Frequency-Selective Single-Photon Detection Using a Double Quantum Dot

We use a double quantum dot as a frequency-tunable on-chip microwave detector to investigate the radiation from electron shot-noise in a near-by quantum point contact. The device is realized by monitoring the inelastic tunneling of electrons between the quantum dots due to photon absorption. The frequency of the absorbed radiation is set by the energy separation between the dots, which is easily tuned with gate voltages. Using time-resolved charge-detection techniques, we can directly relate the detection of a tunneling electron to the absorption of a single photon.

Figure 1

(a) Schematic for operating a double quantum dot (DQD) as a high-frequency noise detector. The tunable level separation $\delta$ of the DQD allows frequency-selective detection. (b) The sample used in the measurement, with two QDs (marked by 1 and 2) and a nearby QPC. (c) Charge stability diagram of the DQD, measured by counting electrons entering the DQD. The numbers in brackets denote the charge population of the two QDs. (d) Typical traces of the detector signal, taken at point I (red) and II (black) in (c).

Figure 2

(a) Electron count rates for a small region close to a triple point (marked by a blue point). The inset shows a sketch of the surrounding hexagon pattern. The dashed line denotes the detuning axis, with zero detuning occurring at the triple point. The data was taken with $V_{\mathrm{QPC}}=-300\mu \mathrm{V}$. (b) Enlargement of the lower-right region of (a), measured for different QPC bias voltages. (c) Rates for electron tunneling into and out of the DQD, measured along the dashed line in (a). ${\Gamma }_{\mathrm{in}}$ falls off rapidly with detuning, while ${\Gamma }_{\mathrm{out}}$ shows only minor variations.

Figure 3

(a) Energy level diagrams for the three states of the DQD. The labels $L$, $R$ and 2 denote the excess charge population. The levels are raised by the intradot charging energy $E_{Ci}$ when the DQD holds two excess electrons. (b) Schematic changes of the detector signal as electrons tunnel into, between and out of the DQD.

Figure 4

Count rate measured versus detuning and QPC bias voltage. The dashed line shows the level separation for a two-level system, with ${\Delta }_{12}=\sqrt{4t^2+{\delta }^2}$. There are only counts in the region where $|e{V}_{\mathrm{QPC}}|>{\Delta }_{12}$. (b) Count rate versus QPC bias for different values of detuning. The solid lines are guides to the eye. (c) DQD absorption spectrum, measured for different QPC bias. The dashed lines are the results of Eq. (4), with parameters given in the text. (d) Noise spectrum of the QPC, extracted from the data in (c). The dashed lines show spectra expected from Eq. (3).