Spin qubits have been successfully realized in electrostatically defined, lateral fewelectron quantum dot circuits [1][2][3][4]. Qubit readout typically involves spin to charge information conversion, followed by a charge measurement made using a nearby biased quantum point contact [1,5,6]. It is critical to understand the back-action disturbances resulting from such a measurement approach [7,8]. Previous studies have indicated that quantum point contact detectors emit phonons which are then absorbed by nearby qubits [9][10][11][12][13]. We report here the observation of a pronounced back-action effect in multiple dot circuits where the absorption of detector-generated phonons is strongly modified by a quantum interference effect, and show that the phenomenon is well described by a theory incorporating both the quantum point contact and coherent phonon absorption. Our combined experimental and theoretical results suggest strategies to suppress back-action during the qubit readout procedure.The back-action process considered in this paper involves deleterious inelastic tunneling events between two adjacent dots in a serial double or triple quantum dot (DQD, TQD). The energy difference ∆ between the initial and final electronic dot states is provided by the absorption of a non-equilibrium acoustic phonon, which itself is generated by the quantum point contact (QPC) detector [12]. Such an absorption process between adjacent dots is constrained by the energy conservation condition ∆ = | q|v ph (v ph is the sound velocity, q the phonon wavevector). More subtly, it is also sensitive to the difference in phase, ∆ϕ = d · q, of the associated phonon wave between the two dot positions, with d being the vector connecting the two dot centers [14,15].This q (and hence ∆) dependent phase difference controls the matrix element for phonon-absorption since it determines whether the electron-phonon couplings in each of the two individual dots add constructively or destructively (see Fig. 1) [16]. The result is an oscillatory probability for inelastic electron-transfer events involving phonon-absorption, with constructive interference occurring when ∆φ = (2n + 1)π (where n is an integer).Data showing a pronounced back-action effect are shown in Fig. 2a, which displays the stability diagram measured in charge detection for a few-electron DQD without a voltage drop between its left and right leads. The charge configuration of the quantum dot structures influences the conductance of a nearby QPC because of the capacitive coupling between the dots and the QPC.In order to serve as a charge detector it is necessary to drive a current through the detector QPC which, in turn, leads to the observed detector back-action. Multiple gates fabricated 85 nm above a high-mobility two-dimensional electron system (2DES) are used to define two dots and two QPCs (Fig. 2d). The differential transconductance dI QPC /dV L of the biased charge detectorDj p = FIG. 1: Interference in quantum dot-phonon interactions. a, The back-action charge fluctuations of...

Charge detection utilizing a highly biased quantum point contact has become the most effective probe for studying few electron quantum dot circuits. Measurements on double and triple quantum dot circuits is performed to clarify a back action role of charge sensing on the confined electrons. The quantum point contact triggers inelastic transitions, which occur quite generally. Under specific device and measurement conditions these transitions manifest themselves as bounded regimes of telegraph noise within a stability diagram. A nonequilibrium transition from artificial atomic to molecular behavior is identified. Consequences for quantum information applications are discussed.

Quantum point contacts (QPCs) are commonly employed to detect capacitively the charge state of coupled quantum dots (QD). An indirect back-action of a biased QPC onto a double QD laterally defined in a GaAs/AlGaAs heterostructure is observed. Energy is emitted by non-equilibrium charge carriers in the leads of the biased QPC. Part of this energy is absorbed by the double QD where it causes charge fluctuations that can be observed under certain conditions in its stability diagram. By investigating the spectrum of the absorbed energy, we identify both acoustic phonons and Coulomb interaction being involved in the back-action, depending on the geometry and coupling constants.PACS numbers: 72.70.+m, 73.21.La, 73.23.Hk Coupled quantum dots (QDs) are promising candidates for applications as qubits in solid state quantum information processing schemes [1]. One important criterion is the scalability of the qubit number. In a complex layout it will pose a great challenge to implement readout techniques that address singe qubits without adding decoherence to the coupled QDs. Direct transport through an array of QDs is limited due to Coulomb blockade. However, a single biased quantum point contact (QPC) in a separate circuit can act as charge detector for several QDs [2,3]. QPCs are straightforward to implement, yield sufficient sensitivity, and can be operated as wide bandwidth detectors [4,5]. The latter is desirable in quantum information processing where a rapid detection scheme is needed. The suitability of QPCs as fast detectors has been demonstrated in single-shot readout [6,7] and counting statistics experiments [8,9]. Increasing the bandwidth, however, requires a high signal-to-noise ratio which makes it necessary to operate the QPC at a relatively high bias voltage.A biased QPC employed as a charge detector causes back-action. Its quantum limit can be traced back to statistical charge fluctuations at the QPC capacitively coupled to QDs [10]. Shot noise only contributes to backaction if the QPC has resistive leads [10]. In addition to these direct Coulomb back-action mechanisms, the solid state environment provides possibilities for indirect backaction [11][12][13][14]. A biased QPC emits non-equilibrium charge carriers into its leads that then relax via electronelectron interaction, the emission of plasmons, or acoustic phonons [15]. Partial reabsorption of the emitted energy can result in charge fluctuations in (coupled) QDs, hence causing indirect back-action. Usually these fluctuations are too fast to be detected in measurements with limited bandwidth, but under certain conditions they can be observed in the stability diagram of coupled QDs [13]. In this Letter we present a systematic investigation of such back-action-induced charge fluctuations in a double QD. We find that both acoustic phonons and Coulomb interaction can play an important role for the back-action in realistic devices.Our device is based on a GaAs/AlGaAs heterostructure containing a two-dimensional electron system 90 nm beneath the surfac...

Quantum dots offer a promising two-level system for applications in solid state based quantum information processing [1]. Within these three-dimensionally confining structures, electrostatically defined quantum dots are a well studied system [2], mostly in III-V materials. A major source of decoherence in such devices is the interaction of the confined electron spin with the surrounding semiconductor host matrix, in particular with the nuclear spin bath [3]. Recently, single electron devices have been reported in materials systems like Si-Ge [4,5] or C [6] which contain a reduced amount of nuclear spins in their natural isotopic composition. As a next step, isotopical purification of the group-IV source materials Si, Ge and C can give access to virtually nuclear spin free materials. In this Letter, we report on the realization of two-dimensional electron systems (2DES) in a nuclear spin free environment. A 2DES forms in a strained 28 Si layer embedded into 28 Si 70 Ge. The ability to control the 2DES via top-gates is demonstrated by the implementation of split-gate structures which are able to locally deplete the 2DES. Suitable voltages allow the complete pinch-off of the narrow conducting channel.Samples are fabricated in solid source molecular beam epitaxy (MBE). The base pressure of the Riber Siva 45 MBE chamber is 1 × 10 -11 mbar. 28 Si and 70 Ge are evaporated from a custom made MBE-Komponenten electron beam source and effusion cell respectively. Figure 1a shows the typical layout of our isotopically engineered Si/SiGe heterostructures. A SiGe virtual substrate of natural isotopic composition is first deposited onto a (100) oriented Si substrate. The virtual substrate is realized by increasing the Ge content linearly by 8%/µm until the desired Ge content is reached. The graded layers are deposited at a substrate temperature of T s = 575 °C. The virtual substrate is fully relaxed within the experimental error of 10%. It typically displays a density of threading dislocations of about 1 × 10 6 cm -2 . The active part is thenWe report on the realization and top-gating of a two-dimensional electron system in a nuclear spin free environment using 28 Si and 70 Ge source material in molecular beam epitaxy. Electron spin decoherence is expected to be minimized in nuclear spin-free materials, making them promising hosts for solid-state based quantum information processing devices. The two-dimensional electron system exhibits a mobility of 18000 cm 2 /(V s) at a sheet carrier density of 4.6 × 10 11 cm -2 at low temperatures. Feasibility of reliable gating is demonstrated by transport through split-gate structures realized with palladium Schottky top-gates which effectively control the two-dimensional electron system underneath. Our work forms the basis for the realization of an electrostatically defined quantum dot in a nuclear spin free environment.