Coherent superposition of photon- and phonon-assisted tunneling in coupled quantum dots

Abstract

We report on electron transport through an artificial molecule formed by two tunnel coupled quantum dots, which are laterally confined in a two-dimensional electron system of an AlxGa1−xAs/GaAs heterostructure. Coherent molecular states in the coupled dots are probed by photon-assisted tunneling (PAT). Above 10 GHz, we observe clear PAT as a result of the resonance between the microwave photons and the molecular states. Below 8 GHz, a pronounced superposition of phononand photon-assisted tunneling is observed. Coherent superposition of molecular states persists under excitation of acoustic phonons. 73.23.Hk;73.21.La;63.20.Kr;03.67.Lx An artificial molecule is defined by coupling two quantum dots, leading to the formation of coherent electronic states in this molecule [1][2][3][4][5][6][7]. As a controllable twolevel system, double dots are proposed for realizing a single quantum bit in solid state systems [8]. One of the obstacles for such an application is the considerable dephasing induced by the semiconductor host materials. The investigation of dissipation processes in these nanostructures is thus of great importance. Operating such a double dot in the regime of weak coupling to the contacts (Γ l,r → 0) minimizes the dephasing of molecular states via the electron-electron interaction and it is then possible to study the interaction between confined molecular states and discrete phonon modes within the dots. Recently, Fujisawa et al. [9] found that at low temperatures vacuum fluctuations indeed result in pronounced transitions between energetically well-separated quantum states, generating discrete phonon modes. A theoretical treatment of this effect was then given by Brandes and Kramer [10]. However, the limitation of this experiment is that it can only be applied at large bias at which coherent molecular states can not be probed.Here, we present transport measurements on a doubledot molecule coupled to both microwave photons and acoustic phonons. An external microwave source generates photons and piezoelectrically induces coherent acoustic phonons in phonon cavities formed by the Schottky gates. At near-zero bias, the formation of molecular states is revealed through single-and double-photon assisted tunneling (PAT). In addition, the combination of PAT with acoustic phonon-assisted tunneling is observed. We verify the persistence of coherent molecular states under phonon excitation.As shown in Fig. 1, five pairs of Schottky gates are defined by electron beam lithography and evaporation of gold on the surface of an Al x Ga 1−x As/GaAs heterostructure. By applying proper negative voltages to these gates two quantum dots are formed in the two-dimensional electron system (2DES) located 90 nm below the surface. The left and right tunnel barriers determine the tunnel coupling of the double dot to the drain and source contact, respectively. The coupling conductance (G c ) between the dots is controlled by the central gate voltage (V t ) (see Fig. 1). At 4.2 K the carrier density of the 2DES is 1.7 × 10 15 m −2 ...

Charge transport through open driven two-level systems with dissipation

“…Here, Eq. (40) demonstrates that a similar summation effectively can be achieved in the AC driven case.…”

“…When the time-dependence is slow, this can give rise to a variety of adiabatic phenomena such as charge pumping 10,11,12,13,14,15,16,17,18,19 , adiabatic control of state vectors 20,21 , or operations relevant for quantum information processing in a condensedmatter setting 22,23,24,25,26,27,28 . Different physics occurs in the high frequency regime where monochromatic timevariation induces photo-excitations, such as for coupling of AC fields to quantum dots 29,30,31,32,33,34 , which has been tested experimentally 35,36,37,38,39,40 recently.…”

Exchange interaction and stability diagram of coupled quantum dots in magnetic fields

“…Meanwhile, recent advances in experimental techniques have made it possible to study coupled QDs in the few-electron regime when each QD contains only one conduction electron (see, e.g., Refs. [6,7,8]). In this case the stability diagram becomes a powerful tool to study inter-dot coupling and electronic transport through double QD systems.…”

“…where V L and V R are the depth of the left and right dots (equivalent to the QD gate voltages in experimental structures 6,7,8 ) which can be independently varied, d is the inter-dot separation, and R is the radius of the dot (R = 30 nm).…”

Adiabatic steering and determination of dephasing rates in double-dot qubits

“…Microscopic coupling constants that determine the dephasing rate γ of the coherenceρ 1,2 , Eq. (4g), for the different dephasing channels (bulk and surface acoustic phonons 19 ) are difficult to calculate, in particular as there might be additional contributions from non-equilibrium phonons induced by the microwave radiation itself 20 . A decisive advantage of coupled dots, however, is the fact that γ ∝ T 2 c can be tuned to values which are orders of magnitude smaller than Γ 0 by varying the tunnel coupling T c between the dots.…”