Comparison of multi-mode Hong-Ou-Mandel interference and multi-slit interference

Hong-Ou-Mandel (HOM) interference of multi-mode frequency entangled states plays a crucial role in quantum metrology. However, as the number of modes increases, the HOM interference pattern becomes increasingly complex, making it challenging to comprehend intuitively. To overcome this problem, we present the theory and simulation of multi-mode-HOM interference (MM-HOMI) and compare it to multi-slit interference (MSI). We find that these two interferences have a strong mapping relationship and are determined by two factors: the envelope factor and the details factor. The envelope factor is contributed by the single-mode HOM interference (single-slit diffraction) for MM-HOMI (MSI). The details factor is given by $\sin(Nx)/ \sin(x)$ ($[\sin(Nv)/\sin(v)]^2$) for MM-HOMI (MSI), where $N$ is the mode (slit) number and $x (v)$ is the phase spacing of two adjacent spectral modes (slits). As a potential application, we demonstrate that the square root of the maximal Fisher information in MM-HOMI increases linearly with the number of modes, indicating that MM-HOMI is a powerful tool for enhancing precision in time estimation. We also discuss multi-mode Mach-Zehnder interference, multi-mode NOON-state interference, and the extended Wiener-Khinchin theorem. This work may provide an intuitive understanding of MM-HOMI patterns and promote the application of MM-HOMI in quantum metrology.


Introduction
Since its discovery in 1987, the Hong-Ou-Mandel (HOM) interference using downconverted biphotons has shown a wide variety of applications in quantum optics [1][2][3][4][5].In traditional HOM interference, the biphotons are usually correlated in one discrete spectral mode.However, the biphotons involved can be correlated in multiple discrete spectral modes, and this two-body high-dimensional entangled state can be called entangled qudits [6][7][8][9].Here, we define the HOM interference using frequency entangled qudits as the multi-mode HOM interference (MM-HOMI).One important characteristic of MM-HOMI is that its interference patterns are significantly narrower than those in single-mode HOM interference.Such narrow interference fringes provide more Fisher information in phase estimation [10][11][12][13][14][15].As a result, MM-HOMI is very promising in quantum metrology.
Recently, many works have been devoted to the study of HOM interference using biphotons in multi-frequency modes.Lingaraju et al. investigated the effect of spectral phase coherence of multi-frequency modes in HOM interference [13].Chen et al. utilized HOM interference as a tool to characterize up to six-mode frequency entangled qudits [14,15].Morrison et al. prepared an eight-mode frequency entangled state in a customized poling crystal and tested its HOM interference patterns [16].In addition, HOM interference using biphoton frequency combs, which have a large number of discrete frequency modes, has also been widely investigated [17][18][19][20].
However, as the mode number increases, the MM-HOMI pattern becomes more and more complicated, which makes it challenging to understand intuitively.To address this issue, we first present the theory and simulation of MM-HOMI and then compare it with a well-known classical interference, the multi-slit interference (MSI) [21][22][23][24][25].We demonstrate that the MM-HOMI and the MSI exhibit a strong mapping relationship.For point , the tilt angle of the incident light is .The interference pattern is observed on the screen.

The theory and simulation of MM-HOMI
The typical setup for HOM interference is shown in Fig. 1 (a).The signal and idler photons generated from a spontaneous parametric downconversion (SPDC) process can be expressed as [26][27][28] where  (  ,   ) is the biphoton's joint spectral amplitude (JSA),  is the angular frequency, and â † is the creation operator.The subscripts  and  represent the signal and idler photons, respectively.In a HOM interference, the two-photon coincidence probability  () can be written as [29,30] where  1 and  2 are the frequencies detected by detectors D1 and D2 in Fig. 1 (a).For simplicity, in the above equation, we have assumed that  ( 1 ,  2 ) is normalized and satisfies the exchanging symmetry of  ( 1 ,  2 ) =  ( 2 ,  1 ).See the Appendix for more details.
In the MM-HOMI, the JSA can be written as: where  0 ( 1 ,  2 ) is an arbitrary distribution function of the single spectral mode,  is the mode number,  represents the mode spacing, and  0 is the mode's central frequency.As calculated in the Appendix,  () can be simplified as where  = 2, which corresponds to the phase spacing caused by two adjacent spectral modes.
corresponds to the envelope of the interference patterns.
We can observe in Eq. ( 4) that the MM-HOMI is determined by two factors: the envelope factor  0 and the details factor sin(   ) sin(  ) . 0 is contributed by the single-mode HOM interference (for N=1). 0 can also be expressed in the form of a Fourier transformation by projecting  0 2 ( 1 ,  2 ) on the axis of  1 −  2 , as shown in Eq. ( 23) in the Appendix.
For simplicity, we can set  ( 1 ,  2 ) to a multi-mode Gaussian distribution [16,31]: where  is the mode number,  represents the mode width.In a real experiment,  and  are determined by the width of the pump and the phase-matching function of the crystal [16].As calculated in the Appendix,  0 () can be simplified as According to Eq. ( 4), Eq. ( 5) and Eq. ( 6), we can plot the interference patterns of MM-HOMI.As shown in Fig. 2, the first column is the JSA of the biphotons, and the mode number is 1, 3, 5, 7, 2, 4, 6, and 8. Here, we choose the unit ellipticity for each mode, since this is the simplest case and has been realized in experiment [16].The second column is the corresponding MM-HOMI pattern.
Firstly, we compare the envelopes of the interference patterns.The odd-number MM-HOMI patterns have an asymmetric envelope, whereas the even-number MM-HOMI patterns are symmetric to the line of  () = 0.5.This is due to the characteristics of the function sin(   ) sin(  ) , which is asymmetric (symmetric) when N is an odd (even) number.
Secondly, we compare the primary and secondary valley (peak) numbers.For the odd-number MM-HOMI patterns, there are (N-3)/2 secondary valleys (for N>3) between two primary valleys, while for the even-number MM-HOMI patterns, there are N-2 secondary valleys (for N>2) between two primary valleys.

Comparison of MM-HOMI and MSI
So far, we have analyzed the properties of the MM-HOMI.However, we can notice that the interference patterns in Fig. 2 are very complicated.One might gain some insight by comparing MM-HOMI with a classical well-known multi-slit interference (MSI).It may be intuitive to understand that the multiple spectra function in a manner similar to the multiple slits.
Next, we deduce the mathematical form of MSI and compare it with MM-HOMI.The typical setup for MSI is shown in Fig. 1 (b).Here, the number of slits is , the slit width is , the interval of the slits is , and  +  = .As calculated in the Appendix, the amplitude of the diffraction pattern at point  is where  0 is a constant determined by the power of the light source, the distance between the slit and the screen, and the size of the slit [21]. =  sin   represents the phase difference in one slit. =  sin   represents the phase difference between two adjacent slits. is the wavelength of the input light and  is the tilt angle of the light, as shown in Fig. 1 (b).For simplicity, we can set  0 = 1.The intensity of the diffraction pattern is: where is the intensity due to one-slit diffraction, which is also the interference pattern's envelope.
According to Eq. ( 8), we can plot the interference patterns of MSI, as shown in the third and fourth columns in Fig. 2. The parameters are listed in detail in the caption of Fig. 2.There are N-2 secondary peaks (for N>2) between two primary peaks in the fourth column of Fig. 2.This is comparable to the even-number MM-HOMI patterns, which also have N-2 secondary valleys (for N>2) between two primary valleys.
Next, we investigate the influence of the mode size in MM-HOMI and MSI. Figure 3 (a1-a3, b1-b3) displays the JSA of the biphotons and the corresponding MM-HOMI.Here, we set  to be fixed at 5 rad•THz, and  to be 0.5 rad•THz, 2.5 rad•THz, and 4.5 rad•THz, respectively.It can be observed that with the increase of , the envelope becomes narrower, but the spacing of adjacent peaks (valleys) does not change.This phenomenon can be well explained by Eq. (4). Figure 3  (c1-c3, d1-d3) shows the 4-slit distributions and the corresponding MSI.It can be noticed that the envelope also becomes narrower with the increase of , which is similar to the phenomenon of MM-HOMI.Then, let us examine the impact of mode spacing on MM-HOMI and MSI. Figure 4 (a1-a3, b1-b3) depicts the JSA and MM-HOMI of biphotons with  fixed at 2 rad•THz and  increasing from 2.5 rad•THz to 5 rad•THz and 7.5 rad•THz.Figure 4 (c1-c3, d1-d3) shows the slit distributions and the corresponding MSI.By comparing (a1-b3) with (c1-d3), we can observe that the envelope remains constant, while the number of peaks and valleys increases as mode spacing increases.These phenomena can also be well explained by Eq. ( 4) and Eq. ( 8).
After analyzing Figs.(4,5,6) and Eqs.(4, 8), we can confirm that the MM-HOMI and MSI have a strong mapping relationship, as summarized in Tab. 1.The phase variable , which accumulates in the time domain, represents the phase spacing between two spectra modes in the MM-HOMI; while the phase variable , which accumulates in the space domain, represents the phase spacing between two slits in the MSI.Both MM-HOMI and MSI are determined by two factors: the envelope factor and the details factor.The details factor includes a common term of sin(   ) sin(  ) with a mode number of .In MM-HOMI, the envelope factor  0 corresponds to the HOM interference of a single spectral mode, and  0 is also related to the Fourier transform of a single spectral mode, as shown in Eq. (23).Similarly, the details factor  0 corresponds to the single-slit diffraction, and it is also contributed by the Fourier transform of a single slit, as explained in Eq. ( 28) in the Appendix.Therefore, we can conclude that the multiple spectra indeed function similarly to the multiple slits, as we expected at the beginning of this section.Consequently, the mapping relationship really can help on the intuitive understanding of the MM-HOMI.

Application of MM-HOMI in quantum meteorology
MSI has numerous applications in optical measurement, with one typical example being the diffraction-grating-based spectrometer.The resolving power of a diffraction grating is proportional to the total number of slits (or grooves) on the grating [21][22][23][24][25]. Inspired by this feature, here we consider the resolving power of a MM-HOMI in quantum metrology by increasing the total number of the spectral modes.The ultimate limit on the precision of time estimation is the Cramér-Rao bound [12,32] , which states that the variance of any unbiased estimator is bounded by where t is the estimator of time , and  is the number of measurement times and   is the Fisher information (FI).For a single measurement, =1.So, the standard deviation (SD) is bound by FI of a single interference fringe can be calculated as [10,33]: By using Eq.( 4) and Eq.( 11), we can obtain the Fisher information of MM-HOMI as Figure 5(a1-a8) displays the simulated FI of the  () in Fig. 2(b1-b8), with the mode number N increasing from 1 to 8. The single valley in Fig. 2(b8) is transferred to a double peak in Fig. 5(a8).Figure 6 summarizes the square root of maximal FI as a function of the mode number N ranging from 1 to 40.It is evident that the square root of maximal FI increases linearly with the increase in mode number.This suggests that increasing the mode number is a powerful method to improve precision in time or phase estimation.

Discussion
From the viewpoint of the extended Wiener-Khinchin theorem (e-WKT) [30], the Fourier transform of the HOM interference pattern is determined by the difference-frequency distribution of the JSI, i.e., the projection of  ( 1 ,  2 ) onto the  1 −  2 axis.The e-WKT is not only applicable to the single-mode case but also explains the multi-mode case in this study.Specifically, the interference patterns of MM-HOMI are determined by the Fourier transform of the difference-frequency distribution of the multi-mode JSI.
To gain a deeper understanding of the multi-mode effect, we also compared the MM-HOMI and MSI with two other important interferences in quantum optics: the multi-mode Mach-Zehnder interference (MM-MZI) and the multi-mode NOON state interference (MM-NOONSI), using the setups shown in Fig. 7 (a, b).Here, the NOON-state is a (|20⟩ + |02⟩)/ √ 2 state, which has a photon number of 2, but with a spectral-mode number of N, as shown in Fig. 7(e1-e8).As calculated in the Appendix, the single count in a MM-MZI can be expressed as and the coincidence counts in a MM-NOONSI can be expressed as In the above two models, Gaussian envelopes were chosen for simplicity.Refer to the Appendix for deductions using arbitrary envelopes.The parameters are listed in detail in the caption of Fig. 7.By comparing the theoretical simulations in Fig. 7 (d1-d8) and (f1-f8), we observe that there are N-2 secondary peaks (for N > 3) between the two main peaks in N-mode Mach-Zehnder interference and N-mode NOON-state interference.By comparing Eq. ( 13), Eq. ( 14), Eq. ( 4), and Eq. ( 8), we observe that the MM-MZI and MM-NOONSI are also contributed by the details factor of sin () /sin (), but multiplied by a factor of cos ( 0 ) or cos (2 0 ).In general, the connection between the MM-HOMI, the MSI, the MM-MZI, and the MM-NOONSI is that all these interferences are contributed by N input modes in physics and determined by the factor of sin () /sin () in mathematics.

Conclusion
In conclusion, we have presented the theory and simulation of MM-HOMI and compared them with MSI.We confirm that both interferences are determined by two factors: the envelope factor and the details factor.For MM-HOMI (MSI), the envelope factor is determined by each mode (slit), while the details factor is determined by the N modes (slits).The mapping relationship between MM-HOMI and MSI may provide an intuitive explanation of MM-HOMI.As an example of its application, we demonstrate that the square root of maximal Fisher information in a MM-HOMI increases linearly with the increase of mode numbers, indicating that increasing the mode number is a potent method for enhancing precision in quantum metrology.

Fig. 1 .
Fig. 1.(a) The typical setup of multi-mode Hong-Ou-Mandel interference (MM-HOMI).The signal (s) and idler (i) photons from an SPDC source impinge on a beam splitter (BS) before the signal is delayed by a time .The output photons from the BS are detected by two single-photon detectors (D1 and D2), which are connected to a coincidence counter (&).For an MM-HOMI, the biphotons have a multi-mode spectral distribution, as shown in the inset in the bottom left corner.(b) The typical setup for a multi-slit interference.The width of a single slit is , and the width of a block is ;  +  = .For point , the tilt angle of the incident light is .The interference pattern is observed on the screen.

Table 1 .
The mapping relationship between MM-HOMI and MSI.