-
A quantum light emitting diode for the standard telecom window around 1550 nm
Authors:
T. Müller,
J. Skiba-Szymanska,
A. Krysa,
J. Huwer,
M. Felle,
M. Anderson,
R. M. Stevenson,
J. Heffernan,
D. A. Ritchie,
A. J. Shields
Abstract:
For the development of long-distance quantum networks, sources of single photons and entangled photon pairs emitting in the low-loss wavelength region around 1550 nm are a crucial building block. Here we show that quantum dot devices based on indium phosphide are capable of electrically injected single photon emission in this wavelength region with multiphoton events suppressed down to 0.11$\pm$0.…
▽ More
For the development of long-distance quantum networks, sources of single photons and entangled photon pairs emitting in the low-loss wavelength region around 1550 nm are a crucial building block. Here we show that quantum dot devices based on indium phosphide are capable of electrically injected single photon emission in this wavelength region with multiphoton events suppressed down to 0.11$\pm$0.02. Using the biexciton cascade mechanism, they further produce entangled photons with a fidelity of 87$\pm$4%, sufficient for the application of one-way error correction protocols. The new material allows for entangled photon generation up to an operating temperature of 93 K, reaching a regime accessible by electric coolers. The quantum photon source can be directly integrated with existing long distance quantum communication and cryptography systems and provides a new material platform for developing future quantum network hardware.
△ Less
Submitted 10 October, 2017;
originally announced October 2017.
-
MOVPE growth, transmission electron microscopy and magneto-optical spectroscopy of individual InAsP/GaInP quantum dots
Authors:
O. Del Pozo-Zamudio,
J. Puebla,
A. B. Krysa,
R. Toro,
A. M. Sanchez,
R. Beanland,
A. I. Tartakovskii,
M. S. Skolnick,
E. A. Chekhovich
Abstract:
We report on growth and characterization of individual InAsP/GaInP quantum dots with variable nominal As molar fraction. Magneto-photoluminescence experiments reveal QD emission in a wide range from 1.3 to 1.8 eV confirming incorporation of As into quantum dots. Transmission electron microscopy reveals a core-cap structure of InAsP quantum dots with an InAs-rich core capped by an InP-rich layer. I…
▽ More
We report on growth and characterization of individual InAsP/GaInP quantum dots with variable nominal As molar fraction. Magneto-photoluminescence experiments reveal QD emission in a wide range from 1.3 to 1.8 eV confirming incorporation of As into quantum dots. Transmission electron microscopy reveals a core-cap structure of InAsP quantum dots with an InAs-rich core capped by an InP-rich layer. Inside the core, an As molar fraction up to x=0.12 is observed. The heavy hole g-factor is found to be strongly dependent on As molar fraction, while the electron g-factor is close to the InP values. This suggests type-II carrier confinement in the studied InAsP dots with holes (electrons) localized in the core (cap) region. Finally, dynamic nuclear polarization is observed which allows for further insight into structural properties using nuclear magnetic resonance.
△ Less
Submitted 5 May, 2017;
originally announced May 2017.
-
Universal growth scheme for entanglement-ready quantum dots
Authors:
Joanna Skiba-Szymanska,
R. Mark Stevenson,
Christiana Varnava,
Martin Felle,
Jan Huwer,
Tina Müller,
Anthony J. Bennett,
James P. Lee,
Ian Farrer,
Andrey Krysa,
Peter Spencer,
Lucy E. Goff,
David A. Ritchie,
Jon Heffernan,
Andrew J. Shields
Abstract:
Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fibre optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure-splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applicatio…
▽ More
Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fibre optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure-splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into 1D optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realise QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs droplet-epitaxy of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at ~900nm, and metal-organic vapour phase epitaxy on InP emitting at 1550 nm, with mean FSS 4x smaller than for Stranski-Krastanow QDs.
△ Less
Submitted 27 September, 2016;
originally announced September 2016.
-
High resolution nuclear magnetic resonance spectroscopy of highly-strained quantum dot nanostructures
Authors:
E. A. Chekhovich,
K. V. Kavokin,
J. Puebla,
A. B. Krysa,
M. Hopkinson,
A. D. Andreev,
A. M. Sanchez,
R. Beanland,
M. S. Skolnick,
A. I. Tartakovskii
Abstract:
Much new solid state technology for single-photon sources, detectors, photovoltaics and quantum computation relies on the fabrication of strained semiconductor nanostructures. Successful development of these devices depends strongly on techniques allowing structural analysis on the nanometer scale. However, commonly used microscopy methods are destructive, leading to the loss of the important link…
▽ More
Much new solid state technology for single-photon sources, detectors, photovoltaics and quantum computation relies on the fabrication of strained semiconductor nanostructures. Successful development of these devices depends strongly on techniques allowing structural analysis on the nanometer scale. However, commonly used microscopy methods are destructive, leading to the loss of the important link between the obtained structural information and the electronic and optical properties of the device. Alternative non-invasive techniques such as optically detected nuclear magnetic resonance (ODNMR) so far proved difficult in semiconductor nano-structures due to significant strain-induced quadrupole broadening of the NMR spectra. Here, we develop new high sensitivity techniques that move ODNMR to a new regime, allowing high resolution spectroscopy of as few as 100000 quadrupole nuclear spins. By applying these techniques to individual strained self-assembled quantum dots, we measure strain distribution and chemical composition in the volume occupied by the confined electron. Furthermore, strain-induced spectral broadening is found to lead to suppression of nuclear spin magnetization fluctuations thus extending spin coherence times. The new ODNMR methods have potential to be applied for non-invasive investigations of a wide range of materials beyond single nano-structures, as well as address the task of understanding and control of nuclear spins on the nanoscale, one of the central problems in quantum information processing.
△ Less
Submitted 17 December, 2011;
originally announced December 2011.
-
Isotope sensitive measurement of the hole-nuclear spin interaction in quantum dots
Authors:
E. A. Chekhovich,
A. B. Krysa,
M. Hopkinson,
P. Senellart,
A. Lemaitre,
M. S. Skolnick,
A. I. Tartakovskii
Abstract:
Decoherence caused by nuclear field fluctuations is a fundamental obstacle to the realization of quantum information processing using single electron spins. Alternative proposals have been made to use spin qubits based on valence band holes having weaker hyperfine coupling. However, it was demonstrated recently both theoretically and experimentally that the hole hyperfine interaction is not neglig…
▽ More
Decoherence caused by nuclear field fluctuations is a fundamental obstacle to the realization of quantum information processing using single electron spins. Alternative proposals have been made to use spin qubits based on valence band holes having weaker hyperfine coupling. However, it was demonstrated recently both theoretically and experimentally that the hole hyperfine interaction is not negligible, although a consistent picture of the mechanism controlling the magnitude of the hole-nuclear coupling is still lacking. Here we address this problem by performing isotope selective measurement of the valence band hyperfine coupling in InGaAs/GaAs, InP/GaInP and GaAs/AlGaAs quantum dots. Contrary to existing models we find that the hole hyperfine constant along the growth direction of the structure (normalized by the electron hyperfine constant) has opposite signs for different isotopes and ranges from -15% to +15%. We attribute such changes in hole hyperfine constants to the competing positive contributions of p-symmetry atomic orbitals and the negative contributions of d-orbitals. Furthermore, we find that the d-symmetry contribution leads to a new mechanism for hole-nuclear spin flips which may play an important role in hole spin decoherence. In addition the measured hyperfine constants enable a fundamentally new approach for verification of the computed Bloch wavefunctions in the vicinity of nuclei in semiconductor nanostructures.
△ Less
Submitted 3 October, 2012; v1 submitted 4 September, 2011;
originally announced September 2011.
-
Charge control in InP/GaInP single quantum dots embedded in Schottky diodes
Authors:
O. D. D. Couto Jr.,
J. Puebla,
E. A. Chekhovich,
I. J. Luxmoore,
C. J. Elliott,
N. Babazadeh,
M. S. Skolnick,
A. I. Tartakovskii,
A. B. Krysa
Abstract:
We demonstrate control by applied electric field of the charge states in single self-assembled InP quantum dots placed in GaInP Schottky structures grown by metalorganic vapor phase epitaxy. This has been enabled by growth optimization leading to suppression of formation of large dots uncontrollably accumulating charge. Using bias- and polarization-dependent micro-photoluminescence, we identify th…
▽ More
We demonstrate control by applied electric field of the charge states in single self-assembled InP quantum dots placed in GaInP Schottky structures grown by metalorganic vapor phase epitaxy. This has been enabled by growth optimization leading to suppression of formation of large dots uncontrollably accumulating charge. Using bias- and polarization-dependent micro-photoluminescence, we identify the exciton multi-particle states and carry out a systematic study of the neutral exciton state dipole moment and polarizability. This analysis allows for the characterization of the exciton wavefunction properties at the single dot level for this type of quantum dots. Photocurrent measurements allow further characterization of exciton properties by electrical means, opening new possibilities for resonant excitation studies for such system.
△ Less
Submitted 13 July, 2011;
originally announced July 2011.
-
Direct measurement of the hole-nuclear spin interaction in single quantum dots
Authors:
E. A. Chekhovich,
A. B. Krysa,
M. S. Skolnick,
A. I. Tartakovskii
Abstract:
We use photoluminescence spectroscopy of ''bright'' and ''dark'' exciton states in single InP/GaInP quantum dots to measure hyperfine interaction of the valence band hole with nuclear spins polarized along the sample growth axis. The ratio of the hyperfine constants for the hole (C) and electron (A) is found to be C/A~-0.11. In InP dots the contribution of spin 1/2 phosphorus nuclei to the hole-nu…
▽ More
We use photoluminescence spectroscopy of ''bright'' and ''dark'' exciton states in single InP/GaInP quantum dots to measure hyperfine interaction of the valence band hole with nuclear spins polarized along the sample growth axis. The ratio of the hyperfine constants for the hole (C) and electron (A) is found to be C/A~-0.11. In InP dots the contribution of spin 1/2 phosphorus nuclei to the hole-nuclear interaction is weak, which enables us to determine experimentally the value of C for spin 9/2 indium nuclei as C_In~-5 micro-eV. This high value of C is in good agreement with recent theoretical predictions and suggests that the hole-nuclear spin interaction has to be taken into account when considering spin qubits based on holes.
△ Less
Submitted 26 August, 2010;
originally announced August 2010.
-
Alignment of nuclear spins in single quantum dots using unpolarized light
Authors:
E. A. Chekhovich,
A. B. Krysa,
M. S. Skolnick,
A. I. Tartakovskii
Abstract:
We report optical pumping of neutral quantum dots leading to nuclear spin alignment with direction insensitive to polarization and wavelength of light. Measurements of photoluminescence of both "dark" and "bright" excitons in single dots reveal that nuclear spin pumping occurs via a virtual spin-flip transition between these states accompanied by photon emission. The sign of the nuclear spin polar…
▽ More
We report optical pumping of neutral quantum dots leading to nuclear spin alignment with direction insensitive to polarization and wavelength of light. Measurements of photoluminescence of both "dark" and "bright" excitons in single dots reveal that nuclear spin pumping occurs via a virtual spin-flip transition between these states accompanied by photon emission. The sign of the nuclear spin polarization is determined by asymmetry in the exciton energy spectrum, rather than by the sign of the exciton spin polarization.
△ Less
Submitted 13 August, 2010; v1 submitted 7 July, 2010;
originally announced July 2010.
-
Pumping of nuclear spins by the optical solid effect in a quantum dot
Authors:
E. A. Chekhovich,
M. N. Makhonin,
K. V. Kavokin,
A. B. Krysa,
M. S. Skolnick,
A. I. Tartakovskii
Abstract:
We demonstrate that efficient optical pumping of nuclear spins in semiconductor quantum dots (QDs) can be achieved by resonant pumping of optically "forbidden" transitions. This process corresponds to one-to-one conversion of a photon absorbed by the dot into a polarized nuclear spin, which also has potential for initialization of hole spin in QDs. Pumping via the "forbidden" transition is a man…
▽ More
We demonstrate that efficient optical pumping of nuclear spins in semiconductor quantum dots (QDs) can be achieved by resonant pumping of optically "forbidden" transitions. This process corresponds to one-to-one conversion of a photon absorbed by the dot into a polarized nuclear spin, which also has potential for initialization of hole spin in QDs. Pumping via the "forbidden" transition is a manifestation of the "optical solid effect", an optical analogue of the effect previously observed in electron spin resonance experiments in the solid state. We find that by employing this effect, nuclear polarization of 65% can be achieved, the highest reported so far in optical orientation studies in QDs. The efficiency of the spin pumping exceeds that employing the allowed transition, which saturates due to the low probability of electron-nuclear spin flip-flop.
△ Less
Submitted 1 October, 2009; v1 submitted 29 June, 2009;
originally announced June 2009.
-
Polarization freezing of 10000 optically-cooled nuclear spins by coupling to a single electron
Authors:
E. A. Chekhovich,
M. N. Makhonin,
J. Skiba-Szymanska,
A. B. Krysa,
V. D. Kulakovskii,
V. I. Fal'ko,
M. S. Skolnick,
A. I. Tartakovskii
Abstract:
The nature of the nano-scale environment presents a major challenge for solid-state implementation of spin-based qubits. In this work, a single electron spin in an optically pumped nanometer-sized III-V semiconductor quantum dot is used to control a macroscopic nuclear spin of several thousand nuclei, freezing its decay and leading to spin life-times exceeding 100 seconds at low temperatures. Fe…
▽ More
The nature of the nano-scale environment presents a major challenge for solid-state implementation of spin-based qubits. In this work, a single electron spin in an optically pumped nanometer-sized III-V semiconductor quantum dot is used to control a macroscopic nuclear spin of several thousand nuclei, freezing its decay and leading to spin life-times exceeding 100 seconds at low temperatures. Few-millisecond-fast optical initialization of the nuclear spin is followed by a slow decay exhibiting random telegraph signals at long delay times, arising from low probability electron jumps out of the dot. The remarkably long spin life-time in a dot surrounded by a densely-packed nuclear spin environment arises from the Knight field created by the resident electron, which leads to suppression of nuclear spin depolarization.
△ Less
Submitted 27 January, 2009;
originally announced January 2009.
-
Overhauser effect in individual InP/GaInP dots
Authors:
J. Skiba-Szymanska,
A. V. Nikolaenko,
E. A. Chekhovich,
A. I. Tartakovskii,
M. N. Makhonin,
I. Drouzas,
M. S. Skolnick,
A. B. Krysa
Abstract:
Sizable nuclear spin polarization is pumped in individual InP/GaInP dots in a wide range of external magnetic fields B_ext=0-5T by circularly polarized optical excitation. We observe nuclear polarization of up to ~40% at Bext=1.5T and corresponding to an Overhauser field of ~1.2T. We find a strong feedback of the nuclear spin on the spin pumping efficiency. This feedback, produced by the Overhau…
▽ More
Sizable nuclear spin polarization is pumped in individual InP/GaInP dots in a wide range of external magnetic fields B_ext=0-5T by circularly polarized optical excitation. We observe nuclear polarization of up to ~40% at Bext=1.5T and corresponding to an Overhauser field of ~1.2T. We find a strong feedback of the nuclear spin on the spin pumping efficiency. This feedback, produced by the Overhauser field, leads to nuclear spin bi-stability at low magnetic fields of Bext=0.5-1.5T. We find that the exciton Zeeman energy increases markedly, when the Overhauser field cancels the external field. This counter-intuitive result is shown to arise from the opposite contribution of the electron and hole Zeeman splittings to the total exciton Zeeman energy.
△ Less
Submitted 14 March, 2008; v1 submitted 15 October, 2007;
originally announced October 2007.
-
Fingerprints of spatial charge transfer in Quantum Cascade Lasers
Authors:
R. Nelander,
A. Wacker,
M. F. Pereira Jr.,
D. G. Revin,
M. R. Soulby,
L. R. Wilson,
J. W. Cockburn,
A. B. Krysa,
J. S. Roberts,
R. J. Airey
Abstract:
We show that mid infrared transmission spectroscopy of a quantum cascade laser provides clear cut information on changes in charge location at different bias. Theoretical simulations of the evolution of the gain/absorption spectrum for the $λ\sim$ 7.4 $μ$m InGaAs/AlInAs/InP quantum cascade laser have been compared with the experimental findings. Transfer of electrons between the ground states in…
▽ More
We show that mid infrared transmission spectroscopy of a quantum cascade laser provides clear cut information on changes in charge location at different bias. Theoretical simulations of the evolution of the gain/absorption spectrum for the $λ\sim$ 7.4 $μ$m InGaAs/AlInAs/InP quantum cascade laser have been compared with the experimental findings. Transfer of electrons between the ground states in the active region and the states in the injector goes in hand with a decrease of discrete intersubband absorption peaks and an increase of broad high-energy absorption towards the continuum delocalised states above the barriers.
△ Less
Submitted 9 August, 2007;
originally announced August 2007.