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quantum key distribution ( qkd ) has been the first practical realization of communication based on the laws of the quantum mechanics . as bennett and
brassard showed with their pioneer protocol bb84
@xcite , nonorthogonal quantum states , together with an unjammable classical channel , can be used to prevent an eavesdropper ( eve ) from gaining information on the key without being revealed .
there are already different working realizations of bb84  @xcite .
some of them  @xcite exploit entanglement in polarization to encode a random string of bits in two unbiased polarization bases of a single photon , thus following the original proposal . however , polarization is not the most suitable degree of freedom for fiber communication because birefringence usually fluctuates randomly in a fiber , making the encoded information impossible to decode .
the most appealing setup for real telecom communications is based on the phase degree of freedom .    as an example we consider the setup shown in the upper part of fig.[bb84 ] , that has been used for a qkd up to 30 km  @xcite . in this arrangement a relative phase @xmath0 between two time - bins of a photon state is realized by the sender ( alice ) with an unbalanced interferometer and a phase modulator .
the receiver ( bob ) measures incoming bins by means of a second interferometer , matched to the sender s one , set to @xmath1 . with a probability of 50% the phase difference between alice and
bob s interferometers will be @xmath2 or @xmath3 , and their measures will be correlated ; in the other half of cases measures will be discarded by the two users .
schematics of two interferometers for bb84 .
the upper one , in blue , has been used for qkd up to 30 km ; the lower one , in red , is the plug - and - play setup used for qkd up to 67 km . ]
another realization of phase encoded qkd , namely the  plug - and - play " setup  @xcite , is reported in the lower part of fig.[bb84 ] , and it has been used for qkd up to a distance of 67 km  @xcite . using an intense pulse ,
bob populates two time - bins with relative phase @xmath4 , and sends them to alice .
alice applies a relative phase @xmath0 , reflects the pulses on a faraday mirror , let them pass through an attenuator , and sends them back to bob .
the two pulses retrace in their backward path all the fluctuations they suffered in the forward path , thus arriving at bob s interferometer ready for being interferometrically revealed . as in the previous scheme bob sets at random @xmath1 , and
measures the outcomes , completing the _
qkd run pertaining to bb84 .
this mechanism of automatic compensation of noise earned this setup the name of  plug - and - play " .
recently bostrm and felbinger  @xcite presented a variant for qkd , and named it  ping - pong " cryptography ( pp ) for its peculiarity of a forward and backward use of the quantum channel .
this peculiarity seems to suit perfectly with the forward and backward dynamics typical of plug - and - play setups , as the one described above , and this motivated the present work .
the main advantage of pp is the deterministic nature of its _ encoding - decoding _ procedure ; its main flow is that it has been proved to be not completely secure  @xcite .
a number of variants , still based on the polarization degree of freedom of a photon , have been proposed to solve this problem , some making use of entanglement  @xcite , and others making no use of it  @xcite .
we envisage here a protocol with phase - encoded information to have a secure , completely deterministic , quantum communication .
a few variants and applications of the protocol are also discussed .
the receiving user , bob , prepares a photon in a superposition of two time - bins with a relative phase @xmath5 randomly chosen between the values @xmath6 .
he sends the photon to alice , the transmitting user , who chooses one of two possible tasks , _ control mode _ ( cm ) or _ message mode _ ( mm ) , with probability @xmath7 and @xmath8 respectively ; the former realizes a control on the security of the channel , the latter the deterministic communication between the users .
alice encodes a bit of information with an unitary operation on the two time - bins : she can either apply the identity operation , choosing @xmath9 and encoding a ` 0 ' , or introduce a ` phase - flip ' , by applying a phase @xmath10 between the time - bins and encoding a ` 1 ' .
the photon is sent back to bob who measures it with his apparatus set with the same phase @xmath5 he prepared initially .
in this way bob s measurement is _ deterministic _ , because the initial phase of the state ( that he does know ) is changed by an amount of 0 or @xmath3 ; he can thus guess alice s operation without any needs of a classical channel .
alice detects the photon with her interferometer randomly set to @xmath9 or @xmath11 .
after that alice prepares a _ new photon state _ with a phase @xmath12 , and sends it back to bob who , analogously to what he did in mm , measures it with the same phase @xmath5 he prepared initially .
we notice that if phase difference between bob s interferometer and alice s one is @xmath2 or @xmath3 then the two users share correlated information , while in the other two cases they do not .
it is straightforward to realize that alice procedure in preparing the new photon let the two users interferometers to be necessarily correlated either in the forward path or in the backward one .
this entails that the cm of our protocol results as deterministic as the mm is , thus achieving that doubling of the whole rate transmission we mentioned before .
none of the qubits , neither destined to mm nor to cm , is discarded .
given un unjammable public channel two users can exchange information in a secure manner by means of the above protocol .
eve can try to gain information by inquiring the phase of the photon both on the forward and in the backward path ; alternatively , she can prevent bob from gaining information ( dos attack  @xcite ) by randomly measuring the state of the traveling photon ; finally , on a lossy channel , she can conceal her presence behind losses  @xcite .
nevertheless no attack can remain undetected by the control procedure since it is akin to a bb84 check test , performed either on the forward path or on the backward one .
this guarantees the protocol is unconditionally secure , as bb84 is , even if the security threshold may be different .
our protocol gives a probability of 25% to detect dos attack as well as a general attack providing eve with full information .
the practical implementation requires to gather the two interferometers of fig.[bb84 ] .
the resulting scheme is shown in fig.[pppp84 ] : the upper interferometer is devoted to cm , the lower to mm , and they are connected by a 1x2 fiber - coupler .
the signal prepared by bob goes at random into cm or into mm with probability @xmath7 and @xmath8 , respectively .
later on , when the photon is on the backward path , this device redirects it to bob .     final setup for deterministic plug - and - play cryptography .
it merges the two interferometers of fig.1 , reported in blue ( cm ) and red ( mm ) . the black represents the common elements between the two , while the green indicates new components .
an automatic switch between mm and cm is performed by a 1x2 coupler . ]
the mm - interferometer resembles the one used for testing bb84 up to 67 km  @xcite .
it is a typical plug - and - play scheme , with the unique difference that the attenuator is inserted just after the laser - diode ( ld ) , at bob s side .
a laser pulse with a mean photon number much lower than unity goes through bob s interferometer and comes out into two time - bins with a relative phase @xmath5 and opposite polarizations . in the plug - and - play setup
an intense pulse is sent to alice to provide a trigger signal for alice s electronics . in our case , as the pulse is already attenuated at bob s side , a further synchronization system , such as usually used in other practical qkd setup  @xcite , is necessary .
after reflection on the faraday mirror , the photon travels back to bob , retracing exactly those paths that let him compensate any undesired phase : this makes unnecessary the stabilization of the polarization . eventually , raileigh backscattering is not a problem in this case because of the low value of the average photon number from the beginning of the protocol .
the cm - interferometer resembles the one used for bb84 qkd up to a distance of 30 km  @xcite . as a time - polarization division technique
is implemented , this task is sensible to random phase changes , and needs adjustments during the protocol runs .
a value of @xmath13   rad / min for slow thermal drift in the interferometer was estimated , thus requiring a compensation step every @xmath14  @xcite .
also in this case alice must synchronize her electronics with detectors , phase - modulator , and laser - diode using a suitable synchronization system .
we proposed a setup that fully exploits the potentialities of the two - way unbalanced interferometers of certain implementations of bb84 to merge the features of plug - and - play setup for bb84 and ping - pong cryptography .
the scheme is completely deterministic , both in the encoding - decoding procedure and in the control one , thus achieving a doubling in the rate of information transmission .
it seems to be quite practical to implement since it is based on existing working implementations .    besides , due to its deterministic nature , several other advantages are available .
bases revelation on the public channel is avoided , and this feature , together with a slightly different alice s preparation procedure during the cm , leads to a qkd more secure than that achieved by means of bb84  @xcite .
furthermore a ` quantum direct communication ' ( qdc ) between users is also possible  @xcite .
finally , we notice that once qdc is available also the novel protocol called ` quantum dialogue '  @xcite can be achieved : if alice delivered a message to bob by means of a secure qdc , then bob can use the message just received as the key of encryption of his answer , and simply communicate it to alice on the public channel .                    g. l. long and x. s. liu , phys .
a * 65 * , 032302 ( 2002 ) ; f .-
deng , _ et al .
a * 68 * , 042317 ( 2003 ) ; i. p. degiovanni , _ et al . _ ,
phys . rev .
a * 69 * , 032310 ( 2004 ) ; q .- y . cai and b .- w .
li , quant - ph/0311168 ( 2003 ) .
we present a scheme for secure deterministic quantum communication without using entanglement , in a plug - and - play fashion . 
 the protocol is completely deterministic , both in the encoding procedure and in the control one , thus doubling the communication rate with respect to other setups ; moreover , deterministic nature of transmission , apart from rendering unnecessary bases revelation on the public channel , allows the realization of protocols like ` direct communication ' and ` quantum dialogue ' . 
 the encoding exploits the phase degree of freedom of a photon , thus paving the way to an optical fiber implementation , feasible with present day technology .