Optical Control of Coherent Interactions between Electron Spins in InGaAs Quantum Dots

Abstract

Coherent interactions between spins in quantum dots are a key requirement for quantum gates. We have performed pump-probe experiments in which pulsed lasers emitting at different photon energies manipulate two distinct subsets of electron spins within an inhomogeneous InGaAs quantum dot ensemble. The spin dynamics are monitored through their precession about an external magnetic field. These measurements demonstrate spin precession phase shifts and modulations of the magnitude of one subset of oriented spins after optical orientation of the second subset. The observations are consistent with results from a model using a Heisenberg-like interaction with µeV-strength.PACS numbers: 78.67. Hc, 78.47.jh Considerable progress has been made recently in establishing optical control of spins confined in semiconductor quantum dots (QDs), a system of interest for quantum bits (qubits) in implementations of quantum information [1]. Single spin decoherence times on the order of microseconds have been demonstrated [2], and methods for spin initialization and readout have been developed [3,4]. Recently, progress in demonstrating optical rotations of single spins has been made [5][6][7][8]. To be useful in quantum information, spin manipulation times must be orders of magnitude faster than decoherence times [1], which is possible using fast optical methods. Interactions between spins in QD systems can provide the mechanism for coherent control in quantum logic but can also complicate their coherent dynamics. The case of coupling between spins in QD molecules has been well studied [9][10][11][12], but long ranged interactions are not yet understood.An ensemble of QDs has the advantage of having strong optical coupling, but ensemble approaches typically have been hampered by inhomogeneities in their properties, particularly spin splittings, which lead to fast spin dephasing. In previous work we have demonstrated nuclear assisted optical techniques for removing some of the effects of these inhomogeneities [2,13]. In these techniques, periodic pulse trains orient spins normal to an external magnetic field, and particular subsets of spins precess in phase with the pulse trains. At rather low magnetic fields, around B = 1 T, a spin ensemble can be put into a state in which only few spin precession modes contribute [14]. This is the system that we study here.In the present work two subsets of spins are selected by spectrally narrow, circularly polarized laser pulse trains of different photon energies. The subsets are oriented by the two laser pulses (pump 1 and pump 2), and precess around a perpendicular magnetic field. The relative phase of the two precessions is controlled by the time difference between the two pulses. We find that after the second pump pulse, the precession associated with the first spin subset acquires a phase shift that depends on the relative orientation of the spins. It emerges smoothly in time after pump 2. In addition, the precession amplitude shows modulations and decreases with time. The major experiment...

Unambiguous determination of spin dephasing times in ZnO by time‐resolved magneto‐optical pump–probe experiments

“…As described above, spin replacement in the pump–pump–probe delay scans only reduces the spin amplitude from the first spin packet, but does not change its phase or precession frequency demonstrating that there is no coherent interaction between both spin packets. We note, however, that coherent control of the magnetic exchange interaction between localized electron spins has recently been achieved in self‐assembled InGaAs/GaAs quantum dots (). Due to an inhomogeneous size distribution of the self‐assembled quantum dots, there is an energy broadening of about 20 meV in the energy level spectrum.…”

Erratum: Quantum model for mode locking in pulsed semiconductor quantum dots [Phys. Rev. B 94 , 245308 (2016)]

“…Typically, the laser radiation is circularly polarized in one single helicity [9,38], so that one of the trion states decouples. We therefore restrict ourselves to a threedimensional Hilbert space for the central spin, with the basis {|↑ , |↓ , |T } where |T ≡ |⇑↑↓ encodes the trion state that is relevant to the dynamics.…”

Nonequilibrium nuclear spin distribution function in quantum dots subject to periodic pulses

“…Furthermore, the different geometries of different QDs yield different electronic confinement potentials and consequently, different laser energies are required to pump the trion states of each QD [14,15]. A monotonous connection between the trion excitation energy and the electron g-factor g e has been experimentally used [16] to address sub-sets of a QD ensemble with differently colored laser light. Since the fluctuations of the Overhauser field define the shorttime dephasing time T * [4,5,17], our calculations can be interpreted as either simulations for an ensemble of identical QDs, or the accumulated time average of many consecutive measurements of the spin-polarisation on a single QD.…”