Abstract
The identification of phenomena able to pinpoint quantum interference is attracting large interest. Indeed, a generalization of the Hong–Ou–Mandel effect valid for any number of photons and optical modes would represent an important leap ahead both from a fundamental perspective and for practical applications, such as certification of photonic quantum devices, whose computational speedup is expected to depend critically on multiparticle interference. Quantum distinctive features have been predicted for many particles injected into multimode interferometers implementing the Fourier transform over the optical modes. Here we develop a scalable approach for the implementation of the fast Fourier transform algorithm using threedimensional photonic integrated interferometers, fabricated via femtosecond laser writing technique. We observe the suppression law for a large number of output states with four and eightmode optical circuits: the experimental results demonstrate genuine quantum interference between the injected photons, thus offering a powerful tool for diagnostic of photonic platforms.
Introduction
The amplitude interference between wavefunctions corresponding to indistinguishable particles lies at the very heart of quantum mechanics. Right after the introduction of laser amplification, the availability of strong coherent pulses allowed to test interference between different light pulses^{1,2}, while generation of pairs of identical photons through parametric fluorescence^{3} led subsequently to the milestone experiment of Hong et al.^{4,5,6,7,8}. Later on, photonic platforms have been demonstrated to be in principle capable to perform universal quantum computing^{9}.
Recently, multiparticle interference effects of many photons in large interferometers are attracting a strong interest, as they should be able to show unprecedented evidences of the superior quantum computational power compared with that of classical devices^{10,11,12}. The main example is given by the boson sampling^{13} computational problem, which consists in sampling from the probability distribution given by the permanents of the n × n submatrices of a given Haar random unitary. The problem is computationally hard (in n) for a classical computer, since calculating the permanent of a complexvalued matrix is a #Phard problem. However, sampling from the output distribution can be efficiently achieved by letting n indistinguishable photons evolve through an optical interferometer implementing the unitary transformation in the Fock space, and by detecting output states with an array of singlephoton detectors. The chance to provide evidences of a postclassical computation with this relatively simple setup has triggered a large experimental effort, leading to smallscale implementations^{14,15,16,17,18,19,20}, as well as theoretical analyses on the effects of experimental imperfections^{21,22} and on possible implementations including alternative schemes^{23,24}.
In the context of searching for experimental evidences against the extended Church–Turing thesis, a boson sampling experiment poses a problem of certification of the result’s correctness in the computationally hard regime^{25}. The very complexity of the bosonsampling computational problem precludes the use of a bruteforce approach, that is, calculating the expected probability distribution at the output and comparing it with the collected data. Efficient statistical techniques able to rule out trivial alternative distributions have been proposed^{26} and tested^{18,19}, but the need for more stringent tests able to rule out less trivial distributions has led, and continues to encourage, additional research efforts in this direction.
In particular, an efficient test able to confirm true nphoton interference in a multimode device has been recently proposed^{27}. The protocol is based on the use of an interferometer implementing the transformation described by the n^{p}dimensional Fourier matrix, with p being any integer. When feeding this device with multiphoton states of a specific symmetry, suppression of many output configurations is observed^{28}, due to granular^{27} manyparticle interference. This effect is able to rule out alternative models requiring only coarsegrained features like the ones present in Bose–Einstein condensates^{29,30,31}. Indeed, the implications of this effect go well beyond the certification of boson sampling devices. As a generalization of the twophoton/twomodes Hong–Ou–Mandel (HOM) effect, the suppression law, also named ZeroTransmission law^{28}, is important at a fundamental level, while at the practical level it could be used as a diagnostic tool for a wide range of photonic platforms^{27,32,33}. During the review process of this work, the implementation of a discrete Fourier transform circuit in a fully reconfigurable chip has been reported^{34}. The ZeroTransmission law for threephoton nobunching events has been demonstrated in this planar sixmode interferometer.
In this article, we report the experimental observation of the recent theoretically proposed^{27} suppression law for Fourier matrices, and its use to validate quantum manybody interference against alternative nontrivial hypotheses resulting in similar output probability distributions. The Fourier matrices have been implemented with an efficient and reliable approach by exploiting the quantum version of the fast Fourier transform (qFFT), an algorithm developed by Barak and BenAryeh^{35} to optimize the number of optical elements required to build the Fourier transform over the optical modes. Here we implement the qFFT on photonic integrated interferometers by exploiting the threedimensional (3D) capabilities of femtosecond laser writing^{36,37}, which makes it possible to fabricate waveguides arranged in 3D structures with arbitrary layouts^{38,39,40}, by adopting an architecture scalable to a larger number of modes. The observations have been carried out with twophoton Fock states injected into fourmode and eightmode qFFT interferometers. The peculiar behaviour of Fock states compared with other kinds of states is investigated, showing in principle the validity of the certification protocol for the identification of true granular nparticle interference, which is the source of a rich landscape of quantum effects such as the computational complexity of boson sampling.
Results
Suppression law in Fourier transform matrices
As a generalization of the HOM effect, it has been pointed out that quantum interference effects in multimode interferometers may determine suppression of a large fraction of the output configurations^{28,31,41}, depending on the specific unitary transformation being implemented and on the symmetry of the input state. In particular^{28,31}, let us consider a cyclic input, that is, an nphoton Fock state over m=n^{p} modes (for some integer p) where the occupied modes are determined by the rule , with r=1, …, n and s=1, …, n^{p−1}. The index s takes into account the fact that there are n^{p−1} possible nphoton arrangements with periodicity n^{p−1}, which simply differ by a translation of the occupational mode labels. For example, for n=2 and m=4 there are 2^{1}=2 possible cyclic states, (1,0,1,0) and (0,1,0,1), while for n=2 and m=8 there are 2^{2}=4 possible (collisionfree) cyclic inputs, that is, the states (1,0,0,0,1,0,0,0), (0,1,0,0,0,1,0,0), (0,0,1,0,0,0,1,0) and (0,0,0,1,0,0,0,1).
We consider the evolution of such states through an interferometer implementing the transformation described by the Fourier matrix
Such evolution results in the suppression of all output configurations not fulfilling the equation
where k_{l} is the output mode of the l^{th} photon. An interesting application of suppression laws is to certify the presence of true manybody granular interference during the evolution in the interferometer, ruling out alternative hypotheses which would result in similar output probability distributions. In particular, in the case of Fourier matrices, the observation of the suppression law (2) allows to certify that the sampled output distribution is not produced by either distinguishable particles or a mean field state (MF)^{27}. The latter is defined as a singleparticle state of the form
with a random set of phases θ_{r} for each state, being a singleparticle state occupying input mode . This state reproduces macroscopic interference effects, such as bunching or bosonic clouding^{19}, and cannot be distinguished from true multiparticle interference with criteria based on these features. Since it is possible to efficiently simulate the evolution of a MF with a classical algorithm^{27}, it is of fundamental importance to assess the ability of a validation scheme to discriminate such a state in the context of an untrusted party claiming to perform a boson sampling experiment. Hence, the MF represents an optimal test bed for the certification protocol based on the suppression law (see Fig. 1).
It is possible to quantify the degree of violation of the suppression law as the number of observed events in forbidden output states divided by the total number of events^{27}. If a Fock state is injected in a Fourier interferometer, a violation would be observed. In the case of distinguishable photons there is no suppression law, and the violation would be simply the fraction of suppressed outputs, each one weighted with the number of possible arrangements of the n distinguishable particles in that output combination. In the case of twophoton states, the weighting factor is 2 for collisionfree outputs and 1 otherwise, and a degree of violation of 1/2 is expected (see Section ‘Observation of the suppression law’). In contrast, in the case of twophoton MF, bunching effects occur leading to an expected degree of violation of half the weighted fraction of suppressed outputs (1/4 for twophoton MF). It has been shown that the fraction of forbidden outputs is always large^{31}. Hence, a comparison of the observed value of with the expected one represents an efficient way, in terms of necessary experimental runs, to discriminate between Fock states, distinguishable particles states and MFs.
Realization of 3D qFFT interferometers
Let us now introduce our experimental implementation of the qFFT. The general method to realize an arbitrary unitary transformation using linear optics was introduced by Reck et al.^{42}, who provided a decomposition of a unitary of dimension m as a sequence of m(m–1)/2 beam splitters and phase shifters. However, in the special case of Fourier matrices a more efficient method has been proposed^{35,43}, which takes advantage of their symmetries to significantly reduce the number of linear optical elements required. On the basis of the classical algorithm of Cooley and Tukey^{44}, who first introduced the fast Fourier transform algorithm as a more efficient way to calculate the discrete Fourier transform, Barak and BenAryeh developed a quantum analogue in the linear optics domain, leading to the concept of qFFT. This approach, valid for 2^{p}dimensional Fourier matrices, requires only (m/2)logm beam splitters and phase shifters, to be compared with the O(m^{2}) elements needed for the more general Reck decomposition, thus enhancing the compactness and scalability of the platform for a more reliable experimental realization. The overall linear transformation on the optical modes implemented by the qFFT circuit is naturally equivalent to the transformation described by the Fourier matrix, hence .
Here we introduce a new methodology for an integrated implementation of the qFFT, which exploits the 3D capabilities of the femtosecond laser writing technique. The sequential structure arising from the decomposition of the mdimensional Fourier matrix using the Barak and BenAryeh algorithm is reproduced by the consecutive layers shown in Fig. 2. The complex arrangement of pairwise interactions necessary for the qFFT method cannot be easily implemented using a planar architecture. However, femtosecond laser writing technique allows to overcome this issue exploiting the third dimension, arranging the waveguides along the bidimensional sections of the integrated chip.
The strategy can be outlined as follows (see also Supplementary Note 1): the 2^{p} modes are ideally placed on the vertices of a pdimensional hypercube; in each step of the algorithm the vertices connected by parallel edges having one specific direction are made to interact by a twomode Hadamard transformation, with proper phase terms. An optical interferometer implementing this procedure is thus composed of log_{2}m=p sections, each employing m/2 balanced beam splitters and phase shifters.
We fabricated waveguide interferometers realizing the Fourier matrix for m=4 and 8 modes in borosilicate glass chips using femtosecond laser micromachining^{36,37}. A schematic representation of these two interferometers is given in Fig. 2. According to the scheme outlined above and by exploiting the 3D capabilities of the fabrication technique, the waveguides are placed, for what concerns the crosssection of the device, on the vertices of a twodimensional projection of the pdimensional hypercube (see also Supplementary Fig. 1). 3D directional couplers, with proper interaction length and distance to achieve a balanced splitting, connect in each step the required vertices. The insets of Fig. 2 show, at each step i, which modes are connected by directional couplers (L_{i}) and the amount of phase shift that needs to be introduced in specific modes (P_{i}). Phase shifters, where needed, are implemented by geometrical deformation of the connecting Sbends. Fanin and fanout sections at the input and output of the devices allows interfacing with 127 μm spaced singlemode fibre arrays. Note that in our device geometry, in each step, the vertices to be connected are all at the same relative distance. This means that, unless geometric deformations are designed where needed, light travelling in different modes does not acquire undesired phase delays. It is worth noting that the geometric construction here developed is scalable to an arbitrary number of modes with a number of elements increasing as mlog_{2}m.
One and twophoton measurements
The two implemented interferometers of m=4 and 8 modes are fed with single and twophoton states. The experimental setup, preparing a biphoton wave packet to be injected into the devices, is shown in Fig. 3. Further details on the photon generation and detection scheme are described in the Methods section. To test the validity of the suppression law, we measured the number of coincidences at each forbidden output combination injecting cyclic inputs with two indistinguishable photons. The degree of violation of the suppression law could simply be evaluated with a counting experiment. Alternatively, the same quantity can be expressed as a function of singlephoton input–output probabilities and of the HOM visibilities, defined as
where is the number of detected coincidences for distinguishable photons and for indistinguishable photons. The subscripts (i,j) are the indexes of the two output modes, for a given input state. The degree of violation can therefore be expressed as
where are the probabilities of having photons in the outputs i,j in the case of indistinguishable (distinguishable) particles. Here can be obtained from singleparticle probabilities. The visibilities are measured by recording the number of coincidences for each output combination as a function of the temporal delay between the two injected photons.
For the fourmode device, we measured the full set of collisionfree input–output combinations, that is, where the two photons exit from different output ports. These contributions have been measured by recording the number of coincidences for each combination of two outputs as a function of the temporal delay between the two input photons. Because of the law given by equation (2), we expect to observe four suppressed outcomes (over six possible output combinations) for the two cyclic input states (1,3) and (2,4). Since distinguishable photons exhibit no interference, HOM dips in the coincidence patterns are expected for the suppressed output states. Conversely, peaks are expected in the nonsuppressed output combinations. The experimental results are shown in Fig. 4a, where the expected pattern of four suppressions and two enhancements is reported, with average visibilities of and for suppression and enhancement, respectively.
For the cyclic inputs, we also measured the interference patterns for the output contributions where the two photons exit from the same mode. These terms have been measured by inserting an additional symmetric beam splitter on each output mode, and by connecting each of its two outputs to a singlephoton detector. These cases correspond to a fullbunching scenario with n=2, and a HOM peak with V=−1 visibility is expected independently from the input state and from the unitary operation^{45}. This feature has been observed for the tested inputs, where an average visibility of has been obtained over all fullbunching combinations. Note that the measured twomode correlation matrix is not compatible with classical light (see Supplementary Note 4).
The existence of a general rule for the prediction of suppressed output combinations when injecting a cyclic Fock state in a Fourier interferometer is due to the intrinsic symmetry of the problem, as opposed to the general boson sampling scenario^{13}. Suppressed outputs for noncyclic inputs can be predicted by calculating the permanent of the submatrix given by the intersection of the columns and rows of U^{F} corresponding to the occupied input and output modes, respectively. The complete set of measured dips and peaks is shown in Fig. 4a, highlighting the symmetry in the Fourier transform interference pattern. The injection of the noncyclic input states has been employed for the complete reconstruction of the chip action , using a data set statistically independent from the one adopted to observe the suppression law. The adopted reconstruction algorithm, which exploits knowledge on the internal structure of the interferometers (specified in Fig. 2), works in two steps. In a first step, the powersplitting ratios measured with classical light are employed to extrapolate the transmissivities of the directional couplers. In a second step, the twophoton visibilities for the noncyclic inputs are used to retrieve the values of the fabrication phases. In both steps the parameters are obtained by minimizing a suitable χ^{2} function. The results are shown in Fig. 4c. The fidelity between the reconstructed unitary and the theoretical Fourier transform is , thus confirming the high quality of the fabrication process. The error in the estimation of the fidelity is obtained through a Monte Carlo simulation, properly accounting for the degree of distinguishability of the photons with a rescaling factor in the visibilities.
For the eightmode chip we recorded all the twophoton coincidence patterns, as a function of the relative delay between the input photons, for each of the four collisionfree cyclic inputs and for one noncyclic input. The reconstruction of the actual unitary transformation implemented has been performed with the same algorithm of the fourmodes, by using the powersplitting ratios measured with classical light and the twophoton visibilities for one noncyclic input. The latter has been chosen in a way to maximize the sensitivity of the measurements with respect to the five fabrication phases. The results are shown in Fig. 5. The fidelity between the reconstructed unitary and the ideal eightmode Fourier transform is . More details on the reconstruction algorithm can be found in the Supplementary Note 3.
Observation of the suppression law
The suppression of events which do not satisfy equation (2) is fulfilled only when two perfectly indistinguishable photons are injected in a cyclic input of a perfect Fourier interferometer. In such a case, we would have the suppression of all output states whose sum of the indexes corresponding to the occupied modes is odd. For the fourmode (eightmode) interferometer, this corresponds to four (16) suppressed and two (12) nonsuppressed collisionfree outputs (each one given by two possible arrangements of the two distinguishable photons), plus four (8) terms with two photons in the same output, each one corresponding to a single possible twophoton path.
The expected violation for distinguishable particles can be obtained from classical considerations. Let us consider the case with n=2. The two distinguishable photons evolve independently from each other, and the output distribution is obtained by classically mixing singleparticle probabilities. All collisionfree terms are equally likely to occur with probability q=2/m^{2}, while fullbunching events occur with probability q′=q/2=1/m^{2}. The degree of violation can then be obtained by multiplying the probability q by the number of forbidden output combinations. As a result, we expect a violation degree of for distinguishable twophoton states. The evaluation of the expected value for a MF state, which is due to singleparticle bosonic statistic effects, requires different calculations^{27}. It can be shown that for n=2 the degree of violation is .
For each of the cyclic input, we have evaluated here the violation degree resulting from collected data. By measuring the coincidence pattern as a function of the path difference Δx=cΔτ between the two photons, and thus by tuning their degree of distinguishability, we could address the transition from distinguishable to indistinguishable particles. The value of as a function of Δx has been obtained as , where and are the number of measured coincidences for a given value of Δx and for distinguishable particles respectively. Two different regions can be identified. For intermediate values of Δx with respect to the coherence length of the photons, the measured data fall below the threshold , and hence the hypothesis of distinguishable particles can be ruled out. Then, for smaller values of the path difference up to Δx→0, true twophoton interference can be certified since both hypothesis of distinguishable particles and MF state can be ruled out. The maximum violation occurring at Δx=0 delay can be evaluated using equation (5). The experimental results retrieved from the protocol are shown in the tables of Fig. 6, in which we compare the values with the expected values for distinguishable particles and for a MF state . As shown for our implementation, the robustness of the protocol is ensured by the high number of s.d. separating the values in each comparison, thus unambiguously confirming the success of the certification protocol. In conclusion, the alternative hypotheses of distinguishable particles and of a MF state can be ruled out for all experiments.
Discussion
We have reported on the experimental observation of the suppression law on specific output combinations of a Fourier transformation due to quantum interference between photons. The observation of the suppression effect allowed us to rule out alternative hypotheses to the Fock state. The use of a novel implementation architecture, enabled by the 3D capabilities of femtosecond laser micromachining, extends the scalability of this technique to larger systems with lower experimental effort with respect to other techniques. While the presented architecture is designed to implement a Fourier matrix for a number of modes equal to m=2^{p}, a generalization of the approach can be obtained by adopting a building block different from the beam splitter. For devices of odd dimension, for instance, such a tool can be provided by the tritter transformation^{39}. At the same time, the universality of a generalized HOM effect with an arbitrary number of particles and modes is expected to make it a pivotal tool in the diagnostic and certification of quantum photonic platforms. Boson sampling represents a key example, since the scalability of the technique is expected to allow efficient certification of devices outperforming their classical counterparts. An interesting open problem is whether the computational hardness of boson sampling is maintained if the Haarrandomness condition is relaxed^{46}, and thus which is the minimal interferometer architecture required for an evidence of postclassical computation.
Fourier matrices can find application in different contexts. For instance, multiport beam splitters described by the Fourier matrix can be employed as building blocks for multiarm interferometers, which can be adopted for quantumenhanced single and multiphase estimation protocols^{47}. This would also allow the measurement of phase gradients with precision lower than the shotnoise limit^{48}. Other fields where Fourier matrices are relevant include quantum communication scenarios^{49}, observation of twophoton correlations as a function of geometric phase^{50}, fundamental quantum information theory including mutually unbiased bases^{51}, as well as entanglement generation^{52}.
Methods
Waveguide device fabrication
Waveguide interferometers are fabricated in EAGLE2000 (Corning Inc.) glass chips. To inscribe the waveguides, laser pulses with 300 fs duration, 220 nJ energy and 1 MHz repetition rate from an Yb:KYW cavity dumped oscillator (wavelength 1,030 nm) are focused in the bulk of the glass, using a 0.6 NA microscope objective. The average depth of the waveguides, in the 3D interferometric structure, is 170 μm under the sample surface. The fabricated waveguides yield singlemode behaviour at 800 nm wavelength, with about 0.5 dB cm^{−1} propagation losses. The central part of the 3D interferometer, which includes all the relevant couplers, have a crosssection of about 50 μm × 50 μm (95 μm × 95 μm) for a length of 9.0 mm (14.7 mm) in the four(eight)modes case. The length of each fanin and fanout section, needed to bring the waveguides at 127 μm relative distance, is 7.8 mm (13.2 mm).
Photon generation and manipulation
The generation of twophoton states is performed by pumping a 2mm long BBO crystal with a 392.5 nm wavelength Ti:Sa pulsed laser, with average power of 750 mW, which generates photons at 785 nm with a type II parametric downconversion process. The two photons are spectrally filtered by means of 3 nm interferential filters, and coupled into singlemode fibres. The indistinguishability of the photons is then ensured by a polarization compensation stage, and by propagation through independent delay lines (used to adjust the degree of temporal distinguishability) before injection within the interferometer via a singlemode fibre array. After the evolution through the integrated devices, photons are collected via a multimode fibre array. The detection system consists of four (8) singlephoton avalanche photodiodes used for the four (eight) modes chip. An electronic data acquisition system allowed us to detect coincidences between all pairs of output modes. Typical coincidence rates for each collisionfree output combination with distinguishable photons amounted to ∼70–80 Hz (for the fourmode chip) and ∼10–20 Hz (for the eightmode chip).
Additional information
How to cite this article: Crespi, A. et al. Suppression law of quantum states in a 3D photonic fast Fourier transform chip. Nat. Commun. 7:10469 doi: 10.1038/ncomms10469 (2016).
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Acknowledgements
We acknowledge technical support from Sandro Giacomini and Giorgio Milani. This work was supported by the ERCStarting Grant 3DQUEST (3DQuantum Integrated Optical Simulation; grant agreement no. 307783): http://www.3dquest.eu, by the PRIN project Advanced Quantum Simulation and Metrology (AQUASIM) and by the H2020FETPROACT2014 Grant QUCHIP (Quantum Simulation on a Photonic Chip; grant agreement no. 641039). F.S. had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
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A.C., R.O., M.B., F.F., N.S., N.V. and F.S. conceived the experimental approach for the observation of the suppression law. A.C. and R.O. developed the technique for threedimensional circuits, and fabricated and characterized the integrated devices using classical optics. M.B., N.V., F.F., N.S., and F.S. carried out the quantum experiments, F.F., L.I., N.S., M.B., N.V. and F.S. elaborated the data. All authors discussed the experimental implementation and results, and contributed to writing the paper.
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Crespi, A., Osellame, R., Ramponi, R. et al. Suppression law of quantum states in a 3D photonic fast Fourier transform chip. Nat Commun 7, 10469 (2016). https://doi.org/10.1038/ncomms10469
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DOI: https://doi.org/10.1038/ncomms10469
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