investigation of SAW reflections
  • Tunnelling behaviour in SAW-defined dynamic quantum dots
  • Numerical modelling of SAW devices

For information on possible PhD projects in this group, please contact Chris Ford or Crispin Barnes

SAW quantum computation

The idea is to use the spin of the single electrons trapped in SAW potential minima as qubits:

• A single electron is trapped in each SAW potential minimum and is transported through a depleted 1D channel
• High-frequency qubit operations can be made by patterned surface gates/nanomagnets laid out on the chip
• A number of identical operations are repeated at the SAW frequency
• A flying qubit scheme on chip
• A scope for quantum information transfer to different qubit schemes (photon/static quantum dots)

Figure 1: SAW quantum computation scheme.

For more information see: Barnes et al., Phys. Rev. B, 62, 8410 (2000).

Quantum information transfer

Qubit states can be transferred to different types of qubits via entanglement. Entanglement between flying and static qubits is especially important for the storage and long-range communication of quantum information.

SAW quantum dot to static quantum dot

• An electron in SAW dot is trapped into a static dot, and is entangled with an electron in another static dot.

Figure 2: SAW dot to static dot transfer.

SAW quantum dot to photon

• Electrons are transported into p-type region, where they recombine with holes to emit photons.
• The spin information of the electrons are transferred into the circular polarisation of the photons.

Figure 3: SAW dot to photon transfer.

Playing ping-pong with single electrons

Figure 4: SAW-driven transfer of an electron between distant quantum dots. The diagram shows the potential energy of an electron as a pulse of surface acoustic waves passes a quantum dot, moving from bottom right to top left.

• A SAW pulse can be used to take an individual electron from one isolated quantum dot to another (from a dot at bottom right in the above figure, to one at top left).
• The quantum dots and the channel joining them are isolated from reservoirs by large barrier potentials.
• The energy levels in the dots and channel are above the Fermi energy, so other electrons have too little energy to enter the system.

Interaction of SAWs with a static quantum dot

• A SAW pulse can be used to populate/depopulate an isolated quantum dot.
• A quantum dot is isolated from reservoirs by large barrier potentials.
• An empty (or occupied) state is set below (or above) the Fermi energy.
• Due to large barrier potentials, the dot stays in this non-equilibrium charge state for ~ 100 sec.
• When a SAW pulse is sent through the dot, the potential modulation of the barrier forces the dot into charge equilibrium, populating (or depopulating) the dot by one electron.
• This method can be used to transfer an electron between a SAW dynamic dot and a gate-defined static dot.

Figure 4: Population and depopulation experiments on an isolated quantum dot using SAW pulses.

For more information see: Kataoka et al., Phys. Rev. Lett., 98, 046801 (2007).

Tunnelling behaviour in SAW-defined dynamic quantum dots

• The electrons in a SAW are confined to dynamic quantum dots (zero-dimensional electronic systems).
• By creating devices with tunnelling barriers alongside the SAW channel, we can probe the quantum states of the dot.
• We observe high-frequency quantum behaviour in the form of coherent charge oscillations, and hope to use these devices for spin-charge conversion.

Figure 6: (a) SEM of device incorporating a tunnel barrier into the SAW channel. (b) Current into and out of device. (c) Tunnelling rates calculated from the device currents. (d) Artistic impression of device operation.

For more information see: Astley et al., Phys. Rev. Lett. 99, 156802 (2007), Astley et al., accepted for publication in Physica E, or contact Masaya Kataoka or Mike Astley.

Investigation of SAW reflections

We have shown that the principal sources of "noise" in the frequency dependence of the acousto-electric current are caused by SAW reflections:

• Reflections come from the edge of the chip and the unused transducer.
• We developed pulse-modulation techniques that modify the effects of different reflections, so we can characterise individual reflection paths and remove them from the experiment.

Figure 5: (a) Noise in the acousto-electric current can be modified by changing the pulse conditions so different reflection paths are present. (b) Fourier-transform analysis shows peaks that correspond to individual reflection paths.

For more information see: Astley et al., Appl. Phys. Lett., 89, 132102 (2006), and Astley et al., Phys. Rev. B, 74, 193302 (2006), or contact Mike Astley.

Numerical modelling of SAW devices

• Electrostatic modelling is used to allow numerical simulation of the experimental devices.
• The time-dependent Schrodinger equation is solved numerically to determine the evolution of an electron in a dynamic quantum dot.

Figure 7: Electrostatic potential in a pair of 1D channels. Dynamic double quantum dots are illustrated in blue.

For more information contact Crispin Barnes or Adam Thorn.