Abstract
Twomode squeezing is a fascinating example of quantum entanglement manifested in crosscorrelations of noncommuting observables between two subsystems. At the same time, these subsystems themselves may contain no quantum signatures in their selfcorrelations. These properties make twomode squeezed (TMS) states an ideal resource for applications in quantum communication. Here, we generate propagating microwave TMS states by a beam splitter distributing single mode squeezing emitted from distinct Josephson parametric amplifiers along two output paths. We experimentally study the fundamental dephasing process of quantum crosscorrelations in continuousvariable propagating TMS microwave states and accurately describe it with a theory model. In this way, we gain the insight into finitetime entanglement limits and predict high fidelities for benchmark quantum communication protocols such as remote state preparation and quantum teleportation.
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Introduction
Propagating quantum microwave signals in the form of squeezed states are natural candidates for quantum communication^{1,2,3} and quantum information processing^{4,5,6,7} with continuous variables. This assessment stems from the fact that they belong to the same frequency range and are generated using the same material technology as quantum information processing platforms based on superconducting circuits^{8,9,10,11}. Utilizing propagating quantum microwaves, one can potentially realize quantum illumination protocols^{12,13,14,15,16}, hybrid computation schemes with continuous variables^{6,17,18} and a highfidelity seamless connection between distant superconducting quantum computers^{19}. In twomode squeezed microwave states, entanglement is expressed in strong correlations between two nonlocal field quadratures^{3,5}. The physical limits of this entanglement are of broad interest regarding both quantum information theory and applications. In this context, finitetime correlation properties of propagating squeezed states provide necessary information about the tolerance to unwanted delays, and therefore, determine whether delay lines are required. From the fundamental point of view, finitetime correlation measurements grant a quantitative physical insight into the dephasing processes of propagating entangled microwave signals.
In this Article, we experimentally show how the entanglement in the twomode squeezed state decays depending on a time delay τ in one of the propagation paths. This entanglement decrease can be attributed to a dephasing process between the two modes propagating along each path. According to our experimental results and the corresponding theoretical model, the dephasing time, characterising the entanglement decay, is inversely proportional to the squeezing level and the measurement filter bandwidth. In contrast to earlier works^{20,21}, we perform direct state tomography of highly symmetric frequencydegenerate microwave TMS states with a record entanglement strength quantified by the negativity of \({\mathscr{N}}\simeq 3.9\).
We use two superconducting fluxdriven Josephson parametric amplifiers (JPAs) operated at f_{0} = 5.323 GHz for the generation of squeezed microwave states (see supplementary information for details). The task of each JPA is to perform a squeezing operation on the incident vacuum state \(\hat{S}(\xi \mathrm{)0}\rangle \), where \(\hat{S}(\xi )=\exp (\frac{1}{2}\xi \ast {\hat{a}}^{2}\frac{1}{2}\xi {({\hat{a}}^{\dagger })}^{2})\) is the squeezing operator, \({\hat{a}}^{\dagger }(\tau )\) and \(\hat{a}(\tau )\) are the creation and annihilation operators of the f_{0} mode, and ξ = re^{iϕ} is the complex squeezing amplitude. Here, the phase ϕ determines the squeezing angle in phase space, while the squeezing factor r parameterizes the amount of squeezing. We conveniently characterize the degree of squeezing of the quantum state in decibels as \(S=\,10\,\,{\mathrm{log}}_{10}[{\sigma }_{{\rm{s}}}^{2}\mathrm{/0.25]}\), where \({\sigma }_{{\rm{s}}}^{2}\) is the variance of the squeezed quadrature and the vacuum variance is 0.25. Positive values of S indicate squeezing below the vacuum level. In experiments, the JPAs are nonideal, meaning that they effectively squeeze a thermal state with a mean noise photon number n. In this scenario, the squeezed and antisqueezed quadratures can be expressed as \({\sigma }_{{\rm{s}}}^{2}=\mathrm{0.25(1}+2n)\,\exp \,(\,\,2r)\) and \({\sigma }_{{\rm{a}}}^{2}=\mathrm{0.25(1}+2n)\,\exp \,\mathrm{(2}r)\), respectively (see supplementary information for details). This expressions allow us to rewrite the squeezing level as \(S=\,10\,{\mathrm{log}}_{10}\mathrm{[(1}+2n)\,\exp \,(\,\,2r)]\). Although n is typically small, it must be accounted for in a quantitative analysis of actual data.
Finitetime correlations of a propagating singlemode squeezed state can be captured using the normalized second order correlation function \({g}^{\mathrm{(2)}}(\tau )=\langle {\hat{a}}^{\dagger }\mathrm{(0)}{\hat{a}}^{\dagger }(\tau )\hat{a}(\tau )\hat{a}\mathrm{(0)}\rangle /{\langle {\hat{a}}^{\dagger }\mathrm{(0)}\hat{a}\mathrm{(0)}\rangle }^{2}\). It describes the decay of the correlations in propagating light and relates an autocorrelation time to this process. To measure g^{(2)}(τ) of propagating squeezed microwaves, we employ the experimental setup shown in Fig. 1a. In order to reconstruct the squeezed vacuum states, we apply a variant of the dualpath reconstruction scheme^{22,23,24}. To this end, the input signal is first distributed over two paths using a hybrid ring beam splitter. After amplification, auto and crosscorrelation measurements are performed on the output paths. In this way, we can correct for the amplifier noise and retrieve all moments of the signal mode incident at the beam splitter up to fourth order in amplitude. An important modification in our setup is the introduction of a digital timedelay τ in one of the detection paths (see Fig. 1). This modification allows us to extend the dualpath technique to measure finitetime correlations g^{(2)}(τ). For further technical details, we refer the reader to the supplemental material.
Figure 2 shows the experimental results. As expected from theory^{25}, the superPoissonian character of squeezed microwave states is demonstrated by g^{(2)}(0) > 1. We observe a smooth decay of g^{(2)}(τ) to the coherentstate limit g^{(2)}(τ) = 1 on a timescale of \(\tau \gtrsim 1\,\mu \)s. In order to describe our results, we use an extended variant of the inputoutput theory presented in ref.^{26}. It yields
where Ω is a width of a measurement bandpass filter centered at f_{0}. By fitting experimental data with Eq. (1), as depicted in Fig. 2a, we extract the experimental filter bandwidth Ω ≃ 420 kHz. This value agrees very well with the bandwidth of the digital finiteimpulse response (FIR) filter used in our measurement protocol, Ω_{FIR} = 430 kHz. As can be seen from Eq. (1), the fitted squeezing factor r and noise photon number n define the value of g^{(2)}(0), but neither one influences the temporal shape of g^{(2)}(τ). A noticeable scatter in g^{(2)}(τ) for low squeezing levels (4–5 dB) is caused by low respective average photon numbers \(\langle {\hat{a}}^{\dagger }\mathrm{(0)}\hat{a}\mathrm{(0)}\rangle \). The latter lead to a decreased experimental signaltonoise ratio and an increased uncertainty of g^{(2)}(τ).
So far, we have investigated the correlations of singlemode squeezed states. Now, in order to evaluate the suitability of our setup for actual quantum microwave communication protocols, we go one step further and study the finitetime behaviour of quantum entanglement between the outputs of the hybrid ring. In general, the amount of quantum entanglement can be assessed using the negativity \({\mathscr{N}}\). It is defined as \({\mathscr{N}}\equiv \,{\rm{\max }}\,\{0,{N}_{k}\}\), where the negativity kernel N_{ k } is a function of the density matrix of the investigated state. In the relevant case of Gaussian states, N_{ k } is expressed in terms of the covariance matrix^{27}, and \({\mathscr{N}}\) constitutes an entanglement measure. Then, condition N_{ k } > 0 implies the presence of entanglement. By varying τ in our experiment, we measure the τdependent negativity kernel N_{ k }(τ) which provides information about the temporal length of the propagating entangled signal. Here, we introduce a maximally acceptable delay τ_{ d } which still allows for existence of entanglement in a TMS state. This condition is given by N_{ k }(τ_{ d }) = 0 for a monotonically decreasing N_{ k }(τ). Assuming all other system properties to be equal, a large τ_{ d } is beneficial in quantum communication. The expression for the negativity kernel N_{ k }(τ) for an arbitrary twomode squeezed state produced by two independent JPAs is given by (see supplementary information for details).
where \(\tilde{n}\equiv \mathrm{(1}+2{n}_{1})(1+2{n}_{2})\), \(C\equiv {\cosh }^{2}({r}_{1}+{r}_{2})\), D ≡ sinh (2r_{1} + 2r_{2}); r_{1} (r_{2}) and n_{1} (n_{2}) are the squeezing factor and number of noise photons of JPA1 (JPA2), respectively. For the scenario depicted in Fig. 1a, where JPA2 is off, we consider r_{2} = n_{2} = 0 and accurately fit the experimental data with Eq. (2) as it can be seen in Fig. 2b. The negativity kernel N_{ k } depends on the squeezing level S and the corresponding values of r since the delay τ_{ d } clearly decreases with increasing S. This is in strong contrast with the behavior of g^{(2)}(τ) where the temporal shape only provides information on Ω. The increasing noise photon numbers n for higher squeezing levels are caused by larger microwave pump powers required to reach these values of S. As a result, stronger microwave fields lead to a stronger coupling to environmental loss channels, a process that increases n.
Finally, we consider the case of propagating symmetric TMS states, which is most relevant for our envisioned applications. To this end, we operate both JPAs with orthogonal squeezing angles as shown in Fig. 1b. The resulting TMS states are essential for quantum microwave communication as they allow for entanglement distribution between separate distant parties. First, we are interested in the full tomography of the pathentangled twomode states. We achieve this goal by using the reference state method^{17,22,28,29} and zero time delay τ = 0. The result is depicted in Fig. 3. We tune the squeezing levels and angles of both JPAs by adjusting the amplitude and phase of the microwave pump tone to obtain symmetric TMS states. In this way, we eliminate residual singlemode squeezing in the marginal distributions in the selfcorrelated subspaces {p_{1}, q_{1}} and {p_{2}, q_{2}} of the resulting Wigner function (see Fig. 3a). We achieve close to perfect local thermal states in both paths. The strong twomode squeezing between them is clearly observed in the crosscorrelation subspaces {q_{2}, p_{1}} and {p_{2}, q_{1}} in Fig. 3b. This result is very different from the case shown in Fig. 1a, where, in addition to the twomode squeezing, also a residual local singlemode squeezing is generated at the hybrid ring outputs. The latter is unwanted in certain communication protocols since it partially reveals the encoded information. To describe the entanglement for this case, we use Eq. (2) in the approximation of identical squeezing factors r_{1} = r_{2} = r (accordingly, S_{1} = S_{2} = S) and noise photon numbers n_{1} = n_{2} = n in both JPAs. Figure 4 describes the entanglement dephasing in the propagating TMS states with respect to the finitetime delay τ of the second path for various input squeezing levels and different filter bandwidths. The fit to the experimental data has been implemented using the squeezing factor and number of noise photons as free fitting parameters while setting the filter bandwidth Ω to the value predefined by FIR characteristics (430 kHz or 770 kHz). Figure 4 shows an excellent agreement between theory and experiment. Moreover, the entanglement dephasing time τ_{ d } strongly decreases with the increasing squeezing levels S, and consequently, the squeezing factors r. Additionally, high squeezing levels lead to a substantial increase in the added noise photons n, which leads to weaker entanglement N_{ k } (τ = 0) but does not affect τ_{ d }. Both contributions may be seen in Eq. (2). Another important experimental observation is that increasing the filter bandwidth Ω leads to shorter dephasing times τ_{ d } which is also described by Eq. (2). In the limit Ω → ∞, τ_{ d } is eventually limited by the internal JPA bandwidth Δf ≃ 5 MHz. Quantum entanglement can be also evaluated using the entanglement of formation N_{ F }. The amount of entanglement in the TMS state for N_{ F } = 1 is equivalent to a maximally entangled pair of qubits^{30,31}. This link allows us to estimate an equivalent entangledbit rate transferred via the propagating TMS states. By measuring the entanglement of formation N_{ F } ≃ 0.86 within the full JPA bandwidth Δf (which corresponds to N_{ k } ≃ 1.08 and S ≃ 5.3 dB), we arrive at the entangledbit rate of 4.3 × 10^{6} ebits · s^{−1} usable in quantum communication protocols.
The demonstrated dephasing process of TMS states allows us to estimate fidelities for two relevant quantum communication protocols based on propagating squeezed microwaves. The first one is remote state preparation (RSP)^{19}. It can be viewed as the creation of a remote quantum state with local operations. Here, we consider the case of Ω = 430 kHz and S_{ e } ≃ 5.7 dB. A straightforward estimation based on the observed N_{ k }(τ) and otherwise ideal conditions provides the drop in fidelity from unity F_{RSP} (τ = 0) = 1 to F_{RSP} (τ) = 0.95 for τ ≃ 90 ns. The situation improves significantly when aiming at the remote preparation with lower squeezing level S_{ e } ≃ 3.8 dB, where the same drop does not happen until τ ≃ 250 ns. A similar analysis for the quantum teleportation (QT) protocol^{19} shows that the initial fidelities are lower (since it ultimately depends on the initial squeezing level and F_{QT} → 1 only for S → ∞), but decrease slower with increasing τ. For S_{ e } = 5.7 dB, the change from F_{QT} (τ = 0) ≃ 0.80 to F_{QT} (τ) ≃ 0.75 occurs in τ ≃ 380 ns. These timescales make the implementation of RSP and QT with propagating squeezed microwaves possible without implementing delay lines, since the required delays due to feedback operations in these protocols are estimated to be around τ ≃ 100–200 ns.
In conclusion, we have confirmed that the autocorrelation time of microwave singlemode squeezed states depends on the minimum filter bandwidth Ω in the measurement setup. In our specific setup, Ω is defined by the bandwidth of the digital FIR filter. We have uncovered that, for propagating microwave TMS states, the entanglement quantified via N_{ k }(τ) survives for finitetime delays τ. We express the negativity kernel N_{ k }(τ) as a function of the squeezing level S and the noise photon number n in addition to Ω. This function is given by Eq. (2) and accurately describes the experimental results. The measurement bandwidth Ω also influences the timescale of entanglement decay due to dephasing in the case of propagating TMS states. High squeezing levels and accompanying large noise photon numbers additionally reduce the entanglement for finite delays between the paths. With respect to applications, a key result of this work is the observation of the characteristic dephasing timescale τ_{ d } for twomode entanglement which can be longer than 1 μs for squeezing levels S ≃ 3 dB. This result confirms that propagating TMS microwave states are suitable for the remote preparation of squeezed states and shorttomedium distance quantum teleportation^{10,11}.
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Acknowledgements
We acknowledge support by the German Research Foundation through FE 1564/11, Spanish MINECO/FEDER FIS201569983P, UPV/EHU PhD grant, Basque Government Grant IT98616, European Project AQuS (Project No. 640800), Elite Network of Bavaria through the program ExQM, the projects JST ERATO (Grant No. JPMJER1601) and JSPS KAKENHI (Grant No. 26220601 and Grant No. 15K17731). E.S. acknowledges support from a TUM AugustWilhelm Scheer Visiting Professorship and hospitality of WaltherMeißnerInstitut and TUM Institute for Advanced Study. We would like to thank K. Kusuyama for assistance with part of the JPA fabrication.
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K.G.F. and F.D. planned the experiment. K.G.F. and S.P. performed the measurements and analysed the data. K.G.F., S.P., P.Y. and P.E. programmed the data acquisition software. U.L.H., M.S., R.D.C., and E.S. provided the theory support. K.I. and Y.N. provided the JPA samples. F.D., A.M. and R.G. supervised the experimental part of this work. K.G.F. wrote the manuscript. All authors contributed to discussions and proofreading the manuscript.
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Fedorov, K.G., Pogorzalek, S., Las Heras, U. et al. Finitetime quantum entanglement in propagating squeezed microwaves. Sci Rep 8, 6416 (2018). https://doi.org/10.1038/s4159801824742z
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DOI: https://doi.org/10.1038/s4159801824742z
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