Abstract
Twinfield (TF) quantum key distribution (QKD) was conjectured to beat the private capacity of a pointtopoint QKD link by using singlephoton interference in a central measuring station. This remarkable conjecture has recently triggered an intense research activity to prove its security. Here, we introduce a TFtype QKD protocol which is conceptually simpler than the original proposal. It relies on the preselection of a global phase, instead of the postselection of a global phase, which significantly simplifies its security analysis and is arguably less demanding experimentally. We demonstrate that the secure key rate of our protocol has a squareroot improvement over the pointtopoint private capacity, as conjectured by the original TF QKD.
Introduction
There is a tremendous research interest towards developing a global quantum internet,^{1,2,3,4,5,6} as this could enable many useful applications of quantum technologies, including, for example, quantum key distribution (QKD),^{7,8} blind quantum computing,^{9,10} distributed quantum metrology^{11,12} and distributed quantum computing.^{13} Among these applications, QKD is certainly the most mature technology today. Experimentally, longdistance QKD has already been performed over 400 km of telecom fibers,^{14,15} as well as over 1000 km of free space through satellite to ground links.^{16,17} Nonetheless, optical loss in telecom fibers (typically about 0.2 dB/km) poses an important limit to the distance of secure QKD without trusted or quantum repeater nodes.^{18,19,20,21,22,23,24,25} Indeed, even with a GHz repetition rate, it would take about 100 years to send a single photon successfully over 1000 km of a telecom fiber.^{20} Besides, fundamental limits for the key rate vs distance for secure pointtopoint QKD have been obtained recently.^{26,27} They essentially state that, in the absence of the repeater nodes, the key rate scales linearly with η, where η is the transmittance of the channel between Alice and Bob.
To overcome these limits, it is necessary to include intermediate nodes in the communication link. A possible solution is to modify the standard measurementdeviceindependent QKD (MDI QKD) protocol^{28} based on twophoton interference. For instance, one could add a feedback mechanism to ensure that the Bellstate measurement is performed between singlephoton pulses from Alice and Bob which actually arrive at the intermediate note. This can be done, for example, by means of quantum memories^{29,30} or by using quantum nondemolition measurements in an alloptical solution.^{31} While these approaches are promising, they are far from practical with current technology.
Remarkably, Lucamarini et al.^{32} have recently proposed a new MDI QKD type protocol, called twinfield (TF) QKD, which is based on a simple singlephoton interferometric measurement in a 50:50 beamsplitter, and is conjectured to beat the fundamental bounds in refs. ^{26,27} TF QKD is conceptually interesting because, for the singlephoton component, it considers a single detection event in the middle node of a photon that has come from either Alice or Bob. In other words, the photon does not even come from a definite party, but bears the interference of the two possibilities that is used to generate a secret key. Indeed, by considering restricted eavesdropping strategies, the authors of ref. ^{32} showed that the secret key rate of TF QKD scales with \(\sqrt \eta\). Very recently, two proofs of security of variants of the seminal TF QKD scheme against general attacks have been proposed,^{33,34} which also show the \(\sqrt \eta\) scaling. However, none of them is entirely satisfactory. They are rather complicated and require a postselection on the matching of the global phase of Alice and Bob, like in the original TF QKD scheme, which leads to nearly an order of magnitude of drop in the secret key rate.
In this paper, we introduce a modified TF QKD protocol and provide a simple proof of its informationtheoretic security. Our protocol removes the requirement of postselection on the matching of the global phase, thus simplifying the security proof and elucidating the concepts behind its security. We draw inspiration from quantum repeaters and connect the security of TF QKD to the study of quantum repeaters. In the key generation basis, the phases are preselected to be either 0 or π. For the security, we invoke a “complementarity” relation^{35} between the “phase” and the “number” of a bosonic mode. This contrasts the phaseencoding MDI QKD protocol introduced in ref.,^{36} which also relies on singlephoton interference at a central station, but uses another complementary relation between rectangular phases. In particular, to prove the security of a bit encoded in the phase value, our protocol considers what happens if Alice and Bob send optical pulses in number states to the central station. Importantly, the statistics related to this scenario can be estimated by using the decoystate method.^{37,38,39} As a result, our protocol can use only local phase randomization together with a preselection of a global phase, instead of postselecting a phase value based on a globalphase matching condition. Our proof also has practical impact as it can deliver nearly an order of magnitude higher secret key rate, compared to the two previous proofs.^{40} Indeed, among the first proofofprinciple experimental demonstrations of TF QKD^{41,42,43,44} reported very recently, most of them^{41,42,43} are based on our Protocol 3 (to be presented below) in the present paper.
Results
The key idea originates from entanglement generation protocols^{19,21,23} based on singlephoton interference in quantum repeaters. In particular, suppose that Alice and Bob are separated over a distance L and there is a station C right in the middle between them. This central station is connected to Alice (Bob) through an optical fiber with transmittance \(\sqrt \eta\). If Alice and Bob implement the original MDI QKD scheme in this scenario, it is clear that the key rate cannot scale better than η, as this protocol requires that twophoton coincidence events with one photon from Alice and one from Bob interfere in the node C. In comparison, TF QKD can provide a key rate scaling with \(\sqrt \eta\) because it only requires singles, i.e., one photon (either from Alice or from Bob) reaches the node C. Indeed, this scaling improvement is wellknown in the field of quantum repeaters. For instance, the performance of entanglement generation protocols in the repeater schemes introduced in^{19,21,23} scales as \(\sqrt \eta\) essentially because they use singlephoton interference in node C. Our starting point is then an ideal version of these entanglement generation protocols with an idealized photon source.
Protocol 1
It consists of the following six steps. (i) Alice (Bob) first prepares an optical pulse a (b) in an entangled state \(\left {\phi _q} \right\rangle _{Aa} = \sqrt q 0\rangle _A0\rangle _a + \sqrt {1  q} 1\rangle _A1\rangle _a\) (ϕ_{q}〉_{Bb}) with 0 ≤ q ≤ 1, where 0〉_{a}_{(}_{b}_{)} is the vacuum state and 1〉_{a}_{(}_{b}_{)} is the singlephoton state for optical pulse a (b), and system A (B) denotes a qubit in Alice’s (Bob’s) hands with {0〉_{A}_{(}_{B}_{)}, 1〉_{A}_{(}_{B}_{)}} representing the Z basis. (ii) Next, Alice and Bob send the optical pulses a and b through optical channels with transmittance \(\sqrt \eta\), respectively, to the middle node C in a synchronized manner. (iii) The node C applies to the incoming pulses a 50:50 beamsplitter, followed by two threshold detectors. Let D_{c} (D_{d}) denote the detector located at the output port c (d) of the beamsplitter associated to constructive (destructive) interference. (iv) The node C announces the measurement outcome k_{c} (k_{d}) corresponding to detector D_{c} (D_{d}), where k_{c} = 0 and k_{c} = 1 (k_{d} = 0 and k_{d} = 1) indicates a noclick event and a click event, respectively. (v) With probability p_{X} Alice (Bob) chooses the X basis \(\{  \pm \rangle _{A(B)}: = (0\rangle _{A(B)} \pm 1\rangle _{A(B)})/\sqrt 2 \}\) as the key generation basis and performs the Xbasis measurement on the qubit A (B), while with probability p_{Z} she (he) chooses the Z basis and performs the Zbasis measurement. As a result, Alice (Bob) obtains the bit value b_{A} (b_{B}), where \((  1)^{b_A} = x\; ((  1)^{b_B} = x)\) for the eigenvalues x = ±1 of the Pauli operators \(\hat X\) and \(\hat Z\). (vi) When node C reports k_{c} = 1 and k_{d} = 0 (k_{c} = 0 and k_{d} = 1) and Alice and Bob choose the X basis, b_{A} and b_{B} (b_{A} and b_{B} ⊕ 1) are regarded as their raw key. Note that in this protocol no phase randomization is applied.
We remark that step (iii) above actually corresponds to performing a “swap test” on the incoming signals. Such a swap test is commonly used in, for example, quantum digital signature schemes^{45} and quantum fingerprinting protocols.^{46,47,48}
For simplicity and for the moment, let us neglect the effect of the dark counts in the detectors D_{c} and D_{d} and assume that their detection efficiency is perfect. Then, it is straightforward to show that the probability r with which node C observes only one click in say detector D_{c} (D_{d}) in step (iv) above is r = r_{1} + r_{2}, where
That is, r_{1} (r_{2}) corresponds to a detection event produced by a singlephoton (twophoton) pulse.
Given only one detection click in say detector D_{c} (D_{d}), the joint state of Alice and Bob’s qubit systems A and B is denoted by \({\hat \rho _{AB}^+}\, (\hat \rho _{AB}^  )\), where
with \(\left {\Psi ^ \pm } \right\rangle _{AB}: = (01\rangle _{AB} \pm 10\rangle _{AB})/\sqrt 2\).
According to Protocol 1, the biterror rate, e_{X}, is defined by the probability with which Alice’s and Bob’s Xbasis measurement outcomes are different (i.e., b_{A} ≠ b_{B}) when k_{c} = 1 and k_{d} = 0, or they are equal (b_{A} = b_{B}) when k_{c} = 0 and k_{d} = 1. On the other hand, the phaseerror rate, e_{Z}, is defined by the probability with which Alice’s and Bob’s measurement outcomes in the Z basis coincide (b_{A} = b_{B}) when k_{c} + k_{d} = 1. From Eq. (3), we obtain that e_{X} and e_{Z} satisfy
The asymptotic key rate formula R_{X} is then given by
where 2r represents the total success probability, f ≥ 1 is an inefficiency function for the error correction process, and h(x) is the binary entropy function, i.e., \(h(x): =  x\,{\mathrm{log}}_2x  (1  x){\mathrm{log}}_2(1  x)\). The parameter q is chosen such that R_{X} is maximized for each given distance.
Protocol 2
We can also consider a prepareandmeasure version of Protocol 1. For this, we note that, without loss of generality, the measurement in step (v) of Protocol 1 can be done soon after its step (i). This is because this measurement operation commutes with all the operations performed in the other steps. So, the ordering of the steps is not relevant to the physics. Hence, Protocol 1 is mathematically equivalent to a prepareandmeasure protocol where one omits step (v) and replaces step (i) with the following step: (i′) Alice (Bob) prepares an optical pulse a (b) in the state \(\left {X_0} \right\rangle _{a(b)}: = \sqrt q 0\rangle _{a(b)} + \sqrt {1  q} 1\rangle _{a(b)}\) for b_{A} = 0 (b_{B} = 0) or in the state \(\left {X_1} \right\rangle _{a(b)}: = \sqrt q 0\rangle _{a(b)}  \sqrt {1  q} 1\rangle _{a(b)}\) for b_{A} = 1 (b_{B} = 1) at random when she (he) chooses the X basis with probability p_{X}, while Alice (Bob) prepares the optical pulse a (b) in the state Z_{0}〉_{a(b)} := 0〉_{a(b)} for b_{A} = 0 (b_{B} = 0) with probability q or in the state Z_{1}〉_{a(b)} := 1〉_{a(b)} for b_{A} = 1 (b_{B} = 1) with probability 1 − q when she (he) chooses the Z basis with probability p_{Z}. That is, Protocol 2 is composed of step (i′), as well as steps (ii)–(iv) and (vi) from Protocol 1.
In Fig. 1, we show the performance of these two protocols by maximizing R_{X} over q as a function of the overall loss between Alice and Bob. According to our computation calculation, the optimal value of q = ∥_{a}〈0∥ϕ_{q}〉_{Aa}∥^{2} starts from about 0.88 at 0 dB, and then monotonically increases with the loss up to a value of about 0.94 at 20 dB, and afterwards remains basically constant. The high value of q suggests that the states X_{k}〉 (k = 0, 1) could be replaced by coherent states (−1)^{k}α〉 by choosing an appropriate amplitude α (> 0), as their good approximation. Also, since the states Z_{k}〉 (k = 0, 1) are number states, Alice and Bob could estimate the phaseerror rate e_{Z} by using phaserandomized coherent states in combination with the decoystate method. These two observations lead to the following practical protocol.
Protocol 3
It is composed of the following modified first step (i″) together with steps (ii)–(iv) and (vi) from Protocol 1: (i″) Alice (Bob) first chooses the X basis with probability p_{X} and the Z basis with probability p_{Z}. If her (his) choice is the X basis, she (he) prepares an optical pulse a (b) in a coherent state α〉_{a(b)} for b_{A} = 0 (b_{B} = 0) or  − α〉_{a(b)} for b_{A} = 1 (b_{B} = 1) at random. If her (his) choice is the Z basis, she (he) prepares an optical pulse a (b) in a phaserandomized coherent state \(\hat \rho _{a,\beta _A}\,(\hat \rho _{b,\beta _B})\) whose amplitude β_{A} (β_{B}) is chosen from a set S = {β_{i}}_{i} of real nonnegative numbers β_{i} ≥ 0, according to a probability distribution \(p_{\beta _A}\,(p_{\beta _B})\).
It is important to note that Protocol 3 requires synchronization of phase references for Alice and Bob. However, since in QKD Alice and Bob may use ancillary strong pulses generated by lasers to establish such a pulse reference, we believe that establishing the phase reference is practical. Indeed, as already mentioned in the introduction, this has already been accomplished in the recent TF QKD experiments reported in.^{41,42,44} In addition, Protocol 3 assumes that all the Xbasis (key generation) states of Alice and Bob are either of the same or opposite phase, but no phase randomization is needed for the key generation states. That is, the global phase of the Xbasis states is preselected by Alice and Bob before the execution of the protocol. This contrasts with the globalphase reconciliation procedure based on a postselection step considered in refs. ^{32,33,34} Furthermore, all the Zbasis states (used for test for tampering) of Alice and Bob have random phases, which allows us to apply the decoystate technique to these states to infer the contributions from the vacuum, singlephoton, and multiphoton components. Also, note that p_{X} can be chosen much higher than p_{Z} to have a high key generation rate.
Security proof of Protocol 3
For simplicity we shall consider the asymptotic scenario where Alice and Bob emit an infinite number of signals, and the eavesdropper, Eve, performs a collective attack. The security against general attacks is presented in the Supplementary Information. We follow the losstolerant approach introduced in ref. ^{49} Also, without loss of generality, we shall assume that the node C is under the full control of Eve. After a QKD run, Alice and Bob can estimate the probability distribution p_{zz}(k_{c}, k_{d}  β_{A}, β_{B}) (p_{xx} (k_{c}, k_{d}  b_{A}, b_{B})) over k_{c} and k_{d} given the choice of β_{A} and β_{B} (b_{A} and b_{B}) and the selection of the Z (X) basis. By noting that
where
we have that the biterror rate, \(e_{X,k_ck_d}\), for Eve’s announcement of k_{c} and k_{d} is defined by
Next we consider the decoystate method. In particular, since when Alice and Bob choose the Z basis in step (i″) of Protocol 3 they prepare phaserandomized coherent states, Eve cannot distinguish this step from the following fictitious scenario: Alice (Bob) prepares an optical pulse a (b) in a number state n_{A}〉_{a} (n_{B}〉_{b}) according to a Poissonian distribution \(P_{\beta _{A}^{2}}(n_{A})\,((P_{\beta _{B}^{2}}(n_{B}))\), where P_{λ}(n) = (e^{–λ}λ^{n})/n!. In this fictitious scenario, Eve needs to return her measurement outcome by performing a measurement on the number states n_{A}〉 and n_{B}〉. This implies that Eve’s announcement of k_{c} and k_{d} follows a probability distribution p_{zz} (k_{c}, k_{d}  n_{A}, n_{B}). Then, we have
for any β_{A} and β_{B}. That is, once Alice and Bob know p_{zz} (k_{c}, k_{d}  β_{A}, β_{B}) for any β_{A} and β_{B}, they can use the decoystate method to estimate p_{zz} (k_{c}, k_{d}  n_{A}, n_{B}) based on their knowledge of \(P_{\beta _A^2}(n_A)\) and \(P_{\beta _B^2}(n_B)\).
The next step is to relate the conditional probabilities p_{zz} (k_{c}, k_{d}  n_{A}, n_{B}) with the phaseerror rate to prove security.^{35} For this, note that if Alice and Bob choose the X basis in step (i″) of Protocol 3, Eve cannot distinguish this step from the following fictitious step: Alice (Bob) prepares an optical pulse a (b) and a qubit A (B) in an entangled state \(\left {\psi _X} \right\rangle _{A{a}} = ( + \rangle _A\alpha \rangle _{a} +   \rangle _A  \alpha \rangle _{a})/\sqrt 2 \; \,(\psi _X\rangle _{Bb})\). By running this, fictitious step together with steps (ii)–(iv) in order, Alice and Bob obtain a state
with probability p_{xx} (k_{c}, k_{d}), where \(\hat M_{k_{c}k_{d}}^{ab}\) is the Kraus operator corresponding to the announcement of k_{c} and k_{d}. The phaseerror rate, \(e_{Z,k_{c}k_{d}}\), is then defined by
Since _{A}〈iψ_{X}〉_{Aa} = C_{i}〉_{a} with unnormalized cat states
for nonnegative coefficients \(c_n^{(i)} \ge 0\), from Eq. (10) and for any i, j = 0, 1, we have
where we have used the CauchySchwarz inequality and \(\left\ {\hat M_{k_{c}k_{d}}^{ab}m_A\rangle _am_B\rangle _b} \right\^2 = p_{ZZ}(k_{c},k_{d}m_A,m_B)\). By combining these results with Eq. (11), we conclude
Notice that in the phaseerror estimation process encapsulated in Eq. (15), it is important to estimate the yields p_{zz} (k_{c}, k_{d}  n_{A}, n_{B}) for various photonnumber components (n_{A}, n_{B}) (and for the various measurement outcomes (k_{c}, k_{d}) of node C). To do so, when Alice and Bob choose the Z basis, a decoystate method is employed. For this reason, phase randomization is performed in the Z basis. The asymptotic key rate formula, \(R_{X,k_{c}k_{d}}\), can then be lower bounded as
which leads to the final key rate formula:
Discussion
The performance of Protocol 3 is illustrated in Fig. 1, where we maximize a further lower bound on \(R_X^{{\mathrm{low}}}\) over α as a function of the overall loss between Alice and Bob. In particular, here we assume the asymptotic scenario where Charlie behaves as he is supposed to do, Alice and Bob use an infinite number of decoy settings, and they can estimate the probabilities p_{zz} (k_{c}, k_{d}  n_{A}, n_{B}), with (n_{A}, n_{B}) = (0, 0), (0, 2), (2, 0), (2, 2), (1, 1), (1, 3), (3, 1), precisely, while the remaining probabilities are simply upper bounded as p_{zz}(k_{c}, k_{d}  n_{A}, n_{B}) ≤ 1 (although, clearly, the more probabilities \(\{ p_{ZZ}(k_{c},k_{d}n_A,n_B)\} _{n_A,n_B}\) Alice and Bob tightly estimate, the higher the resulting key rate is). Notice that, in our protocol, secure key generation has contributions from not only the singlephoton components, but also multiphoton components.^{50} Importantly, Fig. 1 demonstrates that \(R_X^{{\mathrm{low}}}\) has \(\sqrt \eta\) scaling. In the Supplementary Information, it is also confirmed that the use of three decoy states (that is, setting S = {β_{i}}_{i=1,2,3} in Protocol 3), rather than infinite decoy states, is enough for Protocol 3 to achieve a similar performance to Fig. 1. Besides, remarkably, Protocol 3 is quite robust against phase mismatch between AliceC and BobC channels. See Supplementary Information for the details.
The fact that the cases (n_{A}, n_{B}) = (0, 1) or (1, 0) do not contribute at all to the phaseerror rate is remarkable. The reason for this behavior is the following. The even (odd) cat state corresponding to j = 0 (j = 1) in Eq. (12) (Eq. (13)) includes only even (odd) photons. And Eq. (14) considers what happens when Alice’s input and Bob’s input are both (phaserandomized) even cat states or both (phaserandomized) odd cat states. Thus, the terms (0, 1) and (1, 0) never contribute. This means that by lower bounding other contributions (such as (n_{A}, n_{B}) = (0, 0), (0, 2), (2, 0), …) with decoy states, one can severely limit the amount of information Eve has on the sifted key. Moreover, note that the signals contain mainly only one photon or less originating from either Alice or Bob. The net transmittance of the signal is thus of order \(\sqrt \eta\), which leads to a very high key rate for TFtype QKD at long distances. That is, it is mainly the interference between the singlephoton component generated by either Alice or Bob that leads to security.
Finally, we note that since the structure of the security proof of Protocol 3 resembles that for the losstolerant QKD protocol,^{49} its extension to the finitekey scenario could be readily done by using similar techniques like those employed in,^{51,52,53} in combination with the decoystate analysis employed in standard MDI QKD.^{54}
In summary, we have introduced a novel TFtype QKD protocol, together with a simple proof of its security, which can beat the fundamental bounds on the private capacity of pointtopoint QKD over a lossy optical channel presented in.^{26,27} Its secret key rate scales as \(\sqrt \eta\) rather than η, being η the transmittance of the quantum channel. This protocol could also be regarded as a phaseencoding MDI QKD scheme with singlephoton interference. Indeed, it inherits the major advantage of standard MDI QKD, i.e., it is robust against any side channel in the measurement unit. Moreover, it has now been experimentally demonstrated in,^{41,42} thus showing its practicality.
Note added
During the preparation of this paper, three different pieces of research contributions considering variants of the TF QKD protocol have been posted on preprint servers^{55,56} or presented in a conference.^{57} While our formulation and discussion for security have similarities with these results, there are also differences in the methodology and our initial idea was conceived independently of these research contributions. Indeed, the quantum communication part of our protocol is equivalent to that of ref. ^{55} and the main difference between both schemes is merely the technique to prove the security.
Data Availability
Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.
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Acknowledgements
We are especially thankful to G. Kato, M. Koashi, G. C. Lorenzo and Y. Zhang for giving us insightful comments and suggestions about the security proof of this paper, to M. Lucamarini and K. Tamaki for discussions related to the papers,^{32,33} to X. Ma and P. Zeng for discussions related to the paper,^{34} and to N. Lütkenhaus’ group for discussions regarding the results in ref.^{57} K.A. thanks support, in part, from PRESTO, JST JPMJPR1861. M.C. acknowledges support from the Spanish Ministry of Economy and Competitiveness (MINECO), the Fondo Europeo de Desarrollo Regional (FEDER) through grants TEC201454898R and TEC201788243R, and the European Union’s Horizon 2020 research and innovation programme under the Marie SklodowskaCurie grant agreement No 675662 (project QCALL). HK.L. thanks the US Office of Naval Research, NSERC, CFI, ORF, MITACS, Huawei Technologies Canada Co., Ltd, and the Royal Bank of Canada for financial support.
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M.C. and K.A. contributed equally to this work; M.C. contributed more to the protocol design and K.A. to its security proof. HK.L. triggered the consideration of this research project. All authors contributed to the writing and generalization of the ideas.
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Correspondence to Koji Azuma.
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Curty, M., Azuma, K. & Lo, H. Simple security proof of twinfield type quantum key distribution protocol. npj Quantum Inf 5, 64 (2019) doi:10.1038/s4153401901756
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Further reading

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