Single-Port Homodyne Detection in a Squeezed-State Interferometry with Optimal Data Processing

: Performing homodyne detection at a single output port of a squeezed-state light interferometer and then separating the measurement quadrature into several bins can realize superresolving and supersensitive phase measurements. However, the phase resolution and the achievable phase sensitivity depend on the bin size that is adopted in the data processing. By maximizing classical Fisher information, we analytically derive an optimal value of the bin size and the associated best sensitivity for the case of three bins, which can be regarded as a three-outcome measurement. Our results indicate that both the resolution and the achievable sensitivity are better than that of the previous binary–outcome case. Finally, we present an approximate maximum Likelihood estimator to asymptotically saturate the ultimate lower bound of the phase sensitivity.


Introduction
High-precision phase measurement is of importance for multiple areas of scientific research, such as gravitational wave detection [1,2], biological sensing [3,4], atomic clocks [5,6], and magnetometry [7]. For instance, the intensity measurements at the output ports of a coherent-state light interferometer show the interferometric signal ∝ sin 2 (θ/2) or cos 2 (θ/2), where θ is a dimensionless phase shift. Obviously, the signal exhibits the full width at half maximum (FWHM) = π, corresponding to the fringe resolution ∼ λ/2, where λ is the wavelength of the incident light. This is known as the so-called Rayleigh resolution limit [8]. On the other hand, the achievable phase sensitivity is subject to the shot-noise limit (SNL) δθ SNL = 1/ √n , wheren is the number of particles of the input state. As proposed originally by Caves [9], the best sensitivity of the interferometer can beat the SNL by mixing a small amount of squeezed vacuum with the coherent-state light at the input ports of the interferometer (named hereinafter, squeezed-state interferometer). Performing homodyne detections at one output port of the interferometer and following with a suitable data processing, Schäfermeier et al. [10] recently demonstrated that the two classical limits in the resolution and in the sensitivity can be surpassed simultaneously. The data processing method adopted in Ref. [10] is to divide the measurement outcomes into two discrete bins and then construct an inversion estimator associated with one discrete outcome [11]. The inversion estimator is widely used in experiments since its uncertainty simply follows the error-propagation formula [12]. However, the inversion estimator is usually sub-optimal and cannot saturate the ultimate phase estimation precision that determined by the Cramér-Rao lower bound (CRB) [13][14][15][16][17][18][19]: where F (θ) is the classical Fisher information (CFI) determined by the measurement outcome probability distribution P k (θ): Here, the subscript k and f k (θ) represent the k-th measurement outcome and its contribution to the CFI. The data processing method adopted in Ref. [10] can be regarded as a binaryoutcome measurement with k = 0 and ∅, corresponding to the measurement quadrature p ∈ [−a, a] and p / ∈ [−a, a], with the bin size 2a. As a trade-off parameter, the different choice of a is of importance to control the phase resolution and the achievable phase sensitivity [11].
In this paper, we focus on optimal value of the bin size and present the best sensitivity of the three-outcome homodyne detections. By maximizing the CFI of the three-outcome measurement, we derive analytic expressions of the bin size and the best sensitivity. Our analytic results are useful to predict the Heisenberg scaling of the sensitivity δθ CRB ∼ O(1/n). We perform numerical simulations of the three-outcome homodyne detections, using N random numbers at each given phase shift. To saturate the CRB, we adopt the approximate maximum Likelihood estimator (MLE) that developed recently by Ref. [20], which holds for any kind multi-outcome detections with discrete measurement outcomes. Our results show clearly a physical meaning of the MLE and the reason why it can saturate the CRB as N 1.

Homodyne Detection in the Squeezed-State Interferometer with Dataprocessing
As illustrated by Figure 1a, let us consider homodyne detection at one output port of a squeezed-state light interferometer, which is equivalent to measuring the field quadraturê p = (â −â † )/(2i), wherep|p = p|p and p ∈ (−∞, ∞). To improve the phase sensitivity, we choose a coherent state and a squeezed vacuum state with real field amplitudes [9,[21][22][23], corresponding to the input state |ψ in = |α 0 ⊗ |ξ 0 , with α 0 = √n a ∈ R and ξ 0 = −r ∈ R. The average number of photons of the input state is given byn =n a +n b , withn a = α 2 0 andn b = sinh 2 r. The Wigner function of the input state is given by [24][25][26][27] with the parametersμ andν dependent on the input state, and the variables α = x a + ip a and β = x b + ip b . Here, the subscripts "a" and "b" denote two different paths (or the field modes). Following Schäfermeier et al. [10], we consider the squeezed vacuum with the purity < 1 and takeμ = e −2r andν = e 2r , where e −r describes the squeeze factor of |ξ 0 . If other Gaussian state |g is injected from the port b, thenμ andν can take different expressions (e.g.,μ =ν = 1 for the vacuum |0 ). Next, we consider the interferometer that is described by the unitary operator: where exp(−iπĴ y /2) and exp(−iθâ †â ) represent the actions of the 50:50 beam splitter and that of the phase accumulation in the path, withĴ y = (â †b −b †â )/2i andâ,b being bosonic annihilation operators. According to Refs. [24][25][26][27], the Wigner function of the output state |ψ out =Û(θ)|ψ in takes the same form with that of the input state, i.e., Integrating W out (α, β) over {x a , x b , p b }, one can obtain the conditional probability for detecting a measurement outcome [28]: where the subscript of p a has been neglected, and we introduce Note that Equations (3)-(7) hold for the output stateÛ(θ)|α 0 ⊗ |g , where |α 0 denotes a coherent state and |g is an arbitrary Gaussian state, as mentioned above.
In Figure 1b, we show a 3D plot of P(p|θ) against the phase shift θ and the measurement quadrature p. One can find that the conditional probability is nonzero within a region p ∈ (−α 0 /2, α 0 /2) and P(p|θ) = 1 at θ = 0. With the single-port homodyne detection, one can easily obtain the output signal which exhibits the FWHM = 2π/3, and hence the Rayleigh limit in fringe resolution [10,11].
On the other hand, the achievable phase sensitivity is determined by the CFI and hence the CRB 1/ F(θ) (see, e.g., Ref. [28]), where P ≡ ∂P/∂θ. When the coherentstate component dominates over the squeezed vacuum (i.e.,n a n b ), the best sensitivity occurs at the optimal working point θ min = 0, with δθ min ≡ 1/ F(θ min ) ≈ e −r / √n [28], in agreement with the light intensity-difference measurement asn a n b [9,21]. The phase resolution can be improved by a suitable data processing over the measurement outcomes, at the cost of reduced phase sensitivity. Recently, Distante et al. [11] have demonstrated super-resolving phase measurement in a coherent-state light interferometer with the sensitivity close to the SNL. The data-processing method they adopted is to separate the measurement quadrature p ∈ (−∞, ∞) into two bins: where a is to be determined. With such a kind of binary-outcome measurement, Schäfermeier et al. [10] have demonstrated super-sensitive and super-resolving phase measurement in the squeezed-state interferometer. Specifically, a 22-fold improvement in the resolution and a 1.7-fold improvement in the sensitivity can be obtained using realistic experimental parameters [10]. A better phase resolution and an enhanced sensitivity can be obtained by dividing the measurement data into several bins, with an optimal choice of the bin size a.
Using the experimental setup similar to Schäfermeier et al. [10], one can separate the measurement data into three bins: (−∞, −a), [−a, a], and (a, ∞), denoted hereinafter by the outcomes "−", "0", and "+", respectively. Integrating P(p|θ) in Equation (6), one can obtain the occurrence probabilities of each outcome: where Erf[x, y] ≡ erf[y] − erf[x] denotes a generalized error function, and with η θ being defined by Equation (7). Note that the above data-processing method is equivalent to a three-outcome measurement with the observableΠ = ∑ k µ kΠk , where µ k are the eigenvalues associated with the outcomes k = −, 0, and +. Furthermore, the projection operators are given byΠ Using the relation P k (θ) = ψ out |Π k |ψ out , one can easily obtain the averaged output signal where N k denotes the occurrence number of the kth outcome for a single run N independent measurements. Actually, the probability of each outcome can be measured by the occurrence frequency N k /N . Performing N repeated measurements at each given θ ∈ (−π, π), one can obtain the occurrence numbers of all the outcomes {N k }, which can be regarded as a single run. After multiple runs, one can obtain the phase-dependent probabilities from the statistical average of N k /N , and thereby the averaged signal g(θ) ≈ Π (θ) . This can be done by the calibration of the interferometer (see below).
To estimate an unknown phase shift, the inversion estimator is widely used in experiments due to its simplicity, i.e., θ inv = g −1 (∑ k µ k N k /N ), where g −1 denotes the inverse function of the signal. According to the central limit theorem, the phase uncertainty of θ inv follows error-propagation formula [12]: where ∆Π = Π2 − Π 2 denotes the root-mean-square fluctuation of the signal.
The value of a is a trade-off parameter that controls the phase resolution and the achievable sensitivity [10]. In Figure 1c,d, we take µ ± = 0 and µ 0 = 1/erf( √ 2νa) and show the signal (left panel) and the sensitivity (right panel) for different values of a. The resolution is determined by the FWHM of the signal µ 0 P 0 (θ) and the best sensitivity is given by δθ bin,min = 1/ F bin (θ min ). Using the experimental parameters similar to Ref. [10], one can find that the solid lines for a = 0.18 shows a better phase resolution than that of the previous result (a = 0.5, as adopted by Schäfermeier et al. [10]), but with the cost of a reduced phase sensitivity. Furthermore, the phase sensitivity δθ bin diverges at θ = 0, due to P 0 (0) = 0 and hence F bin (0) = 0. This means that, with the binaryoutcome measurement, little phase information can be extracted for the phase shift θ ∼ 0. To avoid such a divergence, we adopt the CRB of the three-outcome measurement as the performance of the sensitivity, i.e., δθ CRB = 1/ F (θ), where the CFI F (θ) has been defined by Equation (2), namely From Equation (10), one can see P ± (0) = 0 and hence F (0) = 0, so the phase information is detectable for θ ∼ 0. As depicted by the red dotted line of Figure 1d, one can see that the best sensitivity occurs at θ = 0 and is better than that of the binary-outcome case.
For the three-outcome measurement, we numerically find that the optimal working point usually occurs at θ min = 0 as long as the coherent-state component dominates (i.e.,n a n b ). In this case, an analytic expression of the best sensitivity δθ CRB,min = 1/ F (θ min ) can be obtained by substituting Equation (10) into Equation (17) and calculating maximum of the CFI at θ = 0, i.e., whereν = e 2r , as defined in Equation (3). If we expand the error function erf(x) up to O(x 3 ), then we further obtain Note that F max can be treated as a function of a and its optimal value satisfies the following equation: which gives This is a quartic equation about a. Discarding three invalid roots, we obtain the optimal value of a, Substituting Equation (22) into Equation (19), we further obtain the best sensitivity When the coherent-state component dominates, i.e., the total photon number of the input staten ≈n a , the best sensitivity can be approximated to 1.11e −r / √ n, which is slightly worse than that of the homodyne detection without any data processing. Note that the above analytic results hold when the best sensitivity occurs at θ = 0. However, this condition may fail ifn b is comparable withn a , for which θ min = 0. Numerically, we obtain the best sensitivity δθ CRB,min againstn a andn b for an optimal value of a ∈ (0, α 0 /2). In Figure 2a,b, we show density plot of the ratios 2π/3 FWHM and 1/ √n δθ CRB,min as functions of the average photon numbern and the bin size a, where δθ CRB,min = 1/ F (θ min ). Using the parameters similar to Schäfermeier et al. [10], one can find that the resolution is optimal as a → 0, while the best sensitivity appears at a ∼ 0.183, as predicted by Equation (22). Next, in Figure 2c,d, we show the validity of Equations (22) and (23)  (the red dotted lines), as long asn b /n 0.55 (i.e.,n b 1.2n a ). For the case < 1, our analytic result can predict the best sensitivity, providedn b /n 0.55 √ (e.g., n b 0.7n a for = 0.58). Whenn b n a 1, our analytic result δθ CRB,ana shows a slight discrepancy with numerical result of the best sensitivity δθ CRB,min . However, it is still useful to predict the Heisenberg scaling of the sensitivity δθ CRB,ana ∼ O(1/n). One can see this by considering the limitn b 1, for which exp(2r) ≈ 4n b (i.e., e −r ≈ 1/ √ 4n b ). If we setn b /n = γ (i.e., n b = γn), then Equation (23) Similar to Ref. [21], let us consider the casen a =n b =n/2 (i.e., γ = 1/2), which yields the sensitivity δθ CRB,ana ≈ 1.11/(n √ ), coincident with the numerical result (see below). To confirm the above results, we show the signal and the sensitivity in Figure 3. In the left panel, we consider the squeezed vacuum withn b n a and the purity = 0.58. One can see that both the resolution and the achievable sensitivity are better than that of the previous work [10], provided that an optimal value of a predicted by Equation (22) is adopted. In the right panel, we consider the casen b =n a =n/2 and compare our results with that of Schäfermeier et al. [10] using the same value of a. From Figure 3b, one can find that the signal and its resolution are the same with the binary-outcome case since we have taken the eigenvalues µ ± = 0 and µ 0 = 1/erf( √ 2νa), similar to Schäfermeier et al. [10]. Remarkably, the solid line of Figure 3d indicates that the best sensitivity can reach the Heisenberg scaling 1.11/(n √ ), as predicted by Equation (24). All for the purity = 0.58.

Approximate Maximum Likelihood Estimation
Finally, there remains a question how to saturate the CRB of the three-outcome measurement. Usually, the estimator by inverting the averaged signal cannot saturate the CRB, except for the case µ − = +1, µ + = −1, and µ 0 = 0 (not shown here). It is therefore important to find out an optimal phase-estimation protocol, which can saturate the CRB. According to Ref. [39], the phase-estimation protocol consists of two steps. The first one is the calibration of the interferometer to obtain {P k (θ)} by measuring the occurrence frequency {N k /N } at each given value of phase shift θ ∈ (−π, π). Using µ ± = 0 and µ 0 = 1/erf( 2 ae r ) in Equation (12), one can obtain the signal within a single run of the calibration. After multiple runs, one can obtain statistical average of the signal and its standard deviation, as depicted by the circles and the bars in Figure 3a,b.
After the calibration, we can perform maximum Likelihood estimation, since the estimator adopted (i.e., the MLE) is well known asymptotically optimal (see, e.g., Ref. [13]). As shown by the red dotted lines of Figure 4a-c, the MLE can be determined by the peak of the likelihood function (which is simply a multinomial distribution): where N k = N k (θ 0 ) denotes the occurrence number of each outcome at a given true value of phase shift θ 0 . When N k ∼ O(N ) and N = ∑ k N k 1, the phase distribution can be approximated as a Gaussian (see the solid lines of Figure 4a-c, and also Ref. [20]) where we have introduced and with the inversion estimator associated with the k-th outcome θ inv,k = P −1 k (N k /N ) and the CFI f k (θ) = [P k (θ)] 2 /P k (θ). The estimator θ est shows clear physical meaning as a linear combination of all the inversion estimators, weighted by the CFI of each outcome. When the CFI of the outcome k = 0 dominates over that of the others, then Equation (27) simply reduces to θ est ≈ θ inv,0 . Furthermore, σ est is a 68.3% confidence interval of the Gaussian around θ est , similar to the MLE [21,38]. To obtain θ est , it requires to measure all the probabilities {P k (θ)} and hence { f k (θ)}, as well as the inversion estimators {θ inv,k }. For each outcome, one has to solve a unique root of the equation P k (θ) = N k /N around θ 0 . This requires some prior information about the true value of phase shift. When the prior knowledge is not sufficient, one can use multi-step estimation protocol [40][41][42], or simply a two-step protocol [43]. As shown by Figure 4d, one can see the inversion estimators of all the outcomes θ inv,k ≈ θ 0 and hence F (θ 0 ) ≈ ∑ k f k (θ inv,k ), which in turn results in σ est ≈ 1/ N F (θ 0 ). This means that the estimator θ est can saturate the CRB for a multi-outcome detection with large enough N . Indeed, Equations (25)- (28) are valid for arbitrary kinds of multi-outcome detection [20]. In Figure 3c,d, we numerically show this result for the three-outcome homodyne detection, using M replicas of N (= 10 3 ) random numbers at each given θ 0 . One can see that statistical averages of {σ

Discussion and Conclusions
From Figure 1c, one can see that the signal shows a better resolution than that of the binary-outcome measurement scheme [10], provided that a relatively small value of a is adopted. However, it results in a reduced sensitivity; see the solid line of Figure 1d. Such a problem can be bypassed using the CRB of a three-outcome measurement; see the red dotted line of Figure 1d. By maximizing the CFI, we derived analytic results of the best sensitivity and the associated optimal value of a. Our analytic results start from the conditionn b n a . Numerically, we show that this condition can be relaxed tō n b 0.7n a for the purity = 0.58. For the casen b n a 1, our analytic results may also be of importance to predict the Heisenberg scaling of the sensitivity; see Equation (24) and also Figure 3d. Finally, we present an optimal phase estimator by approximating the multinomial phase distribution of a multi-outcome measurement by a Gaussian and show the underlying physics of why the MLE can saturate the CRB.
In summary, we have investigated high-precision homodyne measurement and data processing at a single output port of the interferometer fed by a coherent state and a squeezed vacuum of light, where the measurement quadrature is divided into three bins. As a three-outcome measurement, this kind of data processing can further improve the phase resolution and the phase sensitivity beyond the binary-outcome case. By maximizing the Fisher information, we obtain the optimal value of the bin size and the best sensitivity, i.e., Equations (22) and (23). Our analytical results show good agreement with the exact numerical results and are useful to predict the Heisenberg scaling of the sensitivity. Finally, we show an approximate maximum-likelihood estimation with respect to the three-outcome homodyne detections. Our numerical simulations indicate that the phase estimator can saturate the Cramér-Rao lower bound of the sensitivity and hence is asymptotically optimal.