Coherent and robust high-fidelity generation of a biexciton in a quantum dot by rapid adiabatic passage

Abstract

A biexciton in a semiconductor quantum dot is a source of polarization-entangled photons with high potential for implementation in scalable systems. Several approaches for nonresonant, resonant, and quasiresonant biexciton preparation exist, but all have their own disadvantages; for instance, low fidelity, timing jitter, incoherence, or sensitivity to experimental parameters. We demonstrate a coherent and robust technique to generate a biexciton in an InGaAs quantum dot with a fidelity close to 1. The main concept is the application of rapid adiabatic passage to the ground-state-exciton-biexciton system. We reinforce our experimental results with simulations which include a microscopic coupling to phonons. DOI: 10.1103/PhysRevB.95.161302 Entangled photon pairs are a powerful resource, especially for quantum teleportation and quantum key distribution protocols. Spontaneous parametric down-conversion in nonlinear optics is a source of entangled photon pairs [1], but success is not guaranteed-the emission is a probabilistic process-and the error rate is high. In contrast, semiconductor quantum dots (QDs) are bright, on-demand sources of both single photons [2] and entangled photon pairs and hence have enormous potential in quantum computing and quantum cryptography [3].A biexciton in a QD is the starting point for a two-photon cascade: when perfectly prepared, biexciton decay leads to the subsequent emission of two photons [ Fig. 1(f)]. In a QD without a significant fine-structure splitting (FSS), the two photons are polarization entangled [4]. The majority of InGaAs QDs show a FSS due to a reduced symmetry [5][6][7]. However, sophisticated techniques were developed to compensate for the FSS with strain [8], electric [9], or magnetic fields [10,11] and with special growth conditions [7].Several approaches for biexciton preparation have been proposed [12][13][14][15] and demonstrated [4,[16][17][18][19][20]. Resonant two-photon schemes involving Rabi rotations [4,17,18] are sensitive to fluctuations in both laser power and QD optical frequency. They are likely to suffer from an imperfect biexciton preparation resulting in undesired exciton photons unrelated to the cascade process.A more robust scheme using phonon-assisted excitation was reported by several groups recently [18][19][20][21][22]. An impressively high biexciton occupation of up to 95% was demonstrated using this quasiresonant scheme [20]. But the strength here is also a weakness. The scheme relies on the coupling to the phonon bath in the semiconductor environment: it is an inherently incoherent process. Also, a dependence on relaxation processes in the state preparation results in a timing jitter. In some cases, charge-carrier relaxation times can reach values of up to a nanosecond [23].We present here a coherent technique to create a biexciton with high probability, low jitter, and weak dependence on the * timo.kaldewey@unibas.ch excitation and system parameters. The technique is based on rapid adiabatic passage (RAP). RAP allows the robust creatio...

Charge and spin control of ultrafast electron and hole dynamics in single CdSe/ZnSe quantum dots

“…In this context and the context of the single-photon amplifier, it is important to understand the intrinsic relaxation mechanisms which can influence preparation and readout of the few-particle quantum states via ultrashort light pulses. To this end, charge-carrier dynamics in QDs based on III-V semiconductors with shallow quantum confinement has been studied both in large ensembles [18][19][20] and single specimens [7,[21][22][23][24][25][26]. These developments have culminated in demonstrations of optical writing and readout of spin states in single or coupled QDs [13][14][15][16][17]27].…”

Demonstrating the decoupling regime of the electron-phonon interaction in a quantum dot using chirped optical excitation

“…Conversely, in a Rabi oscillation with Gaussian pulses, the splitting between the dressed states increases monotonically from zero such that the full range of phonons is involved in the damping. This second feature also makes the reappearance regime easier to access.Rapid adiabatic passage (RAP) has been demonstrated on single QDs [17,18,24,25], and it has been shown that phonons hinder exciton preparation depending on the sign of the chirp [19,23]. It has been predicted that the reappearance regime translates to a nonmonotonic RAP behavior: At sufficiently high pulse areas RAP improves [11].…”

“…We therefore present compelling evidence that our system enters the reappearance regime where the electron is effectively decoupled from the phonons. We show that RAP is excellent for phonon-free state preparation.We study a self-assembled InGaAs QD embedded in an n-i-p structure [25] as displayed in Fig. 1(a) and described in the Supplemental Material (SM) [26].…”

“…We study a self-assembled InGaAs QD embedded in an n-i-p structure [25] as displayed in Fig. 1(a) and described in the Supplemental Material (SM) [26].…”

“…We switch to an alternative technique here, rapid adiabatic passage (RAP) [17][18][19][20][21][22][23][24][25], and use the full bandwidth of 100 fs pulses. There are two key advantages.…”