Fine structure in the excitonic emission of InAs ∕ GaAs quantum dot molecules

We describe an interaction mechanism between electron spins in a vertically-stacked double quantum dot that can be used for controlled two-qubit operations. This interaction is mediated by excitons confined within, and delocalized over, the double dot. We show that gates equivalent to the √ SWAP gate can be obtained in times much less than the exciton relaxation time and that the negative effects of hole-mixing and spontaneous emission do not seriously affect these results.PACS numbers: 78.67.Hc, 03.67.LxThe spin of an electron confined in a quantum dot (QD) is one of the leading candidates for the realization of a practical qubit. Since the work of Loss and DiVincenzo [1], there have been a number of proposals on how best to achieve the precise manipulations of such spins required for the operation of quantum logic. See, for example, [2,3,4,5].Whilst interest in electrostatic gating remains strong, the use of lasers has several advantages in this role, most notably speed and control. Despite significant theoretical advances in this direction, there has, as yet, been no experimental demonstration of optically-controlled gating between electron spins in QDs.In this paper, we describe an interaction mechanism to achieve just this. This qubit-qubit interaction is mediated by interdot tunnelling of photo-excited carriersan area which has been the subject of significant recent experimental advances [6,7]. Our results are of explicit relevance to the current generation of vertically-stacked self-assembled InAs QDs, but are also easily adaptable to the other dots, including horizontally-coupled ones.The interaction we describe has its origin in the so-called optical-RKKY effect, in which two electron spins are coupled via their exchange interactions with optically-generated excitons in the semiconductor bulk [8]. The coupling effect of these bulk excitons between two electron spins in a double QD was examined in Ref.[9]. Here we consider an interaction mediated, not by bulk excitons, but by a single exciton confined in the same double QD structure. We describe a situation in which the excitonic electron is able to tunnel between the dots and form delocalized 'molecular' states. It is the exchange interaction between this electron and the resident qubit electrons that leads to an optically controlled gating. This gate, although not one of the standard quantum computation (QC) gates, can be used to form a controlled-Z operation when used twice in conjunction with single qubit rotations, and is, in this way, similar to the √ SWAP gate. The main factor limiting the speed with which oper- The lowest electron levels in each dot (labeled 1 and 2) are the two qubit levels and an electron permanently resides in each of them. Laser illumination is tuned such that it creates an exciton in the excited levels of dot A only (levels 3). (c) Due to a tunnel coupling τ34 between the dots and a resonance condition met through the tuning of the gate voltage Vg, the excitonic electron can tunnel back-and-forth. The exchange interaction be...

mentioning

confidence: 70%

Optically controlled logic gates for two spin qubits in vertically coupled quantum dots

Emary

Sham

2007

“…The quantum confinement of the excitons on length scales smaller than the bulk Bohr radius leads to enhanced exciton peaks in optical absorption spectra of lowdimensional nanostructures. [1][2][3][4] Extensive work has been done to investigate the specifics of the excitonic optical properties of quantum wells, 1,2 quantum wires, [3][4][5][6] quantum dots, [7][8][9][10] and superlattices. [11][12][13][14][15][16] The optical spectra of semiconductor structures can be modified dramatically by the application of external static electric and/or magnetic fields.…”

Section: Introductionmentioning

confidence: 99%

Franz-Keldysh effect and dynamical Franz-Keldysh effect of cylindrical quantum wires

Zhang

Zhao

2006

We investigate the interband optical absorption spectra near the band edge of a cylindrical semiconductor quantum wire in the presence of a static electric field and a terahertz electric field polarized along the axis. Optical absorption spectra are nonperturbatively calculated by solving the low-density semiconductor Bloch equations in real space and real time. The influence of the Franz-Keldysh ͑FK͒ effect and dynamical FK effect on the absorption spectrum is investigated. To highlight the physics behind the FK effect and dynamical FK effect, the spatiotemporal dynamics of the polarization wave packet are also presented. Under a reasonable static electric field, substantial and tunable absorption oscillations appear above the band gap. A terahertz field, however, will cause the Autler-Townes splitting of the main exciton peak and the emergence of multiphoton replicas. The presented results suggest that semiconductor quantum wires have potential applications in electro-optical devices.

“…Another interesting application of coupled QDs is generation of entangled photons 7 . It is therefore not surprising that much experimental work has been devoted to proving the existence of coupling in DQD systems 8,9,10,11,12 .…”

Section: Introductionmentioning

confidence: 99%

Theory of nonlinear optical response of ensembles of double quantum dots

Sitek

Machnikowski

2009

We study theoretically the time-resolved four-wave mixing (FWM) response of an ensemble of pairs of quantum dots undergoing radiative recombination. At short (picosecond) delay times, the response signal shows beats that may be dominated by the subensemble of resonant pairs, which gives access to the information of the inter-dot coupling. At longer delay times, the decay of the FWM signal is governed by two rates which result from the collective interaction between the two dots and the radiation modes. The two rates correspond to the subradiant and superradiant components in the radiative decay. Coupling between the dots enhances the collective effects and makes them observable even when the average energy mismatch between the dots is relatively large.