Compact and versatile design of a single photon source

The design of the SPS relies on a compact, portable and ready to use confocal microscope setup [7]. A hemispherical zirconium dioxide (ZrO2) solid immmersion lens (SIL) can be utilized to enhance the collection efficiency of single photons emitted from defect centres in nanodiamonds spin-coated directly on the flat side of the SIL. Details of the fabrication of SILs with NV centres are provided in [117]

or [7]. Figure 22 shows a schematic (a) and a photograph (b) of the source which fits completely on an aluminum plate and has dimensions of only 22.5cm×19cm× 9cm. Thus the SPS is mobile and can easily be integrated in different experimental setups. The setup is robust against mechanical vibrations and thermal drifts due to its small size and compact mounting of all optical components. The generated single photon beam can either be free-space or fibre coupled by removal/addition of a single mirror which is equipped with a magnetic base. The sample unit holding the defect centres can either be a SIL with spin-coated defect centres or another substrate due to a removable sample holder. The setup is equipped with broadband optics and thus suitable for various defect centres, provided their emission wavelength is in the range of 600 nm to 800 nm. Only the exchangeable dichroic mirror has to be adapted together with the suitable excitation source.

The sample holder is mounted on a 3-axes piezo stage. In order to keep track of the absolute position of the stage, sensors capable of detecting changes down to a nanometre are used (SmarAct System). In combination with a numerical aperture (NA) = 0.9 objective it is possible to focus on a well defined position on the sample with very high accuracy and stability, enabling high, constant single photon rates.

Objective NA=0.9 SIL

Fiber for laser excitation

Filter: LP620, SP780 for single phot

ff ons

(600 - 800 nm) Dichroic beam splitter

Single photon beam BS 90/10

(a)

(b)

Nano-diamonds with NV/SiV-centers

Out-Coupler

In-Coupler

Figure 22: (a) Scheme and (b) photo of the compact confocal microscope setup. The excitation laser is focused with a high numerical aperture (NA) objective onto the sample.

The sample contains nanocrystals that contain either NV or SiV defect centres. In the case of NVs centre, a SIL serves as the sample holder, enabling a higher NA for increased photon collection. The emission is collected by the same objective and then filtered by a dichroic beam splitter (exchangeable) and longpass (LP) and shortpass (SP) filters to clean it from residual laser light or fluorescence of the SIL or the substrate. The 90/10 beam splitter (BS) behind the out-coupler of the excitation laser is used to monitor the excitation power. Taken from [171].

In QKD it is favourable to use photons at a well defined instant of time, thus pulsed excitation of the defect centres is used. For the QKD experiment, the maximal excitation rate is limited to frequencies up to 1 MHz by the modulation rate of the EOMs (see below).

Alice uses a green diode laser (PicoQuant LDH-P-FA-530, 531 nm, pulse width

<100 ps) for excitation at a rate of 1 MHz. This yields detected count rates at Bob’s side for an NV⁻centre in a nanodiamond which was spin coated on a SIL of about 8900 cps. That corresponds to an overall photon yield of 0.89 % and a source efficiency of 2.9 %. The latter is defined as the ratio of excitation pulses resulting in a single photon without background in the desired optical mode, here the free-space beam of the QKD experiment. It is determined, for a given overall photon yield, by taking the overall transmissiontsetupof 0.31 of the setup, including the quantum efficiency of∼65 % of the APDs (Perkin Elmer AQR), into account. Ag⁽²⁾(0) value under pulsed excitation of 0.09, indicating high purity single photon emission, is determined (Fig. 23a) using high resolution time-correlation electronics (PicoHarp 300, from PicoQuant). A lifetime of 28.5±1.5 ns is estimated from the exponential decay of the fluorescence in time.

0 0.5 1 1.5 2 2.5 3 3.5 4

1200 1000 800 600 400 200

0

τ [µs]

Counts

(a) (b)

τ [µs]

0 0.5 1 1.5 2 2.5 3 3.5 4

3500 3000 2500 2000 1500 1000 500 0 Counts

Figure 23: Measured intensities as a function of time for NV (a) and SiV (b) emission under pulsed excitation to calculateg⁽²⁾(τ). The excitation rates are 800 kHz and 1 MHz, respectively. The missing peak atτ = 0 indicates single photon emission. From the pulse shape, a lifetime of the excited state of 28.5±1.5 ns for the NV centre and 3±2 ns for the SiV centre is calculated. Taken from [171].

The single SiV centres used here are created during chemical vapour deposition (CVD) growth of randomly oriented nanodiamonds on Iridium (Ir) films [113] and have been supplied by the research group of Christoph Becher from the University

of Saarbr¨ucken. During the growth process, Si atoms in the gas phase of the CVD chamber are incorporated into the diamond lattice. Subsequent annealing yields SiV centres. Excitation of the SiV centre is performed with laser pulses from a red diode laser (PicoQuant diode laser LDH-D-C-690, 687 nm, pulse width <100 ps) also at an excitation frequency of 1 MHz. For the brightest SiV centre a photon count rate of 3700 cps and a g⁽²⁾(0) value of 0.04 is achieved, see fig. 23b. From the exponential decay of the fluorescence peak, a lifetime of 3±2 ns is estimated.

The overall photon yield is 0.37 % and the plain source efficiency is thus 1.2 %.

The achieved count rate per excitation pulse is lower for the SiV centre compared to the NV centre. Knowing that the collection efficiency of emitted photons from SiV centres in nanodiamonds grown on Ir-substrate can be very high [119], this hints at a lower quantum efficiency. In [119], a quantum efficiency between 1-9 % is estimated. However, compared to the NV centre, the excitation frequency could in principle be chosen to be much higher for the SiV centre due to the shorter lifetime, which could compensate for the lower quantum efficiency.