Teleportation simulation of bosonic Gaussian channels:
Strong and uniform convergence
Abstract
We consider the BraunsteinKimble protocol for continuous variable teleportation and its application for the simulation of bosonic channels. We discuss the convergence properties of this protocol under various topologies (strong, uniform, and boundeduniform) clarifying some typical misinterpretations in the literature. We then show that the teleportation simulation of an arbitrary singlemode Gaussian channel is uniformly convergent to the channel if and only if its noise matrix has full rank. The various forms of convergence are then discussed within adaptive protocols, where the simulation error must be propagated to the output of the protocol by means of a “peeling” argument, following techniques from PLOB [arXiv:1510.08863]. Finally, as an application of the peeling argument and the various topologies of convergence, we provide complete rigorous proofs for recentlyclaimed strong converse bounds for private communication over Gaussian channels.
I Introduction
Quantum teleportation Tele1 (); Tele2 (); telereview (); oldrev (); teleCV2 () is a fundamental operation in quantum information theory NielsenChuang (); CVbook (); RMP () and quantum Shannon theory HolevoBOOK (); Hayashi (). It is a central tool for simulating quantum channels with direct applications to quantum/private communications TQC () and quantum metrology ReviewMETRO (). In a seminal paper, Bennett et al. ref1 () showed how to simulate Pauli channels and reduce quantum communication protocols into entanglement distillation. Similar ideas can be found in a number of other investigations ref2 (); ref3 (); ref4 (); ref5 (); ref6 (); ref7 (); ref8 (); ref9 (); ref10 (); ref11 (); ref12 (); ref13 (); ref14 (); ref15 () (see Ref. (TQC, , Sec. IX) for a detailed discussion of the literature on channel simulation). More recently, in 2015, PirandolaLaurenzaOttavianiBanchi (PLOB) PLOB () showed how to transform these precursory ideas into a completely general formulation.
PLOB showed how to simulate an arbitrary quantum channel (in arbitrary dimension) by means of local operations and classical communication (LOCC) applied to the channel input and a suitable resource state. For instance, this approach allowed one to deterministically simulate the amplitude damping channel for the very first time. The LOCC simulation of a quantum channel is then exploited in the technique of teleportation stretching PLOB (), where an arbitrary adaptive protocol (i.e., based on the use of feedback) is simplified into a simpler block version, where no feedback is involved.
Teleportation stretching is a very flexible technique whose combination with suitable entanglement measures (such as the relative entropy of entanglement REE1 (); REE2 (); REE3 ()) and other functionals (such as the quantum Fisher information qfi1 (); qfi2 (); qfi3 (); qfi4 (); qfi5 ()) has recently led to the discovery of a number of results. For instance, PLOB established the twoway assisted quantum/private capacities of various fundamental channels, such as the lossy channel, the quantumlimited amplifier, dephasing and erasure channels PLOB (). In particular, the PLOB bound of bits per use of a lossy channel with transmissivity sets the ultimate limit of pointtopoint quantum communications or, equivalently, a fundamental benchmark for quantum repeaters Briegel (); Rep2 (); Rep3 (); Rep4 (); Rep5 (); Rep6 (); Rep7 (); Rep8 (); Rep9 (); Rep10 (); Rep12 (); Rep13 (); Rep13bis (); Rep14 (); Rep15 (); Rep16 (); Rep17 (); Rep18 (); Rep19 (). In the setting of quantum metrology, Ref. PirCo () used teleportation stretching to show that parameter estimation with teleportationcovariant channels cannot beat the standard quantum limit, establishing the adaptive limits achievable in many scenarios. Other results were established for quantum networks netPAPER (), such as a quantum version of the max flow/min cut theorem. See also Refs. next1 (); next2 (); next3 (); next4 () for other studies.
It is clear that continuous variable (CV) quantum teleportation telereview (), also known as the BraunsteinKimble (BK) protocol Tele2 (), is central in many of the previous results and in several other important applications. The BK protocol is a tool for optical quantum communications, from realistic implementations of quantum key distribution, e.g., via swapping in untrusted relays MDI1 (); CVMDIQKD (); CVMDIQKD2 (); CVMDIQKD3 () to more ambitious goals such as the design of a future quantum Internet HybridINTERNET (); Kimble2008 (). That being said, the BK protocol is still the subject of misunderstandings by some authors. Typical misuses arise from confusing the different forms of convergence that can be associated with this protocol, an error which is connected with a specific order of the limits to be carefully considered when teleportation is performed within an infinitedimensional Hilbert space.
In this work, we discuss and clarify the convergence properties of the BK protocol and its consequences for the simulation of bosonic channels. As a specific case, we investigate the simulation of singlemode bosonic Gaussian channels, which can be fully classified in different canonical forms HolevoCanonical (); Caruso (); HolevoVittorio () up to input/output Gaussian unitaries. We show that the teleportation simulation of a singlemode Gaussian channel uniformly converges to the channel as long as its noise matrix has full rank. This matrix is generally connected with the covariance matrix of the Gaussian state describing the environment in a singlemode symplectic dilation of the quantum channel.
Assuming various topologies of convergence (strong, uniform, and boundeduniform), we then study the teleportation simulation of bosonic channels in adaptive protocols. Here we discuss the crucial role of a peeling argument that connects the channel simulation error, associated with the single channel transmissions, to the overall simulation error accumulated on the final quantum state at the output of the protocol. This argument is needed in order to rigorously prove strong converse upper bounds for twoway assisted private capacities. As a direct application of our analysis, we then provide various complete proofs for the strong converse bounds claimed in WildeTomamichelBerta (WTB) WildeFollowup (). In particular, we show how the bounds claimed in WTB can be rigorously proven for adaptive protocols, and how their illness (divergence to infinity) is fixed by a correct use of the BK teleportation protocol. In this regard, our study extends the one already given in Ref. TQC () to also include the topologies of strong and uniform convergence.
The paper is organized as follows. In Sec. II, we provide some preliminary notions on bosonic systems, Gaussian states, and Gaussian channels, including the classification in canonical forms HolevoCanonical (); Caruso (); HolevoVittorio (), as revisited in terms of matrix ranks in Ref. RMP (). In Sec. III, we discuss the convergence properties of the BK protocol for CV teleportation, also discussing the interplay between the different limits associated with this protocol. In Sec. IV, we consider the teleportation simulation of bosonic channels under the topologies of strong and boundeduniform convergence. In Sec. V, we present the main result of our work, which is the necessary and sufficient condition for the uniform convergence of the teleportation simulation of a Gaussian channel. In Sec. V, we present the peeling argument for adaptive protocols, considering the various forms of convergence. Next, in Sec. VI, we present implications for quantum/private communications, showing the rigorous proofs of the claims presented in WTB. Finally, Sec. VII is for conclusions.
Ii Preliminaries
ii.1 Bosonic systems and Gaussian states
CV systems have an infinitedimensional Hilbert space . The most important example of CV systems is given by the bosonic modes of the radiation field. In general, a bosonic system of modes is described by a tensor product Hilbert space and a vector of quadrature operators satisfying the commutation relations
(1) 
where is the symplectic form
(2) 
An arbitrary bosonic state is characterized by a density operator or, equivalently, by its Wigner representation. Introducing the Weyl operator Weyl ()
(3) 
an arbitrary is equivalent to a characteristic function
(4) 
or to a Wigner function
(5) 
where the continuous variables span the real symplectic space which is called the phase space.
The most relevant quantities that characterize the Wigner representations are the statistical moments. In particular, the first moment is the mean value
(6) 
and the second moment is the covariance matrix (CM) , whose arbitrary element is defined by
(7) 
where and is the anticommutator. The CM is a , real symmetric matrix which must satisfy the uncertainty principle
(8) 
coming directly from Eq. (1). For a particular class of states, the first two moments are sufficient for a complete characterization. These are the Gaussian states which, by definition, are those bosonic states whose Wigner representation ( or ) is Gaussian, i.e.,
(9)  
(10) 
It is also very important to identify the quantum operations that preserve the Gaussian character of such quantum states. In the Heisenberg picture, Gaussian unitaries correspond to canonical linear unitary Bogoliubov transformations, i.e., affine real maps of the quadratures
(11) 
that preserve the commutation relations of Eq. (1). It is easy to show that such a preservation occurs when the matrix is symplectic, i.e., when it satisfies
(12) 
By applying the map of Eq. (11) to the Weyl operator of Eq. (3), we find the corresponding transformations for the Wigner representations. In particular, the arbitrary vector of the phase space undergoes exactly the same affine map as above
(13) 
In other words, an arbitrary Gaussian unitary acting on the Hilbert space of the system is equivalent to a symplectic affine map acting on the corresponding phase space . Notice that such a map is composed by two different elements, i.e., the phasespace displacement which corresponds to a displacement operator , and the symplectic transformation which corresponds to a canonical unitary in the Hilbert space. In particular, the phasespace displacement does not affect the second moments of the quantum state since the CM is transformed by the simple congruence
(14) 
Fundamental properties of the bosonic states can be easily expressed via the symplectic manipulation of their CM. In fact, according to the Williamson’s theorem Williamson (); Arnold (); Alex (), any CM can be diagonalized by a symplectic transformation. This means that there always exists a symplectic matrix such that
(15) 
where the set is called the symplectic spectrum and satisfies (since for symplectic ). By applying the symplectic diagonalization of Eq. (15) to Eq. (8), one can write the uncertainty principle in the simple form of RMP ()
(16) 
ii.2 Gaussian channels and canonical forms
A singlemode bosonic channel is a completely positive trace preserving (CPTP) map acting on the density matrix of a single bosonic mode. In particular, it is Gaussian () if it transforms Gaussian states into Gaussian states. The general form of a singlemode Gaussian channel can be expressed by the following transformation of the characteristic function HolevoCanonical ()
(17) 
where is a displacement, while and are real matrices, with and
(18) 
These are the transmission matrix and the noise matrix . At the level of the first two statistical moments, the transformation of Eq. (17) takes the simple form
(19) 
Any singlemode Gaussian channel can be transformed into a simpler canonical form HolevoCanonical (); Caruso (); HolevoVittorio () via unitary transformations at the input and the output (see Fig. 1). In fact, for any physical there are (nonunique) finiteenergy Gaussian unitaries and such that
(20) 
where the canonical form is the CPTP map
(21) 
characterized by zero displacement () and diagonal matrices and .
Depending on the values of the symplectic invariants , rank() and rank(), we have six different expressions for the diagonal matrices and, therefore, six inequivalent classes of canonical forms , which are denoted by and . From Ref. HolevoVittorio () we report the classification of these forms in Table 1, where , the identity matrix, and the zero matrix. In this table is the (generalized) transmissivity, while is the thermal number of the environment and is additive noise notation ().

Let us also introduce the symplectic invariant
(22) 
that we call the rank of the Gaussian channel formsREF (); RMP (). Then, every class is simply determined by the pair according to the refined Table 2. Note that classes and have been divided into subclasses. In fact, class includes the identity channel (for ), while class describes an attenuator (amplifier) channel for (). In common terminology the forms , and are known as phaseinsensitive, because they act symmetrically on the two input quadratures. By contrast, the forms , and (conjugate of the amplifier) are all phasesensitive. The form is an additive form. In fact it is also known as the additivenoise Gaussian channel, which is a direct generalization of the classical Gaussian channel in the quantum setting.
ii.3 Singlemode dilation of a canonical form
All the nonadditive forms admit a simple singlemode physical representation where the degrees of freedom of the input bosonic mode “” unitarily interacts with the degrees of freedom of a single environmental bosonic mode “” described by a mixed state Caruso (); HolevoVittorio () (see Fig. 2). In particular, such a physical representation can always be chosen to be Gaussian. This means that can be represented by a canonical unitary mixing the input state with a thermal state , i.e.,
(23) 
where
(24) 
with symplectic and is a thermal state with CM (see Fig. 2).
In fact, by writing in the blockform
(25) 
so that
(26)  
(27) 
one finds that Eq. (23) corresponds to the following inputoutput transformation for the characteristic function
(28) 
Then, by setting and
(29) 
one easily verifies that Eq. (28) has the form of Eq. (21), where the bona fide condition of Eq. (18) is assured by the symplectic nature of NoteSympl (). In Eq. (29) the orthogonal transformation is chosen in a way to preserve the symplectic condition for . Such a condition also restricts the possible forms of the remaining blocks and , which can be fixed up to a canonical local unitary.
Altogether, any nonadditive canonical form can be described by a singlemode physical representation where the type of symplectic transformation is determined by its class while the thermal noise only characterizes the environmental state. From the point of view of the second order statistical moments, the CM of an input state undergoes the transformation
(30) 
where the partial trace must be interpreted as deletion of rows and columns associated with mode .
In particular, one has the following symplectic matrices for the various forms HolevoVittorio ()
(31) 
describing a beamsplitter,
(32) 
describing an amplifier,
(33) 
describing the complementary of an amplifier. Finally HolevoVittorio ()
(34)  
(35)  
(36) 
ii.4 Asymptotic dilation of the additive form
The additivenoise Gaussian channel or canonical form can be dilated into a twomode environment HolevoVittorio (). Another possibility is to describe this form by means of an asymptotic singlemode dilation. In fact, consider the dilation of the attenuator channel, which is a beamsplitter with transmissivity coupling the input mode with an environmental mode prepared in a thermal state with mean photons. In this dilation, let us consider a thermal state with so that we realize . Then, taking the limit for (so that ), we represent the canonical form as
(37) 
In fact it is clear that, in this way, we may realize the asymptotic transformations and .
Iii Convergence of CV teleportation
iii.1 BraunsteinKimble teleportation protocol
Let us review the BK protocol for CV quantum teleportation Tele2 (); telereview (). Alice and Bob share a resource state which is a twomode squeezed vacuum (TMSV) state . Recall that this is a zeromean Gaussian state with CM RMP ()
(38) 
Here the variance parameter determines both the squeezing (or entanglement) and the energy associated with the state. In particular, we may write , where is the mean number of photons in each mode, (for Alice) and (for Bob).
Then, Alice has an input bipartite state , where is an arbitrary multimode system while is a single mode that she wants to teleport to Bob. To teleport, she combines modes and in a joint CV Bell detection, whose complex outcome is classically communicated to Bob (this can be realized by a balanced beam splitter followed by two conjugate homodyne detectors telereview ()). Finally, Bob applies a displacement on his mode , so that the output state is the teleported version of the input .
One has perfect teleportation in the limit of infinite squeezing . In other words, for any input state (with finite energy) we may write the trace norm limit
(39) 
or equivalently, we may write
(40) 
where is the Bures fidelity. This is a well known result which has been proven in Ref. Tele2 ().
iii.2 Strong convergence of CV teleportation
Let us denote by the overall LOCC associated with the BK protocol, as in Fig. 3(a). The application of this LOCC onto a finiteenergy TMSV state generates a teleportation channel which is not the bosonic identity channel but a pointwise (local) approximation of . In other words, for any (energybounded) input state , we may consider the output
(41) 
and write the tracenorm limit
(42) 
It is clear that this pointwise limit immediately implies the convergence in the strong topology Tele2 ()
(43) 
Similarly, we may introduce the Bures distance qfi1 ()
(44) 
and write the previous limit as
(45) 
Remark 1
Let us stress that the strong convergence of the BK protocol is known since 1998. It is wellknown that, for any given energyconstrained input state, if we send the squeezing of the resource state (TMSV state) to infinite, then we can perfectly teleport the input state. In Eqs. (4) and (8) of Ref. Tele2 (), there is a convolution between the Wigner function of an arbitrary normalized input state and the Gaussian kernel , where goes to zero for increasing squeezing (and ideal homodyne detectors). Taking the limit for large , the teleportation fidelity goes to as we can also see from Eq. (11) of Ref. Tele2 (). This is just a standard deltalike limit that does not really need explicit steps to be shown and fully provides the (strong) convergence of the BK protocol.
iii.3 Boundeduniform convergence of CV teleportation
Consider an energyconstrained alphabet of states
(46) 
where is the total number operator associated with the input mode and the reference modes . Then, we define an energyconstrained diamond distance PLOB (); TQC () between two arbitrary bosonic channels and , as
(47) 
See also Ref. MaximNORM (); WinterNORM () for an alternate definition of energyconstrained diamond norm. It is easy to show that, for any finite energy , one may write PLOB ()
(48) 
so that the BK channel converges to the identity channel in the boundeduniform topology. In fact, this comes from the pointwise limit in Eq. (42) combined with the fact that is a compact set HolevoCOMPACT (); HolBook (); Werner ().
iii.4 Nonuniform convergence of CV teleportation
Can we relax the energy constraint in Eq. (48)? The answer is no. As already discussed in Ref. TQC (), we have
(49) 
where
(50)  
(51) 
is the standard diamond distance. In fact, Ref. TQC () provided a simple proof that the BK protocol does not uniformly converge to the identity channel. For this proof, it is sufficient to take the input state to be a TMSV state with diverging energy . Then, Eq. (49) is implied by the fact that, for any energy BK protocol, we have
(52) 
which is equivalent to for any .
In order to show Eq. (52) we directly report the steps given in Ref. TQC () but adapted to our different notation. The first observation is that, when applied to an energyconstrained quantum state (i.e., a “point”), the energy BK channel is locally equivalent to an additivenoise Gaussian channel (form ) with added noise
(53) 
For instance, see Refs. GerLimited (); next3 (). Then, from the CM of , it is easy to compute the CM of the output state yielding
(54) 
Using the formula for the quantum fidelity between arbitrary Gaussian states banchiPRL2015 (), we compute
(55)  
Here we notice the expansion at any fixed . Now using the Fuchsvan de Graaf relations Fuchs ()
(56) 
we get, for any finite , the following expansion
(57) 
which implies Eq. (52).
Here it is important to observe the radically different behavior of the teleportation protocol with respect to exchanging the limits in the energy of the resource state and in the energy of the input state . In fact, by taking the limit in before the one in in Eq. (55), we get
(58) 
Because of the noncommutation between these two limits
(59) 
we have a difference between the strong convergence in Eq. (43) and the uniform nonconvergence in Eq. (49). This also means that joint limits such as
(60) 
are not defined. While this problem has been known since the early days of CV teleportation, technical errors related to this issue can still be found in recent literature (see the “case study” discussed in Sec. VII D).
Iv Teleportation simulation of bosonic channels
The BK teleportation protocol is a fundamental tool for the simulation of bosonic channels (not necessarily Gaussian). Consider a teleportationcovariant bosonic channel PLOB (). This means that, for any random displacement , we may write
(61) 
where is an output unitary. If this is the case, then the bosonic channel can be simulated by teleporting the input state with a modified teleportation LOCC over the (asymptotic) Choi matrix of the channel . In particular, Eq. (61) is true for Gaussian channels, for which is just another displacement.
In order to correctly formulate this type of simulation, we need to start from an imperfect finiteenergy simulation and then take the asymptotic limit for large energy. Therefore, let us consider a energy BK protocol generating a BK channel at the input of a bosonic channel . Let us consider the composite channel
(62) 
As shown in Fig. 3(b), for any input state , we may write the output state as
(63) 
If the bosonic channel is teleportation covariant, then we can swap it with the displacements , up to redefining the teleportation corrections as . On the one hand this changes the teleportation LOCC , on the other hand the resource state becomes a quasiChoi state
(64) 
Therefore, as depicted in Fig. 3(c), we may rewrite the teleportation simulation of the output as
(65) 
Now, using Eq. (62) and the monotonicity of the trace distance under CPTP maps, we may write
(66) 
where we exploit Eq. (42) in the last step. Therefore, for any bipartite (energyconstrained) input state , we may write the pointwise limit
(67) 
iv.1 Strong convergence in the teleportation simulation of bosonic channels
The strong convergence in the simulation of (teleportationcovariant) bosonic channels (not necessarily Gaussian) is an immediate consequence of the pointwise limit in Eq. (67). In fact, because Eq. (67) holds for any bipartite (energyconstrained) input state , we may write
(68) 
or similarly in terms of the Bures distance
(69) 
In other words, the teleportation simulation of a bosonic channel , strongly converges to it in the limit of large .
iv.2 Boundeduniform convergence in the teleportation simulation of bosonic channels
Consider now an energy constrained input alphabet as in Eq. (46) and the energyconstrained diamond distance defined in Eq. (47). Given an arbitrary (teleportationcovariant) bosonic channel and its teleportation simulation as in Eq. (65), we define the simulation error as PLOB (); TQC ()
(70) 
Because of the monotonicity of the tracedistance under CPTP maps, we may certainly write
(71)  
(72)  
(73) 
Therefore, from Eq. (48) we have that, for any finite energy , we may write
(74) 
In other words, for any (telecovariant) bosonic channel , its teleportation simulation converges to in energybounded diamond norm. The question is: Can we remove the energy constraint? In the next section we completely characterize the condition that a bosonic Gaussian channel needs to satisfy in order to be simulated by teleportation according to the uniform topology (unconstrained diamond norm).
V Uniform convergence in the teleportation simulation of bosonic Gaussian channels
Let us now consider the convergence of the teleportation simulation in the uniform topology, i.e., according to the unconstrained diamond norm (). As we already know, this is a property that only certain bosonic channels may have. The simplest counterexample is certainly the identity channel for which the teleportation simulation via the BK protocol strongly but not uniformly converges. See Eqs. (43) and (49). As we will see below, this is also a problem for many Gaussian channels, including all the channels that can be represented as Gaussian unitaries, and those that can be reduced to the canonical form via unitary transformations. The theorem below establishes the exact condition that a singlemode Gaussian channel must have in order to be simulated by teleportation according to the uniform topology.
Theorem 2
Consider a singlemode bosonic Gaussian channel and its teleportation simulation
(75) 
where is the LOCCof a modified BK protocol implemented over the resource state, with being a TMSV state with energy . Then, we have uniform convergence
(76) 
if and only if the noise matrix of the Gaussian channel has full rank, i.e., .
Proof. Let us start by showing the implication
(77) 
Consider an arbitrary singlemode Gaussian channel , so that it transforms the statistical moments as in Eq. (19). As we know from Eq. (63), for any input state , we may write
(78)  
(79)  
(80) 
where is the LOCC of the standard BK protocol and is the BK channel, which is locally equivalent to an additivenoise Gaussian channel ( form) with added noise as in Eq. (53). Therefore, for the Gaussian channel we may write the modified transformations
(81) 
As we can see, the transformation of the first moments is identical. By contrast, the transformation of the second moments is characterized by the modified noise matrix
(82) 
In order words, we may write .
Because and have the same displacement, we can set without losing generality. Consider the unitary reduction of into the corresponding canonical form by means of two Gaussian unitaries and as in Eq. (20). Because , we may assume that these unitaries are canonical (i.e., with zero displacement), so that they are onetoone with two symplectic transformations, and , in the phase space. To simplify the notation define the Gaussian channels
(83) 
Then we may write
(84)  
(85) 
Then notice that we may rewrite
(86) 
where we have defined
(87) 
In Appendix A we prove the following.
Lemma 3
Consider a Gaussian channel with and . Then and have the same unitary dilation but different environmental states and , i.e., for any input state we may write
(88) 
where with unitary. Furthermore
(89) 
Using this lemma in Eqs. (84) and (86) leads to
(90)  
(91) 
Clearly these relations can be extended to the presence of a reference system , so that for any input , we may write
(92)  
(93) 
As a result for any , we may bound the trace distance as follows
(94)  
(95)  
(96)  
(97) 
where we use: (1) the monotonicity under CPTP maps (including the partial trace) (2) multiplicity over tensor products; and (3) one of the Fuchsvan der Graaf relations. This is a very typical computation in teleportation stretching PLOB () which has been adopted by several other authors in followup analyses.
As we can see the upperbound in Eq. (97) does not depend on the input state . Therefore, we may extend the result to the supremum and write
(98) 
Now, using Eq. (89), we obtain
(99) 
proving the result for and , i.e.,
(100) 
Let us now remove the assumption . Note that the Gaussian channels with and are those unitarily equivalent to the form with added noise