2.1

PhotonBox - a low-cost lab container

The PhotonBox is a low-cost container laboratory and part of 1.7 km intra-city link in Jena, Germany. It is built from a standard 10-foot shipping container that was lifted onto the roof (6th floor) of the local energy provider. Figure 1 shows the floor plan of the PhotonBox as well as a photograph of the outside view. The container has one door, one front window which is equipped with a 450 mm x 450 mm optical fused silica substrate and one window with an electrical shutter on the roof. Electricity, which must be provided from an external source as well as internet connection is feed into the container through a sealed hole. For an easy access 50 sockets are distributed around the walls. An air-condition together with 100-mm-thick insulation ensure room temperature with acceptable stability also at high or low outside temperatures. Basis for the experiments is a science desk or an optical table close to the front window. The desk with a size of 1000 mm x 900 mm is decoupled from the containers floor to prevent vibration or any swinging effect when operators move while an experiment is running. The legs from the desk are extended with aluminium posts that are feet through four wholes in the containers floor. Thus, the legs can be positioned directly on the roof the building. The remaining space inside is filled with racks for the quantum communications equipment and two computer workstations.

Figure 1.

Drawing of the PhotonBox floor plan and a photograph from outside, while installation on the roof of the building.

2.2

QuBus - The concept of a high performance mobile platform for quantum communications

The QuBus is a more sophisticated version of the PhotonBox which is described in section 2.1. While the PhotonBox is developed as a low-cost laboratory providing the basic laboratory infrastructure like appropriate humidity, temperature, electricity and internet, the QuBus is designed as a robust experiment vehicle for ad-hoc field measurement campaigns in quantum communications. Basis for the QuBus is a 15-foot shipping container that can be transported by a low-bed trailer. Main difference to the PhotonBox is how the light of the experiments is feed out of the container and directed to the target. In the PhotonBox, the light coming from the telescope breadboard must be directed by moving the breadboard itself. This is done by using a commercially available motorized mount (see section 3.1). In contrast, in the QuBus the telescope breadboard and all the optical and electrical equipment can be fixed onto an optical table rigidly, which reduces limitations in weight and geometry of the equipment and enables a faster change of all devices even while operation. The light is then directed by a highly stable, precise periscope mechanics on the roof of the QuBus (see Figure 2). While the coarse alignment of the beam coming from the PhotonBox must be done by positioning the container itself in the right way, the alignment of the QuBus’ beam is totally independent from the vehicles orientation. The periscope enables a -5° to 90° angle in altitude and a 360° angle in azimuth. The diameter of the periscope is 250 mm and matches the aperture of the telescope of the breadboard described in section 3.

Figure 2.

3D-Model of the QuBus. View from outside.

For secure transportation operation, the entire equipment inside and outside the container is installed according to military standards. The QuBus is currently under construction. First field tests are planned for summer 2022.

3.1

Opto-mechanical transmitter and receiver module

The opto-mechanical transmitter and receiver modules consist of two distinct modules. The optical beam paths inside the two modules are shown in Figure 3. The basis of each module is an off-the-shelf 750 mm x 750 mm optical breadboard braced by aluminium profiles. The modules provide the basic infrastructure for flexible quantum experiments and therefore include numerous functionalities.

Each module includes an obscuration-free, off-axis mirror telescope, which provides a large transmitting and receiving aperture with a diameter of 200 mm, respectively. Each telescope is characterized by a 20x optical magnification and features multiple aspherical mirror surfaces with a protected gold coating. The respective design is optimized to provide a diffraction limited imaging performance over a large field of view up to 3.5 mrad. In addition, the individual mirror surfaces provide a minimized angle of incidence variation across their aperture in order to minimize their effect on the polarization state of the laser beams. The ultra-precision manufactured mirrors are integrated in a housing, manufactured from aluminum alloy and containing pattern of ribs for bracing purposes. The housing is manufactured from a single piece, providing highly precise reference surfaces for the mirrors. Three bipods mount the housing strainlessly to the breadboard. In addition to their main telescope, each module includes an active 4D beam stabilization system. This adaptive optical system corrects for the influence of atmospheric turbulences during the laser beam propagation between the transmitting and receiving aperture, i.e. tip/tilt wavefront aberrations as well as optical beam wander. The system consists of two tip-tilt mirrors (TTMs), which are based on commercially available, ultra-fast piezoelectric platforms from Physik Instrumente. The plattforms are integrated into a customized mirror mount that provides the needed dynamic stiffness. The TTMs are controlled in a closed loop using a commercially available control system (TEM Aligna). The system utilizes the signal of a set of two position sensitive devices (PSDs). The PSDs measure the position and angle of an incoming beacon laser beam in order to stabilize the communication beam. The control loop provides a low response time of less than two milliseconds to counteract the rapidly changing wavefront aberrations. The telescope’s bipods as well as the TTM mounts are screwed directly into the breadboard since this provides the needed stability.

As can be seen in Figrue 4, the telescope and the beam stabilization in the transmitter and the receiver module are followed by a combination of broad-band, high-pass, low-pass and narrow-band beam splitters. This combination facilitates the separation of the different beam paths depending on their wavelength in order to feed the different interfaces for the quantum sources and analysis equipment. A live-view camera for coarse subsystem alignment at 653 nm is implemented using a long-pass beam splitter located between the two TTMs. After the second TTM, the beam paths for the quantum source and the quantum receiver module at 810 nm and 1550 nm, respectively, the PSD as well as a photo diode for the time synchronization at 970 nm or 1064 nm, respectively, are isolated according to the schematic overview in Figure 3. Note that the transmission properties of all beam-splitting elements are optimized for a minimum polarization sensitivity at the QKD wavelengths. The respective opto-mechanical components can be flexibly arranged depending on the particular experiment using the standard bore pattern of the optical breadboard. The combination of a multitude of functionalities with respect to the QKD wavelengths requires a large installation space, which we reduced by using a 3D beam path arrangement. After passing the mirror telescope and the first TTM, the light is guided through an elongated hole in the telescopes housing and the breadboard. In doing so, both sides of the breadboard can be fully used and the overall assembly is kept compact. To prevent the breadboard from bending under the heavy weight, a frame of aluminum profiles and one additional profile on the opposite side of the ASA mount interface braces the breadboard. Four legs can be added to both sides of the frame, respectively, to align the optical system on an optical table. For a reliable functionality of the breadboard mount, the center of gravity (COG) of the moved mass must be close to its spin axis. Since the position of the COG depends on the chosen setup, we integrated an adjustable mechanism to balance the breadboard over the breadboard mount. One of the two transceiver systems with the the mounted telescope breadboard is shown in Figure 5.

Figure 4.

3D beam path arrangement of a transmitter (ALICE) and receiver (BOB) module.

Figure 5.

Image of the fully integrated Telescope Breadboard.

3.2

Quantum Sources

Entangled photon pairs are generated via spontaneous parametric down-conversion (SPDC) in a nonlinear crystal. Here one can imagine that pump photons from a continuous-wave laser spontaneously decay into a pair of photons with lower energy. Energy- and momentum conservation lead phase matching conditions. These are acquired by a specific crystal-design with periodically poling and temperature adjustment. The temperature stabilized non-linear crystal is placed into a Sagnac-loop, in this configuration polarization entangled photon pairs can be created very efficiently. The specific source used for the free-space link creates pairs with a rate of 70kHZ at a wavelength of 810 nm. The quality of the entanglement can be estimated by the visibilities the different polarization bases H/V and D/A with the present source values of 98.2% and 98.6% can be reached respectively. Further details about the source can be found in.¹

3.3

Receiver Modules

The receiver for wavelengths around 810 nm is designed for polarization encoded quantum communication, compatible with most standard QKD protocols such as BB84² and BBM92.³ These protocols require polarization measurements in two mutually unbiased bases, namely horizontal-vertical (HV-basis) and diagonal-antidiagonal (DA-basis), and the selection of the basis for each incoming photon needs to be randomly selected. In our polarization analysis module (see Fig. 6), this is passively realized with a beam-splitter, of which one output is directly followed by a Wollaston prism to split H- and V-polarized photons, while the half-wave plate at the other output first rotates polarization by 45°, making the subsequent Wollaston prism split D- and A-polarized photons. Each of the four detection paths is finally coupled into a multi-mode fibre, which guides the light from the telescope board to the four single-photon avalanche detectors (SPADs) located in the rack.

Figure 6.

Polarization analysis module. Incoming photons are randomly measured either in horizontal-vertical or in diagonal-antidiagonal basis. Polarization controller ensures matching of transmitter and receiver polarization coordinate systems.

To ensure matching of polarization coordinate systems at transmitter and receiver, a motorized polarization controller consisting of a stack of half- and quarter-wave plates was developed and integrated into the module, allowing for automatic basis alignment. Furthermore, the module incorporates two exchangeable filter slots that can be used to seamlessly replace the desired filter combination to match the wavelength of the source. Strong rejection of other wavelengths is of utmost importance, since the signal levels may be as low as in the order of 105 photons per second, amounting only to tens of femtowatts of power at 810 nm. For the wavelengths around 1550 nm, a rather different approach was chosen. These wavelengths are particularly interesting for the fibre-based quantum communication systems. For this reason, instead of directly performing polarization mode projection in free-space, the light is coupled into a single-mode fibre, opening the possibility to use the QuNET system as an interface between a free-space and a fibre-based communication channel. Once the light is coupled, it can be guided to any analysis system, allowing also for other encoding-schemes and protocols.

3.4

Active beam stabilization

Active beam stabilization is realized by utilizing a combination of two silver mirrors along the beam path that are mounted on fast piezo-based tip/tilt platforms. Together with a detection module at the end of the beam path, which provides the feedback signal for the piezo drivers, they form a closed-loop system. The combined control of the two mirrors provides a 4-dimensional beam stabilization in angle and position. The detection module consists of a beam splitter and two position-sensitive-detectors – one directly measures the centroid of the beam, while the other one positioned behind a lens in turn measures the angle of the beam. The system was designed to cope with beam wander and angle-of-arrival fluctuations induced by turbulences with Fried parameter of about r₀ = 2 cm. The bandwidth of the entire correction system is estimated to be about 250 Hz. These specifications are expected to suffice for beam stabilization in most conditions of the Jena free-space link. The system was tested on an actual turbulent 50 m-link by running a measurement from 6 pm until 10 am the next day. In addition to varying turbulence strength, the system was also exposed to relatively large but slow temperature changes, characteristic of a late November night in Jena. Figure 7 shows the four piezo drive signals used to control tilts of the mirrors (top row), and the corresponding residual beam angle/position error (bottom row). The solid black lines represent 5-minute moving averages, indicating long-term changes in the system beam path. It can clearly be seen from piezo driver signals that the system mostly stayed well within its dynamical range of ±10 (abstract units) over the course of 16 h, compensating for short-term turbulence effects as well as for long-term thermal drifts. Standard deviations (STDs) of the residual errors throughout the test are shown in Table 1. Note that these values correspond to angles and positions on the telescope breadboard – in order to translate them to the free-space link in between the two telescopes, angles need to be demagnified and positions magnified by 20, which is the magnification factor of the telescopes.

Figure 7.

Overnight beam-stabilization. The plots in the top row show the closed-loop piezo signals and the plots in the bottom row show the corresponding residual errors of beam’s angle and centroid. The solid black line is a 5-minute moving average, indicating the long-term changes in the system beam path.

Table 1.

Standard deviations of residual beam angle and position errors on the telescope breadboard during the 16 h overnight measurement.