# Twisted photon entanglement through turbulent air across Vienna

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Contributed by Anton Zeilinger, September 14, 2015 (sent for review August 5, 2015)

## Significance

The spatial structure of photons provides access to a very large state space. It enables the encoding of more information per photon, useful for (quantum) communication with large alphabets and fundamental studies of high-dimensional entanglement. However, the question of the distribution of such photons has not been settled yet, as they are significantly influenced by atmospheric turbulence in free-space transmissions. In the present paper we show that it is possible to distribute quantum entanglement of spatially structured photons over a free-space intracity link. We demonstrate the access to four orthogonal quantum channels in which entanglement can be distributed over large distances. Furthermore, already available technology could provide access to even larger quantum state spaces.

## Abstract

Photons with a twisted phase front can carry a discrete, in principle, unbounded amount of orbital angular momentum (OAM). The large state space allows for complex types of entanglement, interesting both for quantum communication and for fundamental tests of quantum theory. However, the distribution of such entangled states over large distances was thought to be infeasible due to influence of atmospheric turbulence, indicating a serious limitation on their usefulness. Here we show that it is possible to distribute quantum entanglement encoded in OAM over a turbulent intracity link of 3 km. We confirm quantum entanglement of the first two higher-order levels (with OAM=

- quantum entanglement
- photonic orbital angular momentum
- quantum communication
- large Hilbert space
- photonic spatial modes

Long-distance quantum entanglement with photons opens up the possibility to test fundamental properties of quantum physics in regimes not accessible in laboratory-scale experiments, it can be used for quantum communication between widely separated parties, and it is the basis of quantum repeaters as nodes in a global quantum network. As the polarization of photons is easily controllable and resistant against atmospheric turbulences, it has been successfully used in a variety of different long-distance quantum experiments (1⇓⇓–4). However, polarization of photons resides in a two-dimensional state space, restricting the complexity of entangled states both for certain quantum communication tasks and for fundamental tests.

In contrast with polarization, the orbital angular momentum (OAM) modes of photons have an unbounded state space. Photons carrying OAM have a twisted wave front with a phase that varies from 0 to 2πℓ in the azimuthal direction. Here, ℓ is an integer which stands for the topological charge, and

## Results

Recently, in two experiments the classical transmission of OAM modes in a long-distance outdoor environment has been investigated. The first experiment was performed over a 3-km intracity link in Vienna (the same link that is being used in the experiment presented here). Superpositions of OAM modes have been used, which can be categorized by their intensity structure. A pattern recognition algorithm distinguished the different modes with high quality. The results also indicated that the phase of OAM superpositions is well conserved during the transmission, hinting that the distribution of quantum entanglement encoded in OAM might be possible (26). Shortly after that, a second experiment was performed over a 1.6-km intracity link in Erlangen (27). There, an OAM mode sorter (28) has been used to categorize different states from ℓ = −2 to ℓ = +2. A significant broadening of the OAM spectra has been observed. Here, we present the results of an experiment in which we confirm the indication of the first experiment mentioned above: We show that quantum entanglement distribution with spatial modes is possible over a turbulent intracity link.

The experimental setup can be divided into four main parts (Fig. 1): the source of polarization entanglement, the transfer of one photon from polarization to the OAM degree of freedom, Alice’s polarization analysis, and Bob’s OAM measurement after transmission. The sender (Alice) and the receiver (Bob) are at different physical locations 3 km apart. The sender is located in an *Zentralanstalt für Metrologie und Geodynamik* (ZAMG: Central Institute for Metrology and Geodynamics). There we use a high-fidelity, high-brightness polarization entanglement source (30, 31) with an uncorrected average visibility of ∼97.5%. Photon A is unchanged, whereas photon B’s polarization state is transferred interferometrically to an OAM state (5). After the transfer the generated hybrid-entangled quantum state can be written as*.* The OAM-encoded photon B is magnified to a Gaussian beam waist of 11 mm and sent through turbulent atmosphere with a high-quality lens (best-form f/4 doublet lens, with low spherical aberrations) using a focal length of f = 30 cm to the receiver 3 km away.

At the receiver on the rooftop of our institute [Institute for Quantum Optics and Quantum Information (IQOQI) Vienna], we use a Newton-type telescope with a primary mirror of 37-cm diameter and a focal length of f = 1.2m. In front of the primary mirror, we use an absorptive mask with a transparent, symmetric slit pattern to measure the modes (Fig. 2). The technique (5) allows us to measure visibilities in OAM-superposition bases, which is sufficient to verify entanglement. The masks are 40 cm in diameter and have a slit opening angle of 16° and 5.6° (for ℓ = 1 and ℓ = 2, respectively). The transmitted light is then detected on an avalanche photon detector (APD) with an active area of 500-µm diameter. Similarly to the detection of the polarized partner photons, the arrival times are time tagged with a second TTM. To synchronize the time stamps on the two remote locations in the subnanosecond regime, we directly use the time correlation of the photon pairs (which is inherently below 1 ps), as explained in ref. 32.

In the experiment, we perform visibility measurements in two mutually unbiased bases (MUBs). For photon A, the bases are diagonal or antidiagonal and right- or left-circular polarization (*σ*_{x} and *σ*_{y} bases, which correspond to the *σ*_{x} and *σ*_{y} bases of photon A (5). Specifically, we measure fringes in coincidence counts for changing angular positions of the mask. We find minima and maxima of coincidences (Fig. 3), and calculate the visibility:

In the first measurement, we use the first higher-order mode with ℓ = 1. We accumulate coincidence detections over 20 s at 20 different angular positions of the mask with a resolution of 9° (Fig. 3). The coincidence window is 2.5 ns. Without any corrections (such as accidental coincidence subtraction) and without any assumption about the photon statistics, we get**2** for each of the 20 sections individually. From the resulting 20 values for W_{ℓ} _{=} _{1} (see the *Supporting Information*), we calculate the mean value and its uncertainty. To calculate the uncertainty of W_{ℓ} _{=} _{1}, we did not need to assume any specific photon statistics. In many cases, Poissonian distribution is a good approximation of the photon statistics. However, it neglects additional sources of fluctuations, which can become relevant in experiments without controlled environments, such as free-space experiments. If we had assumed Poissonian distribution in our experiment, we would have underestimated the uncertainty significantly by around 70%, which is mainly due to atmospheric turbulences and instabilities at the sender. If we subtract accidental counts (

In a second experiment, we transfer the photon to ℓ = 2 before transmission, send it to the receiver, and measure coincidence counts for 20 different mask positions, each 4.5° rotated, for 40 seconds per setting. Here we get_{ℓ = 2} (see the *Supporting Information*), from which the mean and its error was obtained. Note that again we did not assume any information about the photon statistics. However, we had to subtract accidental coincidence counts (*Supporting Information*), because the signal-to-noise ratio was significantly higher (5.5% for *Supporting Information*). As we collect timing information of the photons at the sending and receiving stations, we can access the number of accidental counts directly: At the offset between the time-tagging clocks, which correspond to the arrival times of the photons from a pair at the two locations, real coincidence counts from the entangled pairs are found. At every other offset, accidental coincidences can be seen (see the *Supporting Information*).

In both experiments for ℓ = 1 and ℓ = 2, we find visibilities smaller than unity. The reasons are an imperfect entanglement source (due to lack of temperature and vibration stabilization), imperfect detection method (the mask method can only give unity visibility for infinitesimal small slits), imperfect polarization compensation in fibers, accidental coincidence counts, and atmospheric influence. The first-order atmospheric influences are tip and tilt of the beam, which leads to relative misalignment between the mode and the mask. As the detection method is axis-dependent, it results in a significant drop in visibility (see the *Supporting Information*), which is larger for higher-order modes. However, that effect could be compensated with readily available adaptive optics.

Having confirmed that entanglement encoded in OAM can be transmitted over an intracity link, we estimate the number of different orthogonal quantum channels we have access to in principle. As OAM modes grow for higher numbers of ℓ and our receiver telescope has a finite size, there is a maximum number of ℓ that can be detected. For that, we transfer photon B to different values of ℓ and (from ℓ = 0–15) and record the number of coincidences with photon A. Thus, photon A is a trigger for the higher-order ℓ modes of photon B after sending it across the city. Here, no mask is in front of the telescope. The detected coincidence rates in Fig. 4 show that photons up to ℓ = 5 can be distinguished from the background. The graph can be described very well by the geometry of our telescope, which cuts the incoming beam both at the primary and secondary mirror (*Supporting Information*). We consider the counts of high order (ℓ ≥ 10) as background, as they reach an asymptotic value (*Supporting Information*). With our sender and receiver, we have access to roughly 11 quantum channels of OAM (ℓ = 0 to ℓ = ±5).

## Discussion

In conclusion, we are able to verify quantum entanglement of photon pairs with spatial modes over a turbulent, real-world link of 3 km across Vienna. It shows that the spatial phase structure of single photons is preserved sufficiently well to be used in quantum optical experiments involving entanglement. By using the first two higher-order structures (ℓ = ±1, ℓ = ±2), we show that at least four additional orthogonal channels [in addition to the zeroth-order Gaussian (1) case for ℓ = 0] permit long-distance quantum communication. Although we still use two-dimensional subspaces, our result clearly shows that entanglement encoded in OAM can be identified after long-distance transmission. It is not fundamentally limited by atmospheric turbulences, as expected in some recent investigation, and thus could be a feasible way to distribute high-dimensional entanglement.

We also show that our quantum link allows up to 11 orthogonal channels of OAM. The restrictive factors toward a higher number of channels and higher quality of entanglement detection are technical limitations. The number of accessible channels can be increased by using optimal generations of the modes (leading to smaller intensity structures) (34, 35) and larger sending and receiving telescopes. The quality of disturbed spatial modes can be improved with well-established adaptive phase-correction algorithms (24, 36, 37), which might lead to significantly larger quality in the entanglement identification. Adaptive measurement algorithms form another method to improve the entanglement detection, by adjusting the measurements according to the turbulence (38).

Entanglement of high-order spatially encoded modes over long distance opens up several interesting directions: Firstly, twisted photons have a large state space, and thus can carry more information than the well-studied case of polarization. Higher information capacity could be interesting for both classical and quantum communication, for example to increase the data rate. Additionally, in quantum key distribution (39⇓–41) it could be used for increasing the robustness against noise, or improving the security against advanced eavesdroppers (42). Secondly, OAM of photons permits complex types of entanglement due to their large state space. It also represents a physical quantity which can be (in principle) arbitrarily large, thus it might be a very interesting testbed for fundamental tests. As such, curious phenomena such as the coupling of OAM modes with the space–time metric have been proposed (43). We believe that our results will motivate both further theoretical and experimental research into the promising novel direction of long-distance quantum experiments with twisted photons.

## Weather Conditions During the Measurement Nights

Fig. S1 depicts the detailed weather data from the measurement nights. Specifically important is Fig. S1*A*, the metrological visibility. It is a measure of the distance at which a black object can be clearly identified from the background and is defined as the distance x_{V}, where the contrast _{BG} and F_{O} are intensities of the background and the black object, respectively. At the position of the object (at x = 0), F_{O}(0) = 0, thus C_{V}(0) = 1. Under very good conditions, x_{V} can reach *F*). The change in temperature (Fig. S1*B*) as well as the average and maximum wind (Fig. S1 *D* and *E*) are influencing the turbulences of the atmosphere.

In the measurement of entanglement involving ℓ = 2, the metrological visibility was significantly smaller, which was one reason for the lower fringe visibility in Fig. 2. Temperature, wind speed, humidity, and cloud base were similar in the two nights.

## Single-Photon Counts and Accidental Coincidence Counts

In the measurement of ℓ = 1 and ℓ = 2, we detect on average the single-photon counts shown in Table S1.

In the measurement of entanglement with ℓ = 2, we needed to subtract accidental coincidence counts. The reason was a large number of background counts compared with the small number of photons from the entangled pairs, with a signal-to-noise ratio of 0.055. The smaller ratio (compared with ℓ = 1) can be understood by the smaller metrological visibility (as explained above), less efficient collection at the telescope (Fig. 4), and a smaller slit opening angle of the mask.

As we record the timing information of arriving photons (with TTMs), we know the accidental count rates very accurately. The reason is that we implicitly measure the rate (for every real coincidence measurement) very often: Consider that we set the relative timing delay between the different time-tagging clocks to zero; we see correlated photons. However, if we compare any other timing interval, we only see the accidental coincident counts. An example can be seen in Fig. S2. (The implicit assumption is that the accidental counts are independent of Δt.)

## Mode Misalignment at Mask

The detection method used here is axis-dependent, which means that the mode and the mask have to be aligned well (Fig. 2*B*). Misalignments such as shifts of the mode relative to the mask will reduce the observable visibility. The misalignment can be introduced by first-order atmospheric influences such as tip and tilt. The effect can be simulated (Fig. S3), and we find that higher modes are more sensitive to alignment, which is one reason why the observed visibility for our ℓ = ±2 measurement was lower than for the ℓ = ±1 measurement.

In the experiment with ℓ = 1, we had an average visibility of roughly 68.2%. With accidental subtraction we have 84.2% visibility, and accounting for nonperfect visibility from the entanglement source (which was roughly 97.5%), and imperfect polarization compensation in fibers, we would expect a visibility of ∼89.0%. The remaining decrease in visibility could be explained by atmospheric turbulence. The first-order effect is a tip–tilt, which reduces the visibility as seen in Fig. S3. The loss of ∼11% can be explained by an average misalignment of the beam relative to the mask by 25% of the beam waist. For higher-order modes the reduction is more prominent. However, this effect can be compensated with readily available adaptive optics systems.

## Witness Values for 1-s Intervals

The statistical error is calculated by separating the measured data into 1-s intervals, calculating the witness according to Eq. **2** and calculating the SD of the mean.

For ℓ = 1, we have 20 witness values, because we measured each value for 20 s (sorted):

W = 1.4354, 1.4214, 1.3942, 1.3940, 1.3928, 1.3858, 1.3848, 1.3841, 1.3804, 1.3692, 1.3633, 1.3560, 1.3522, 1.3472, 1.3366, 1.3361, 1.3346, 1.3279, 1.3173, 1.2743.

For ℓ = 2, we have 40 witness values, because we measured each value for 40 s (sorted):

W = 1.4218, 1.4201, 1.4106, 1.3708, 1.2581, 1.2570, 1.2452, 1.2338, 1.2245, 1.2180, 1.2158, 1.2142, 1.2055, 1.2051, 1.2023, 1.2023, 1.1919, 1.1738, 1.1454, 1.1312, 1.1235, 1.1091, 1.0995, 1.0950, 1.0778, 1.0702, 1.0695, 1.0524, 1.0492, 1.0410, 1.0283, 1.0248, 1.0230, 1.0190, 1.0141, 0.9927, 0.9926, 0.9789, 0.8963, 0.8635.

With our method, we not only account for photon-counting statistics, but more general systematics such as laser-power fluctuations or other instabilities in the experiment; or fluctuations due to atmospheric turbulences.

## Telescope Geometry and OAM-Dependent Receiving of Photons

The telescope cuts the beam in two different ways, which result in a reduced number of detected photons: Firstly, the primary mirror has a finite diameter of roughly 37 cm. As OAM modes increase in size if one increases the l value, higher-order modes are more significantly cut. Secondly, the secondary mirror of the telescope is in the center of the beam path. The Gauss mode has the maximal intensity there, thus it is substantially cut at the secondary mirror. The effect can be illustrated graphically; see Fig. S4.

In the experiment, we prepare OAM modes with phase-only holograms. The size of those modes scales linearly with ℓ. There are no closed-form solutions for such holograms; thus, we calculate them numerically by applying a Fourier transformation of a Gaussian beam with a helical phase (which is introduced by the SLM):_{0} is the beam waist, and *N* is a normalization constant. The expected intensities for different modes can be easily calculated with the given boundary conditions. The function_{0} (beam waist at telescope) being the fit parameters. Here, the background bg has been calculated from the measured data (the mean of the counts from ℓ = 10 to ℓ = 15), and PM (size of primary mirror) has been measured to be roughly 18.5 cm. The obtained fit values resemble very well the physical dimensions of the telescope (SM = 1.2 cm) and of the observed classical alignment beam (w_{0} = 8.5 cm) in Fig. 2.

## Triggered Single Photons and Background

In Fig. 4, we show the potential of our telescope to detect high-order modes, which is restricted due to the finite size of the primary and secondary mirror (see Fig. S3). In that experiment, we are not interested in entanglement, but only in the number of detected triggered single photons in higher-order modes. Therefore, we removed the polarizer after the transfer setup to double the number of photons that are sent. We create the state

The background in Fig. 4 originates mainly from two effects: The first effect is accidental coincidence counts because of large count rates (1,500,000 single counts per s at the receiver, which led to

## Acknowledgments

We thank Roland Potzmann and ZAMG for providing access to the radar tower and detailed weather information. We also thank Mehul Malik for help with the experiment, Thomas Scheidl, Thomas Herbst, and Rupert Ursin for useful discussions, and Nora Tischler for useful comments on the manuscript. This project was supported by the Austrian Academy of Sciences, the European Research Council (SIQS Grant 600645 EU-FP7-ICT), the Austrian Science Fund with SFB F40 (FOQUS), and the Austrian Federal Ministry of Science, Research and Economy.

## Footnotes

- ↵
^{1}To whom correspondence may be addressed. Email: mario.krenn{at}univie.ac.at or anton.zeilinger{at}univie.ac.at. ↵

^{2}Present address: Department of Physics and Max Planck Centre for Extreme and Quantum Photonics, University of Ottawa, Ottawa, Canada K1N 6N5.

Author contributions: M.K., R.F., and A.Z. designed research; M.K., J.H., M.F., and R.F. performed research; M.K., J.H., M.F., R.F., and A.Z. contributed new reagents/analytic tools; M.K. analyzed data; and M.K., J.H., M.F., R.F., and A.Z. wrote the paper.

The authors declare no conflict of interest.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1517574112/-/DCSupplemental.

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