QUROPE Quantum Information Processing and Communication in Europe

A. Physical approach and perspective

III-V Semiconductor heterostructures (e.g. GaAs, InP, InAs, etc) form the backbone of today’s opto-electronics combining ultrafast electronics (e.g. HEMT), low-power optics together with the conversion between electronics and optics. The industrial development of this material class has also been fruitfully utilized in the field of QIPC. Employing nanofabrication and/or self-assembling techniques, quantum dots have been defined that can be addressed electrically and/or optically. Each quantum dot contains one electron, the spin of which serves as the qubit (earlier quantum dot work on electron charge qubits and on excitonic qubits has been phased out, because of the short coherence times). The emerging field of quantum opto-electronics can provide an interface between solid state qubits and single-photon quantum optics.

Currently, quantum dot (QD) spin based quantum information processing (QIP) is pursued by ~20 groups worldwide, 11 of which are located in Europe [L. Kouwenhoven (Delft, NL), L. Vandersypen (Delft, NL), K. Ensslin (ETH-Zurich, CH), J. Finley (TU-Munich, DE), M. Bayer (Dortmund, DE), M. Atature (Cambridge, UK), D. Zumbuhl (Basel, CH) R. Warburton (Basel, CH) and A. Imamoglu (ETH-Zurich, CH)], as well as G. Burkard (Konstanz, DE), D. Loss (Basel, CH) and Y. Nazarov (Delft, NL) on the theory side.

B. State of the art

Two main technologies are used to form quantum dots, self-assembly and nanofabrication. Self-assembled quantum dots are controlled and detected mostly by optical means; lithographically defined quantum dots are controlled and detected electrically. Despite these differences, much of the underlying physics is the same in these two systems. The state-of-the art is as follows:

Lithographically defined quantum dots

• Quantum dot circuits with up to three quantum dots have been controllably loaded with electrons;
• Single-shot read-out of a single spin state was demonstrated;
• Single-spin coherent rotations have been demonstrated both using magnetic and using electrical driving;
• Coherent exchange of two spins in a double quantum has been demonstrated;
• Relaxation times (T1) from milliseconds to one second have been observed, and the relaxation mechanism has been established;
• Spin coherence times of ~1 microsecond have been measured, and the main decoherence mechanism has been established;
• Partial control of the nuclear spin environment (the main source of decoherence) has been achieved.

Self-assembled quantum dots

• High fidelity initialization of an electron spin was achieved using optical pumping;
• Single spin measurement using Faraday roation has been demonstrated;
• Long electron spin lifetime has been measured;
• Quantum nature of light generated by a strongly coupled quantum dot cavity system has been demonstrated;
• Optical pumping of a single hole and coherent population trapping have been demonstrated;
• Coherent rotation of a single spin has been achieved;
• Photon blockade in a quantum dot cavity system has been demonstrated;
• Optically controlled exchange interaction between two quantum dots has been realized;
• Partial control of nuclear spin environment has been achieved.

C. Short-term goals (3-5 years)

• Integrate electrically controlled single-qubit gates, two-qubit gates and single-shot read-out into a single device;
• Demonstrate optically controlled single- and two-qubit gates;
• Realize coupling between spins on a chip, via striplines or on-chip cavities;
• Interconvert between single electron spins and single-photon polarization (standing qubit to flying qubit conversion);
• Make control of the otherwise random nuclear Overhauser field routine, in order to extend the dephasing times;
• Extend system size from two qubits to three;
• Implement simple quantum algorithms, error correction protocols, etc.;
• Explore and compare alternative semiconductor materials for quantum dots.

D. Long-term goals (10 years and beyond)

• Develop multi-qubit circuits in a scalable architecture;
• Improve fidelity to the level needed for fault tolerance;
• Demonstrate a quantum repeater (photon to spin to photon conversion).

E. Key references

[1] D. Loss and D. DiVincenzo, ‘‘Quantum computation with quantum dots’’, Phys. Rev. A 57, 120–126 (1998)
[2] R. Hanson, L.P Kouwenhoven, J.R. Petta, S. Tarucha, and L.M.K. Vandersypen, "Spins in few-electron quantum dots", Reviews of Modern Physics 79, 1217 (2007)
[3] R. Hanson and D.D. Awschalom, "Coherent manipulation of single spins in semiconductors ", Nature 453, 1043 (2008)