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Edward Flagg
Department of Physics and Astronomy

Spin Measurement

Quantum information processing promises to make computationally intractable problems – such as prime factorization – efficiently solvable [DiVincenzo, 1995], but the physical implementation of a quantum computer faces many challenges. Quantum bits, or qubits, are the fundamental building blocks of a quantum computer. One candidate qubit system is the spin degree of freedom of a single electron trapped in a quantum dot (QD) [Liu, 2010]. QDs are often called “artificial atoms” because electrons trapped within them have discrete energy levels in the fashion of single atoms. Because of their discrete energies and their spin, when placed in a magnetic field, an electron in a QD behaves like a canonical two-level quantum system suitable to function as a qubit. The necessary capabilities of a single qubit are initialization, manipulation, and measurement of its quantum state [DiVincenzo, 2000]. Unfortunately, with the current state of the field, the experimental conditions for initialization and manipulation of a spin in a QD are incompatible with a single-shot measurement of the spin [Flagg & Solomon, 2015]. Therefore, there is a critical need for a method to perform a single-shot measurement of the electron spin-state while maintaining the initialization and manipulation capabilities. Achievement of this capability is an important problem because without the ability to perform all three operations, optically active QDs will not be viable quantum bits.

The central hypothesis driving the proposal is that the effect of the magnetic field necessary to allow spin measurement can instead be mimicked by the AC Stark effect of a laser. The AC Stark effect causes a shift of the energy levels of a QD, and it is capable of modifying the allowed optical transitions, resulting in spin-preserving cycling transitions that could be used for a single-shot measurement. The hypothesis is supported by published evidence that the AC Stark effect of an off-resonant laser can both modify the transition energies of a QD and manipulate the electron spin [Unold, 2004; Berezovsky, 2008]. The rationale for this approach is that the AC Stark effect can be switched on and off rapidly simply by turning a laser on and off, which is a much faster process – by about 9 orders of magnitude – than changing the direction of a magnetic field.

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This research is funded by the U.S. Department of Energy Office of Science, grant number DE-SC0016848  

Related publications

  1. Wilkinson TA, Maurer CE, Flood CJ, Lander G, Chafin S, Flagg EB. Complete Stokes vector analysis with a compact, portable rotating waveplate polarimeter. Review of Scientific Instruments, 92(9):093101 (2021).
  2. Wilkinson T.A., Cottrill D.J., Cramlet J.M., Maurer C.E., Flood C.J., Bracker A.S., Yakes M., Gammon D., Flagg E.B. Dynamic nuclear polarization in a charged quantum dot induced by the AC Stark effect. SPIE Nanoscience + Engineering Proceedings. Quantum Nanophotonic Materials, Devices, and Systems 2019, 11091:110910I (2019). doi:10.1117/12.2529455
  3. Wilkinson, T. A., Cottrill, D. J., Cramlet, J. M., Maurer, C. E., Flood, C. J., Bracker, A. S., Yakes, M., Gammon, D., Flagg, E. B. Spin-selective AC Stark shifts in a charged quantum dot.
    Applied Physics Letters 114, 133104 (2019). doi:10.1063/1.5084244
  4. Flagg, E. B. & Solomon, G. S. Optical spin readout method in a quantum dot using the ac Stark effect. Phys. Rev. B 92, 245309 (2015). doi:10.1103/PhysRevB.92.245309 


  1. Berezovsky, J., Mikkelsen, M. H., Stoltz, N. G., Coldren, L. A., and Awschalom, D. D. Picosecond Coherent Optical Manipulation of a Single Electron Spin in a Quantum DotScience 320, no. 5874 (2008): 349–352. doi:10.1126/science.1154798
  2. DiVincenzo, D. P. “Quantum ComputationScience 270, no. 5234 (1995): 255–261. doi:10.1126/science.270.5234.255
  3. DiVincenzo, D. P. “The Physical Implementation of Quantum ComputationFortschritte der Physik 48, no. 9–11 (2000): 771–783. doi:10.1002/1521-3978(200009)48:9/11<771::AID-PROP771>3.0.CO;2-E
  4. Flagg, E. B. and Solomon, G. S. “Optical Spin Readout Method in a Quantum Dot Using the Ac Stark EffectPhysical Review B 92, no. 24 (2015): 245309. doi:10.1103/PhysRevB.92.245309
  5. Unold, T., Mueller, K., Lienau, C., Elsaesser, T., and Wieck, A. D. “Optical Stark Effect in a Quantum Dot: Ultrafast Control of Single Exciton PolarizationsPhysical Review Letters 92, no. 15 (2004): 157401. doi:10.1103/PhysRevLett.92.157401