Long Lived Coherence in Self-Assembled Quantum Dots

Birkedal, D.; Leósson, Kristján; Hvam, Jørn Märcher

doi:10.1103/physrevlett.87.227401

Cited by 220 publications

(173 citation statements)

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“…We note that although pure dephasing does not play an important role in IFQDs [15,16], its manifestation has been reported in SAQDs [17].…”

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confidence: 99%

“…This behaviour also rules out the mechanism resulting from coupling to delocalized excitons, proposed in [5] for interface fluctuation QDs (IFQDs) since that mechanism will take the excitonic state out of the QDs and will give rise to totally different overall behaviour. This is not surprising since the energy confinement in SAQDs is much higher than that in IFQDs.We note that although pure dephasing does not play an important role in IFQDs [15,16], its manifestation has been reported in SAQDs [17].What could be the underlying mechanism? The lattice mediated dephasing model proposed in [18], showed that the RO amplitudes decrease with the laser intensity.…”

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Decoherence processes during optical manipulation of excitonic qubits in semiconductor quantum dots

et al. 2005

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Using photoluminescence spectroscopy, we have investigated the nature of Rabi oscillation damping during active manipulation of excitonic qubits in self-assembled quantum dots. Rabi oscillations were recorded by varying the pulse amplitude for fixed pulse durations between 4 ps and 10 ps. Up to 5 periods are visible, making it possible to quantify the excitation dependent damping. We find that this damping is more pronounced for shorter pulse widths and show that its origin is the non-resonant excitation of carriers in the wetting layer, most likely involving bound-to-continuum and continuum-to-bound transitions. Hz, 78.47.+p, 78.55.Cr The current topic of quantum computation presents a wide range of challenges to physical science [1], particularly the search for candidates for solid-state quantum bits (qubits). Semiconductor quantum dots (QDs) are attractive because they possess energy structures and coherent optical properties similar to, and dipole moments larger than, those of atoms [2,3]. Efforts in the past few years have led to successful observations of Rabi oscillations (ROs) of excitonic states [4][5][6][7][8][9], the hallmark for active manipulation of qubits in QDs. However, all found that ROs damped out very quickly when the external field is increased. Because QDs contain a macroscopic number of atoms, this strong decoherence process must be due to unwanted coupling to other degrees of freedom.Identification of the underlying mechanism is difficult precisely because of this macroscopic nature. Yet such understanding plays the most crucial role in future development of quantum information technology in semiconductors. Through manipulations of high quality factor excitonic qubits in InGaAs QDs, we have studied the underlying mechanism for decoherence processes during active manipulation. More specifically, we have found that this strong decoherence process is manifested through indirect excitations of carriers in the wetting layer whose composition is highly fluctuating.We study In 0.5 Ga 0.5 As self-assembled QD (SAQD) samples grown by molecular beam epitaxy (MBE). The details of growth processes are given in [10]. These QDs are 3 3 embedded in a GaAs matrix with a wetting layer of roughly 5 monolayers thickness. The dots have an average lateral size, height, and dot-to-dot distance of 20-40 nm, 4.5 nm and 100 nm, respectively, characterized using cross-sectional scanning tunnelling microscopy. There are three excitonic levels involved: The exciton vacuum (labelled as |0Ú) when there is no electron-hole pair present, the single exciton ground state, (labelled as |2Ú), and the first excited state of the exciton (labelled as |1Ú). The qubit is based on the two level system formed by |0Ú and |1Ú. The exciton ground state |2Ú is a spectator state used to monitor the population of state |1Ú. This is possible because |1Ú decays nonradiatively to |2Ú long before it can radiatively decay to |0Ú. The state |1Ú then decays radiatively to |0Ú and is detected as the photoluminescence (PL) signal as summariz...

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“…We note that although pure dephasing does not play an important role in IFQDs [15,16], its manifestation has been reported in SAQDs [17].…”

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confidence: 99%

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confidence: 99%

Decoherence processes during optical manipulation of excitonic qubits in semiconductor quantum dots

et al. 2005

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“…This entailed a severe decrease in the dephasing time of the qubit, as the non-radiative decay from the excited state to the exciton ground state (necessary for our detection scheme to work) puts an upper bound in the coherence time of the exciton 26 . This upper bound is significant, since measured dephasing times for excitonic ground states are in the order of hundreds of picoseconds 22,23,24 while those for carefully chosen excited states (i.e. no further than approximately 20 meV apart from the corresponding ground state) range in the tens of picoseconds.…”

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confidence: 99%

Experimental realization of the one qubit Deutsch-Jozsa algorithm in a quantum dot

et al. 2004

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We perform quantum interference experiments on a single self-assembled semiconductor quantum dot. The presence or absence of a single exciton in the dot provides a qubit that we control with femtosecond time resolution. We combine a set of quantum operations to realize the singlequbit Deutsch-Jozsa algorithm. The results show the feasibility of single qubit quantum logic in a semiconductor quantum dot using ultrafast optical control.PACS numbers: 78.55. Cr,03.67.Lx Time-resolved optical spectroscopy in semiconductor quantum dots has recently progressed toward the full quantum control of excitons trapped inside a single dot. 1,2,3,4 These advances have stimulated proposals to use excitons in quantum dots as quantum bits 5,6,7 for implementation of quantum computing. Very recently, the ability to operate a two-qubit gate using exciton and biexciton states was demonstrated in a single quantum dot. 8 These achievements represent a step toward an alloptical implementation of quantum computing using excitonic qubits. The first algorithm that comes to mind in order to check the feasibility of quantum computation in this context is the Deutsch-Jozsa (DJ) algorithm. 9 This algorithm is one of the simplest quantum algorithms that provides an exponential speed-up with respect to classical algorithms. As such, it has been extensively studied and has been used in experimental demonstrations of simple quantum computation in a variety of systems. 10,11,12 In this Rapid Communication we report the experimental realization of the DJ algorithm for a single qubit using an optimized version of the algorithm 13 .The Deutsch problem 9 involves global properties of binary functions on a subset of the natural numbers. Given a natural number N , we can define a set called X N with all the natural numbers that can be represented with N bits. A binary function f : X N → {0, 1} is called balanced if it returns 0 for exactly half of the elements of X N and 1 for the other half. Given a function that is either balanced or constant, the Deutsch problem consists of finding out which type it is. A general classical algorithm requires evaluating the function on more than half of the elements, requiring at least 2 N −1 + 1 evaluations. This causes the classical run time to grow exponentially with the input size. The Deutsch-Jozsa algorithm provides a way to solve the Deutsch problem on a quantum computer using a quantum subroutine that evaluates f . The problem and its solution provide an example of Oracle-based quantum computation. 14,15 It is assumed that a quantum subroutine or Oracle contains the information about the unknown function. The algorithm gives a recipe on how to prepare (encoding) and read out (decoding) the qubit in an efficient way. In an experimental demonstration, we have not only to implement the algorithm (encoding and decoding operations), but we also have to build the Oracle. The specific structure of the Oracle, encoding and decoding is not unique and several versions can be found in the literature. 9,13,16,17 . The on...

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“…[21][22][23][24] The laser driving strength, ⍀, is chosen such that population within the excited states in the 11 subspace is only 1%.…”

Section: Conditional Dynamics With Imperfect Detectorsmentioning

confidence: 99%

Measurement-based approach to entanglement generation in coupled quantum dots

et al. 2009

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The remarkable phenomenon of measurement-induced quantum entanglement has recently been demonstrated between noninteracting atomic systems ͓D. L. Moehring et al., Nature ͑London͒ 449, 68 ͑2007͔͒. In the solid state, the technique may offer a new means of harnessing the strong interactions between neighboring units without the need for precise control over interactions. Recently, we proposed a method for optical parity measurements in a coupled quantum dot system ͓A. Kolli et al., Phys. Rev. Lett. 97, 250504 ͑2006͔͒. Here we perform a comprehensive analytic and numerical study to determine the feasibility of realizing this method using existing technology. We calculate the effects of possible error sources including nonideal photon detectors, ineffective spin-selective excitation, and dot distinguishability ͑both spatial and spectral͒. Furthermore, we present an experimental approach for verifying the success of the parity measurement. We conclude that experimental realization of the process should be feasible immediately.

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Long Lived Coherence in Self-Assembled Quantum Dots

Cited by 220 publications

References 21 publications

Decoherence processes during optical manipulation of excitonic qubits in semiconductor quantum dots

Decoherence processes during optical manipulation of excitonic qubits in semiconductor quantum dots

Experimental realization of the one qubit Deutsch-Jozsa algorithm in a quantum dot

Measurement-based approach to entanglement generation in coupled quantum dots

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