Ultrafast optical rotations of electron spins in quantum dots

Abstract

Coherent manipulation of quantum bits (qubits) on timescales much shorter than the coherence time 1,2 is a key prerequisite for quantum information processing. Electron spins in quantum dots are particularly attractive for implementations of qubits, and efficient optical methods for initialization and readout of spins have been developed in recent years 3,4 . Spin coherence times in the microsecond range have been demonstrated 5 . Therefore, spin control by picosecond optical pulses would be highly desirable so that a large number of spin rotations could be carried out while coherence is maintained. A major remaining challenge is demonstration of such rotations with high fidelity. Here, we use an ensemble of quantum-dot electron spins focused into a small number of precession modes about a magnetic field by periodic optical pumping. We demonstrate ultrafast optical rotations of spins about arbitrary axes on a picosecond timescale using laser pulses as control fields.Spin rotations have been obtained for gated GaAs transport quantum dots using radiofrequency fields. But such manipulations are slow, with times comparable to the spin coherence time 6 . Fast optical rotation operations on spins in quantum dots are being sought actively, but progress so far has been limited. In an all-optical experiment on shallow interface-fluctuation quantum dots, weak confinement precluded full unitary spin rotations 7 . Evidence for spin rotations was reported for a single GaAs interface-fluctuation quantum dot using very intense optical excitation needed for spins to be rotated by precession about the effective laser magnetic field 8 . Very recently, convincing rotations in a single InAs self-organized quantum dot were reported 9 . This work on single quantum dots 8,9 used pulses far from resonance, which could lead to excitation of other spins. This could inhibit the use of spins with separations less than the laser spot size for two-quantum-bit entanglement. In general, optical coupling to a spin in a single quantum dot is weak, and thus spin rotations on single quantum dots give weak responses. This generally necessitates many optical pulses in time to obtain sufficient fidelity, and it can lead to extra dephasing.An ensemble of quantum dots has the advantage of increasing the optical coupling and can decrease the loss of information from the loss of a few spins. However, ensemble approaches typically are hampered by the inhomogeneities in quantum dot properties, particularly in the spin-splitting g -factor, and lead to dephasing of the spin precession about a magnetic field 10,11 . Recently we have demonstrated techniques for eliminating part of the inhomogeneity in the electron spin precession in ensembles of singly charged quantum dots 12 . We have found that at low magnetic fields the spin ensemble can be put into a Zeeman spectrum 13 that is very close to a single precession mode. This is the system that we study S about the magnetic field. b, Spin polarization is generated by laser pulses resonant with the trion (two e...

Quantum teleportation from a propagating photon to a solid-state spin qubit

“…On the other hand, single spins confined in QDs have been extensively studied as promising solid-state qubits with long coherence time and a potential for integration on a chip [26][27][28][29][30][31] . By realizing teleportation of the quantum state of a photonic qubit generated by one QD to the spin state of another QD located in a different cryostat, we demonstrate that these two auspicious features of QDs can be combined to realize an elementary process relevant for a quantum network 32,33 .…”

Scalable qubit architecture based on holes in quantum dot molecules

“…1 Spins confined in III-V semiconductor self-assembled quantum dots (QDs) have received a great deal of attention because they interact strongly with light and provide the opportunity for ultrafast all-optical implementation of logic operations. [2][3][4][5] There has been dramatic progress in the initialization, coherent manipulation and readout of single spins in GaAs and InGaAs QDs, [6][7][8][9][10][11][12][13] but many challenges to the creation of a scalable quantum logic device based on optical control of single spins remain. One serious obstacle is the natural inhomogeneous distribution of energy levels in a quantum dot ensemble.…”

Quantum control of electron spins in the two-dimensional electron gas of a CdTe quantum well with a pair of Raman-resonant phase-locked laser pulses

“…1 results from individual laser pulses. This term corresponds to spin rotations induced by a DC magnetic field along the x-axis, as extensively investigated in earlier studies [7][8][9][10][11][12][13][14] . The second term in Eq.…”

“…The coherent optical interactions can also be exploited for the generation of spin-photon quantum entanglement in atomic as well as solid-state spin systems [3][4][5][6] . Optical spin control using ultrafast laser pulses has been demonstrated in various semiconductor systems [7][8][9][10][11][12][13] and also in trapped ions 14 . In these experiments, individual off-resonant laser pulses act like an effective DC magnetic field along the optical axis, inducing a spin rotation about the fixed axis.…”

“…The full quantum control of spins, however, requires spin rotation about two orthogonal axes 15 . For spin rotations about an axis orthogonal to the optical axis, recent studies have exploited spin precession about an external magnetic field 9,12 and have also attempted the use of additional laser pulses to induce a geometrical phase shift 16,17 .…”