Multiplexed Single Photons from Deterministically Positioned Nanowire Quantum Dots

Solid-state quantum emitters are excellent sources of on-demand indistinguishable or entangled photons and can host long-lived spin memories, crucial resources for photonic quantum-information applications. However, their scalability remains an outstanding challenge. Here, we present a scalable technique to multiplex streams of photons from multiple independent quantum dots, on chip, into a fiber network for use “off chip.” Multiplexing is achieved by incorporating a multicore fiber into a confocal microscope and spatially matching the multiple foci, seven in this case, to quantum dots in an array of deterministically positioned nanowires. As a proof-of-principle demonstration, we perform parallel spectroscopy on the nanowire array to identify two nearly identical quantum dots at different positions, which are subsequently tuned into resonance with an external magnetic field. Multiplexing of background-free single photons from these two quantum dots is then achieved. Our approach, applicable to all types of quantum emitters, can readily be integrated into scalable photonic based quantum technologies.

Figure 1

The multispot microscope and nanowire sample structure. (a) A schematic of the lens configuration of the multispot microscope: col, collimating lens; rot, rotatable fiber mount; BS, 96:4 beam splitter; RL, relay lens; obj, microscope objective; CCD, charged-coupled device. (b) An image of the collection or excitation spots on the CCD device. (c) A SEM image of the tilted sample. The nanowires are arranged in a hexagonal lattice (the geometry is outlined by the dashed lines) with lattice spacing $a$.

Figure 2

The multiplexed spectroscopy. (a) A schematic of the setup for fluorescence detection on the spectrometer. The MCF fan-outs (labeled from 1 to 7) are coupled into an imaging spectrometer via a fiber array holder (FAH). The multispot signals are imaged on a CCD detector array ($1340\times 100$ pixels), with a separate region of interest, labeled from $a$ to $f$, indicated on the CCD. (b) The crosstalk matrix between the input fiber fan-out and the output pixels on the spectrometers. The adjacent channels show crosstalk of approximately $-15\mathrm{dB}$, while the rest of the channels show crosstalk of approximately $-35\mathrm{dB}$, approaching the dark counts of the spectrometer. (c) An example of the emission spectra from three nanowires (corresponding to fluorescence at channels $b$, $d$, and $g$) measured simultaneously.

Figure 3

The spectroscopy of a nanowire QD. (a) Emission spectra under above-band excitation (at 1.4938 eV), showing the emission lines from the $X$, $XX$, and $X^{1-}$ transitions. The insets show the emission energy as a function of the polarization angle, confirming the nature of each emission line ($X$, blue; $XX$, green; $X^{1-}$, purple). (b) The lifetime measurement of the biexciton-exciton cascade under resonant two-photon excitation. (c) The power dependence of the $X$ and $XX$ emissions under cw resonant two-photon excitation.

Figure 4

Multiplexing single photons from independent quantum emitters. (a),(b) Emission spectra of the original (solid line) and filtered (dashed line) from wires at spot 2 (c) and spot 7 (d), under above-band excitation. The inset figure indicates the excitation and collection spots for wires at spots 2 and 7. (c) The emission energy of the filtered QDs as a function of the external magnetic field, $B_{\mathrm{ext}}$. The fits give a $g$ factor and a diamagnetic shift coefficient of $1.53(1)$ and $7.99(7)\mu \mathrm{eV}/{\mathrm{T}}^2$ for the QDs at spot 2 (solid line) and $1.49(1)$ and $9.65(8)\mu \mathrm{eV}/{\mathrm{T}}^2$ at spot 7 (dashed line). (d),(e) The normalized $g^{(2)}$ for the emission from nanowires at spot 2 (f) and spot 7 (g), at $B_{\mathrm{ext}}=0$, showing a suppressed multiphoton emission of $g^{(2)}(0)\sim 0$.

Figure 5

The spectroscopy of a nanowire QD under resonant two-photon excitation (TPE). (a) An energy-level schematic of the biexciton-exciton ($XX$-$X$) cascaded system: $\mathrm{\Delta }$, exciton fine-structure splitting; $E_B$, biexciton binding energy; ${\omega }_0$, excitation energy under resonant two-photon excitation; ${\omega }_{X,XX}$, emission energy for exciton $X$ and biexciton $XX$. (b) Emission spectra under resonant TPE (at ${\omega }_0=1.3455\mathrm{eV}$) reveals cleaner emission lines of $X$, $XX$, and $X^{1-}$. (c) The power dependence of the emission under cw TPE reveals an Autler-Townes splitting for both $X$ and $XX$ emissions. (d) The second-order intensity correlation histogram, $g^{(2)}$, of the $X$ and $XX$ emissions [at a power $1.17\mu \mathrm{W}$ indicated by the dashed line in (c)]. The inset shows the same $g^{(2)}$ of the $X$ emission, up to $5\mu \mathrm{s}$. (e) The power dependence of the $X$ and $XX$ emissions under pulsed TPE reveals Rabi oscillations, with the $\pi$ pulse given by the black dashed line at $1.6\mu \mathrm{W}$. (f) The $g^{(2)}$ data for $X$ and $XX$ emissions under $\pi$-pulse excitation [with the power indicated by the dashed line in (e)] in log scale. The $g^{(2)}$ data, normalized by the raw coincidence at $5\mu \mathrm{s}$, for $XX$ emission in (d) and (f), are artificially shifted by $-3$ ns for clarity.