Final Report Summary - QON (Quantum optics using nanostructures: from many-body physics to quantum information processing)
A principal achievement of this project is the demonstration that resonant optical spectroscopy can be used to explore non-equilibrium many-body physics that could not be accessed using transport spectroscopy. A second, equally significant achievement is the demonstration of decoherence-free semiconductor qubits together with the realization of quantum entanglement between a confined spin and a propagating photon.
Interactions between electrons in a solid lead to "emergent" physical phenomena that cannot be simply understood by investigating the properties of individual parts. Best known examples for these so-called strongly correlated systems include superconductivity and fractional quantum hall states. Another important example is the Kondo effect, where free electrons dress up to screen the tiny magnetic moment produced by a single localized spin. In most prior studies of Kondo systems, the emphasis has been on determining equilibrium properties by measuring the electrical resistance of the material. A major achievement of the project is the experimental observation of non-equilibrium signatures of Kondo physics using optical spectroscopy. The abrupt turn-off or quantum quench of exchange interactions responsible for Kondo correlations upon single-photon absorption demonstrates the intimate connection between the Kondo physics and the Anderson orthogonality catastrophe; the latter is a consequence of a vanishing overlap between the initial and final many-body eigenstates and is evidenced by a power-law singularity characterizing the optical absorption line-shape. The techniques we have developed also allowed us to experimentally study interplay between Anderson orthogonality physics responsible for the Fermi-edge singularity and Fano-type quantum interference effects. The highlight of our theoretical work is the discovery of a new correlated many-body state that emerges due to simultaneous presence of Kondo correlations and non-perturbative laser coupling. The broader significance of our results stems from the fact that they provide an unequivocal demonstration of how quantum optics enables the investigation of non-equilibrium many-body phenomena not accessible to transport spectroscopy.
The observation of a nuclear-spin mediated locking of quantum dot optical resonances to an incident laser frequency is another highlight of the first period. This locking effect is enabled by a precise amount of nuclear spin polarization that leads to an effective magnetic (Overhauser) field, whose magnitude adjusts/determines the optical transition frequency. Conversely, one could consider this process as fixing the value of the nuclear Overhauser field using the laser frequency. Given that fluctuations in the Overhauser field produced by 100,000 quantum dot nuclear spins is the principal source of electron spin decoherence, the findings of the project are of central importance for solid-state quantum information processing.
Despite intense theoretical activity, the experimental progress in the emerging field of strongly correlated photons has been hindered by a lack of systems exhibiting strong photon-photon interactions. The observation of the photon blockade effect on a chip and the ultra-fast interaction between two single-photon pulses of different colour demonstrated that requisite nonlinearities are achievable in integrated semiconductor photonic structures.
Decoherence has strongly limited the use of quantum dot spins in quantum information processing tasks. Our demonstration of decoherence-free spin qubits based on singlet-triplet states of an optically active quantum dot molecule paves the way for realization of protocols exploiting the long coherence times and the superior optical properties of these structures. As a first step towards this goal, we have realized quantum entanglement between a confined spin and a propagating photon. We then used this quantum interface to demonstrate teleportation of quantum information from a photonic qubit to a quantum dot spin.
Interactions between electrons in a solid lead to "emergent" physical phenomena that cannot be simply understood by investigating the properties of individual parts. Best known examples for these so-called strongly correlated systems include superconductivity and fractional quantum hall states. Another important example is the Kondo effect, where free electrons dress up to screen the tiny magnetic moment produced by a single localized spin. In most prior studies of Kondo systems, the emphasis has been on determining equilibrium properties by measuring the electrical resistance of the material. A major achievement of the project is the experimental observation of non-equilibrium signatures of Kondo physics using optical spectroscopy. The abrupt turn-off or quantum quench of exchange interactions responsible for Kondo correlations upon single-photon absorption demonstrates the intimate connection between the Kondo physics and the Anderson orthogonality catastrophe; the latter is a consequence of a vanishing overlap between the initial and final many-body eigenstates and is evidenced by a power-law singularity characterizing the optical absorption line-shape. The techniques we have developed also allowed us to experimentally study interplay between Anderson orthogonality physics responsible for the Fermi-edge singularity and Fano-type quantum interference effects. The highlight of our theoretical work is the discovery of a new correlated many-body state that emerges due to simultaneous presence of Kondo correlations and non-perturbative laser coupling. The broader significance of our results stems from the fact that they provide an unequivocal demonstration of how quantum optics enables the investigation of non-equilibrium many-body phenomena not accessible to transport spectroscopy.
The observation of a nuclear-spin mediated locking of quantum dot optical resonances to an incident laser frequency is another highlight of the first period. This locking effect is enabled by a precise amount of nuclear spin polarization that leads to an effective magnetic (Overhauser) field, whose magnitude adjusts/determines the optical transition frequency. Conversely, one could consider this process as fixing the value of the nuclear Overhauser field using the laser frequency. Given that fluctuations in the Overhauser field produced by 100,000 quantum dot nuclear spins is the principal source of electron spin decoherence, the findings of the project are of central importance for solid-state quantum information processing.
Despite intense theoretical activity, the experimental progress in the emerging field of strongly correlated photons has been hindered by a lack of systems exhibiting strong photon-photon interactions. The observation of the photon blockade effect on a chip and the ultra-fast interaction between two single-photon pulses of different colour demonstrated that requisite nonlinearities are achievable in integrated semiconductor photonic structures.
Decoherence has strongly limited the use of quantum dot spins in quantum information processing tasks. Our demonstration of decoherence-free spin qubits based on singlet-triplet states of an optically active quantum dot molecule paves the way for realization of protocols exploiting the long coherence times and the superior optical properties of these structures. As a first step towards this goal, we have realized quantum entanglement between a confined spin and a propagating photon. We then used this quantum interface to demonstrate teleportation of quantum information from a photonic qubit to a quantum dot spin.