document: a quantum computer will allow to perform some algorithms much faster than in classical computers e.g. shor algorithm for the factorization the numbers @xcite . 
   the basic elements in the quantum computation are qubits and quantum logical gates , which allow to construct any circuit to quantum algorithms . 
   the good candidates to realization of qubits are semiconductor quantum dots with controlled electron numbers . 
   the qubit state can be encoded using an electron charge or , which is also promising , an electron spin @xcite . 
   the spin qubits are characterized by longer decoherence times necessary in the quantum computation @xcite . however to prepare that qubit one needs to apply a magnetic field and removed the degeneracy between spin up and down . 
   the manipulation of the qubit can be done by electron spin resonance and the read - out via currents in spin - polarized leads @xcite . another concept to encode the qubit 
   is based on the singlet - triplet states in a double quantum dot ( dqd ) . in this case 
   the magnetic field is not necessary and the qubit preparation is performed by electrical control of the exchange interactions @xcite . 
   the qubit states can be controlled by e.g. an external magnetic field @xcite , spin - orbit @xcite or hyperfine interaction @xcite . for the read - out of the qubit state one can use current measurement and the effect of pauli spin blockade @xcite . in the pauli blockade regime 
   the current flows only for the singlet , which gives information about the qubit states . 
   divincenzo _ et al _ @xcite suggested to build the qubit in more complex system , namely in three coherently couplet quantum dots ( tqd ) . 
   the qubit states are encoded in the doublet subspace and can be controlled by exchange interactions . 
   this subspace was pointed as a decoherence - free subspace ( dfs ) @xcite , which is immune to decoherence processes . 
   another advantage of this proposal is the purely electrical control of the exchange interactions by gate potentials which act locally and provide much faster operations . in the tqd system , in the contrast to the dqd qubit , one can modify more than one exchange interaction between the spins and perform full unitary rotation of the qubit states @xcite . 
   the three spin qubit has also more complicated energy spectrum which provides operations on more states in contrast to the two spin system . 
   recently experimental efforts were undertaken @xcite to get coherent spin manipulations in a linear tqd system according to the scheme proposed by divincenzo _ 
   et al _ @xcite . 
   the initialization , coherent exchange and decoherence of the qubit states were shown in the doublet @xcite and doublet - quadruple subspace @xcite . 
   the read - out of the qubit state was performed , like in dqd , by means of the pauli blockade @xcite . 
   _ @xcite observed a quadruplet blockade effect which is based on reducing leakage current from quadruplet to triplet states in the presence of magnetic field . 
   @xcite showed that divincenzo s proposal can be realized on double quantum dots with many levels and three spin system controlled by gate potentials .    in this paper 
   we demonstrate that tqd in a triangular geometry can work as a qubit . 
   this kind of tqd was already fabricated experimentally by local anodic oxidation with the atomic force microscope @xcite and the electron - beam lithography @xcite . in the triangular tqd 
   qubit exchange interactions between all spins are always on and very important is symmetry of the system . 
   trif et al . 
   @xcite and tsukerblat @xcite studied an influence of the electric field on the symmetry of triangular molecular magnets and spin configurations in the presence of a spin - orbit interaction . 
   divincenzo s scheme to encode the qubit in triangular tqd was considered by hawrylak and korkusinski @xcite where one of the exchange coupling was modified by gate potential . 
   recently georgeot and mila @xcite suggested to build the qubit on two opposite chiral states generated by a magnetic flux penetrating the triangular tqd . 
   one can use also a special configuration of magnetic fields ( one in - plane and perpendicular to the tqd system ) to encode a qubit in chirality states @xcite . 
   recent progres in theory and experiment with tqd system was reported in @xcite . 
   our origin idea is to use the fully electrical control of the symmetry of tqd to encode and manipulate the qubit in the doublet subspace . 
   the doublets are vulnerable to change the symmetry of tqd , which will be use to prepare and manipulate the qubit ( sec . 
   [ preparation ] ) . 
   the crucial aspect in quantum computations is to read - out the qubit states . 
   here we propose a new detection method , namely , a doublet blockade effect which manifests itself in currents for a special configuration of the local potential gates . 
   we show ( sec . [ detection ] ) that the doublet blockade is related with an asymmetry of a tunnel rates from source and drain electrodes to tqd and the inter - channel coulomb blockade . 
   the method is fully compatible with purely electrical manipulations of the qubit . 
   next we present studies of dynamics of the qubit and demonstrate the coherent and rabi oscillations ( sec . 
   [ dynamics ] ) . 
   the studies take into account relaxation and decoherence processes due to coupling with the electrodes as well as leakage from the doublet subspace in the measurement of current flowing through the system . 
   we derive characteristic times which describe all relaxation processes . 
   our model is general and can be used for a qubits encoded also in the linear tqd , which is a one of the cases of broken symmetry in the triangular tqd . 
   our system is a triangular artificial molecule built of three coherently coupled quantum dots with a single electron spin on each dot ( see . 
   fig.[fig1 ] ) . 
   interactions between the spins are described by an effective heisenberg hamiltonian @xmath0 where the zeeman term is included to show splitting by an external magnetic field @xmath1 ( @xmath2 is the bohr magneton , _ g _ is the electron g - factor ) and @xmath3 is an exchange interaction between electrons on sites @xmath4 and @xmath5 
   .    the exchange parameter can be calculated by heitler - london and hund - mulliken method . for a defined confinement potential 
   one can find the parameter as a function of the interdot distance , the potential barrier and the magnetic field @xcite .    for the system with three spins 
   there are two subspaces , one of them is a quadruplet with the total spin @xmath6 and @xmath7 . 
   the quadruplet states are given by : @xmath8 . 
   the second subspace is formed by doublet states with @xmath9 and @xmath10 . the doublet state for @xmath11 
   can be expressed as : @xmath12 @xmath13 and @xmath14 denote a singlet and triplet state on the @xmath15 bond , respectively . here 
   we assume large coulomb intradot interactions and ignore double electron occupancy . 
   in the doublet subspace ( [ d1])-([d2 ] ) one can express the hamiltonian ( [ heisenberg ] ) as @xmath16 using the pauli matrix representation . 
   the parameters are given by @xmath17 the eigenvalues of ( [ matrix2 ] ) are : @xmath18 where @xmath19 is the doublet splitting and @xmath20 describes the energy differences between the doublet and the quadruplet subspace . 
   two other parameters @xmath21 and @xmath22 can be interpreted as an effective magnetic field in the _ z _ and _ x _ direction , respectively @xcite .    in gaas 
   / algaas quantum dots the exchange interaction @xmath20 is estimated in the range @xmath23 mev @xcite and in a molecular magnet as @xmath24 mev @xcite . the doublet splitting for a linear tqd is the order @xmath25 
   @xmath26ev @xcite . 
   this parameter can be even larger @xmath27 @xmath26ev in si / sige quantum dots @xcite . 
   in this paper we assume that the exchange couplings @xmath3 can be manipulated by local potential gates @xmath28 , which change potential barriers and modify electron hopping as well as local covalency between the quantum dots . 
   the exchange coupling can be expressed in the linear approximation as @xmath29 , where @xmath30 describes sensitivity of the exchange coupling to the gate voltage . for our analysis of the symmetry breaking in tqd , 
   it is more suitable to parameterize the gate potentials as @xmath31 $ ] with @xmath32 , some amplitude @xmath33 and angle @xmath34 . 
   this parametrization corresponds to influence of an effective electric field @xmath35 on the bond polarization and covalency . for a small value of * e * 
   the exchange couplings can be expressed as @xmath36,\end{aligned}\ ] ] where @xmath37 , @xmath38 is a parameter describing sensitivity of the exchange coupling to the electric field , @xmath39 - the elementary electron charge , 
   @xmath40 - the vector showing the position of the @xmath4-th quantum dot , @xmath34 is the angle between @xmath35 and the axis @xmath41 ( see fig.[fig1 ] ) . 
   a similar relation was obtained for the triangular molecule in the electric field which changed chirality of the spin system @xcite . 
   let us stress that because the electric field is taken as the small parameter , single electron occupancy of each dot is conserved and the ground state is always the doublet .    in the tqd system one can also consider superexchange processes through excited double occupied states . applying local potential gates to the quantum dots 
   one can shift their energy levels and modify the superexchange couplings @xcite . because a parameter of inter - dot electron hopping is relatively small with respect to a intra - dot coulomb interaction , the modifications of the superexchange couplings are very small and 
   will not be discussed in the paper . 
   let us consider how to encode the qubit in the doublet subspace with the spin @xmath11 , expressed by @xmath42 . 
   this state is isolated from the doublet @xmath43 and the quadruplets states for a moderate magnetic field , @xmath44 . 
   the encoded qubit states @xmath45 and @xmath46 correspond to the doublets @xmath47 and @xmath48 , eq.([d1 ] ) and ( [ d2 ] ) ( the spin index @xmath49 is omitted to simplify the notation ) . in the further considerations the hyperfine and spin - orbit interactions 
   are ignored . 
   the qubit is prepared by a proper orientation @xmath34 of the effective electric field @xmath50 which changes the symmetry of the system . 
   [ eigenvectors ] presents density matrix elements @xmath51 and @xmath52 as a function of @xmath34 . 
   one can see that the qubit is prepared in the state @xmath47 for @xmath53 ( the electric field is oriented from the quantum dot 1 ) . for this symmetry 
   the exchange parameters are @xmath54 and from eq . 
   ( [ delta ] ) and ( [ gamma ] ) one gets @xmath55 and the mixing between the doublets @xmath56 . for @xmath57 
   the electric field points to the quantum dot 1 and @xmath58 , @xmath59 and @xmath56 . in this case 
   the qubit is prepared in the state @xmath48 . 
   we would like to emphasize that in the triangular tqd the both qubit states are equivalent and can be easily achieved only by change the symmetry of the system . 
   this is the main advantage in comparison with the linear tqd where the qubit can be prepare usually only in one of the doublet state . 
   ( solid black curve ) and @xmath52 ( dashed red curve ) vs @xmath34 for the ground state at a moderate value of the electric field @xmath60 . 
   the left and right inserts shows direction of the electric field when the qubit is prepared in the states @xmath48 and @xmath47 , respectively . 
   these two opposite directions of the electric field corresponds to a minimal ( thin line ) and maximal ( thick line ) bond polarization between the dots 2 and 3.,scaledwidth=30.0% ]    now we show how one can perform one qubit operations by means of the electric field . 
   after preparation of the qubit in one of the states @xmath47 or @xmath48 we change rapidly the angle @xmath34 to perform a dynamic rotation of the qubit state . 
   the qubit dynamics is described by the time - dependent schrdinger equation with the hamiltonian ( [ matrix2 ] ) . 
   we can show two basic quantum gates . 
   for @xmath61 the pseudo - spin rotates around the x - axis on the bloch sphere , which is the pauli - x quantum gate and the solution of the schrdinger is given by @xmath62 $ ] is an unitary operator of rotation around the x - axis . 
   second quantum gate we get for @xmath56 with the solution given by ( [ h2 ] ) but now instead @xmath63 we have @xmath64 $ ] which is an unitary operator of rotation around the z - axis . 
   these two rotations can be use to get to any point on the bloch sphere . 
   it is clearly seen that by modification of the parameters @xmath22 and @xmath21 one can get full control of the qubit operations ( see also @xcite ) . 
   in the linear tqd the read - out of the qubit state is possible due to charge - spin conversion in the regime of the pauli spin blockade . 
   a detuning voltage is applied between two outermost dots , which drives the system from single occupied configuration ( 1,1,1 ) to double occupied e.g. ( 2,0,1 ) . 
   this transfer is possible for an electron with the opposite spin orientation and can be detected by a quantum point contact ( qpc ) @xcite .    in this section 
   we would like to show a new method to read - out the qubit state which is based on a measurement of currents flowing through the system . 
   the detection is compatible with electrical control of qubit state and the charge - spin conversion is not necessary . 
   we assume that tqd is coupled by tunnel junctions to the electrodes , where the first and the second dot are connected to the left and right electrode , respectively . 
   application of this method in an experimental setup is a similar technical complexity as qpc . 
   the electron transport through the tunnel junctions is studied within the sequential tunneling regime . 
   transfer rates from the left ( l ) and the right ( r ) electrode to tqd are given by : @xmath65 here we assume that both tunnel barriers are characterized by the same parameter @xmath66 , the reduced planck constant is taken @xmath67 , @xmath68 and @xmath69 denote the states with two and three electrons with the corresponding energies @xmath70 and @xmath71 , @xmath72 is an electron creation operator on the dot 1 ( 2 ) with spin @xmath73 . _ 
   f _ denotes the fermi distribution function , the electrochemical potentials in the left and the right electrode are @xmath74 and @xmath75 , where @xmath76 is the fermi energy and @xmath77 is an applied bias voltage . by analogy 
   one can define transfer rates @xmath78 from tqd to the electrodes . 
   we confine our considerations to a voltage window with transitions between the states with three and two electrons , but a similar situation one can expect for transitions between three and four electron states . 
   two electron states @xmath68 can be either the singlet @xmath79 or triplet @xmath80 . for a high intra - dot coulomb interaction one can neglect double occupied states and confine considerations to the states with single electron occupancy only . the singlet can be then expressed as a linear superposition : @xmath81 , where @xmath82 denotes the singlet on the @xmath83 pair of dots . 
   calculating the elements of the transfer matrices one can find net transfer rates between the doublet @xmath84 , @xmath85 and the singlet @xmath79 : latexmath:[\ ] ]    @xmath195 has a lorentzian dependence and reaches its maximum for a resonance condition @xmath196 . in this case the doublet blockade is removed and higher current flows through the system .     at the resonance condition ( @xmath196 ) . 
   the notation and the parameters are the same as in fig . 
   [ singdynamic ] . 
   notice that the results are presented in the rotating frame , whereas fig . 
   [ singdynamic ] presents the evolution in the laboratory frame . 
   the relaxation rates are @xmath149 , and @xmath151 . ] 
   [ singdynamicrez ] shows the density matrix elements , the currents and the oscillations on the bloch sphere for the driven case in the resonance ( @xmath196 ) . 
   we take the same parameters as for fig . 
   [ singdynamic ] but now the results are presented in the rotating frame . 
   one can see the large rabi oscillations of @xmath51 and @xmath52 . 
   the population of the states is changed alternately between @xmath47 and @xmath48 with the frequency @xmath197 . 
   it is also seen in fig . 
   [ singdynamicrez]c which presents the rotation of the pseudo - spin on the bloch sphere . 
   the rotation is in the @xmath198-@xmath199 plane on the spiral with periodic transfers between the states @xmath51 and @xmath52 . in the stationary limit one gets @xmath200 and @xmath201 , which is a higher value than in the non - driven case with @xmath202 ( see fig . [ singdynamic ] ) . 
   the relaxation times @xmath166 and @xmath167 are almost the same as for the time independent case due to their weak dependance on @xmath22 and @xmath21 . 
   the current plots in fig . 
   [ singdynamicrez]b ) present strong oscillations which corresponds to coherent switching between the doublet states ( rabi oscillations ) . 
   the leakage current flowing through the right junction @xmath203 also shows some oscillations . the stationary current ( dashed line ) is larger than for the non - driven case as one may expect when the doublet blockade is removed . 
   switched on at @xmath152 . 
   panel a ) presents the time evolution of the density matrix elements , while b ) presents the current flowing through the left ( blue curve ) and right junction ( black curve ) . 
   the black - dashed line corresponds to the current in the stationary limit . 
   panel c ) shows the evolution of the pseudo - spin on the bloch sphere . 
   the initial state is @xmath48 ( the black arrow ) , and the final state is represented by the green arrow . 
   the parameters taken in the calculations : @xmath204 , @xmath205 the other parameters are the same as in fig.[blockadet ] . 
   the relaxation rates for these parameters are @xmath206 , @xmath207 , @xmath208 , and @xmath209 . 
   notice that in the present case the currents are smaller than those ones in fig.[singdynamic]b because now the transfer rates are different . ] 
   one can expect similar dynamics when the two - electron ground state is triplet . 
   here we still confine ourselves to the doublets with @xmath11 and ignore spin - flip processes . now in the voltage window ( see the inset in fig.[trypdynamic ] ) we have the states @xmath210 , @xmath211 and @xmath212 whereas @xmath213 is below the chemical potential in the left electrode . 
   the master equation ( [ dme ] ) is rewritten in the form @xmath214 here the population of the states is built - up by the transfers @xmath215 , @xmath216 and @xmath217 from the right electrode . 
   electrons escape from the system to the left electrode which is described by @xmath218 and @xmath219 . 
   the currents flowing from the right and the left electrode are expressed as @xmath220 notice that now the role of the doublet states @xmath47 and @xmath48 is reversed , and current flows through @xmath48 while @xmath47 is the dark state and blocks electron transport - see eq.([dtot1 ] ) . in the bloch space we have : @xmath221 one can easily find the main contributions to the relaxation rates : @xmath222 , which is qualitatively similar to the previously considered case with the singlet state but now the transfer rates are different [ compare eq.([dtos])-([dtos2 ] ) with eq.([dtot1])-([dtot4 ] ) ] . 
   the leakage to the triplet state is given by @xmath223 which has an additional term @xmath224 describing transfer from the triplet to quadruplet state . 
   the last row in eq.([master - dt ] ) and ( [ bloch - trip ] ) describes another leakage process with the relaxation rate @xmath225 . 
   this process changes the population of the triplet and the quadruplet state and indirectly influences dynamics in the doublet subspace .    solving these equations we determine the occupation probabilities and consequently the currents : @xmath183 and @xmath180 . 
   the results are presented in fig . 
   [ trypdynamic ] for the time independent mixing term , @xmath226 for @xmath154 and @xmath48 as the initial state . on the top panel 
   one can see coherent oscillations for @xmath51 and @xmath52 similar as for the case with the singlet state . 
   however , due to the quadruplet state the dynamic of the doublets and their final occupation is different than for the singlet case . in the short time 
   range the population @xmath227 and @xmath228 increases due to leakage from the doublet subspace with characteristics rates @xmath167 and @xmath229 . 
   in the longer time scale the population @xmath227 goes to @xmath228 ( in the stationary limit @xmath230 ) . the influence of these states on the doublet dynamics is clearly seen in the currents @xmath231 and @xmath203 presented in fig . 
   [ trypdynamic]b . 
   @xmath183 shows different behavior than in the singlet case , because the doublet blockade is modified by the quadruplet state which gives an additional contribution to the current . 
   the increase of the current @xmath180 at the short time scale is related with the leakage to the triplet state . for longer times 
   @xmath180 is diminished but the quadruplet contribution makes the drop less pronounced than in the singlet case .    the bottom panel , fig . 
   [ trypdynamic]c , presents the doublet dynamics on the bloch sphere . 
   the behavior of the bloch vector is similar as in the singlet case with some quantitative differences . 
   the phase and the amplitude damping is smaller which is the result of the longer relaxation times in the considered case .      in the considerations above 
   we have taken into account only charge fluctuations on the evolution of the qubit . 
   let us now extend the studies and include spin - flip processes . 
   a spin of an electron captured on a quantum dot can interact with nuclear spins of many atoms confined in the area of the quantum dot , which can lead to decoherence of the qubit states . 
   the decoherence processes due to hyperfine interaction in triangular spin clusters has been already investigated by troiani et al . 
   @xcite . here 
   we consider another decoherence process caused by the spin relaxation in the electrodes . 
   the electrodes connected to tqd are paramagnetic and electron can be injected with spin up to the state @xmath232 or with spin down to @xmath233 . 
   this stochastic process leads to mixing between two doublet subspaces . 
   the evolution of the qubit is studied in the absence of the magnetic field . 
   we assume that the qubit is prepared in the state @xmath210 with the spin @xmath141 and the singlet is the ground state for two electrons . 
   similarly as in in the previous cases the qubit dynamics is govern by the master equation ( [ dme ] ) , but now we take into account states with different spin orientation @xmath234 , @xmath235 and @xmath79    @xmath236    where the transfer rates are the same for both spin orientations : @xmath237 and @xmath238 . 
   one can see that the master equation ( [ master - spin - flip ] ) represents dynamics of two doublet subspaces with @xmath11 and @xmath239 . 
   the subspaces are mixed with each other by transfers to the singlet state [ see fourth equation in ( [ master - spin - flip ] ) ] . 
   these two subspaces correspond to two pseudo - spin vectors on two bloch spheres . 
   each of the subspace is described by the master equation eq . 
   ( [ bloch - sing ] ) but the relaxation rates are now : @xmath240 and the leakage process @xmath241 , which is twice larger than in the case without spin - flip due states degeneracy . 
   we make the laplace transformation of the master equation ( [ master - spin - flip ] ) and find the relaxation rates from the poles of the polynomial @xmath242 . here 
   the polynomial @xmath243 is the same as for the previous case [ described by eq . 
   ( [ bloch - sing ] ) ] with the transfer rates including degeneracy of the doublet states . 
   the second polynomial @xmath244 $ ] is related with the spin flip - processes which mix two doublet subspaces . from @xmath245 one 
   finds the spin - flip relaxation times @xmath246 in the limit of weak coupling with the electrodes . 
   the rate @xmath247 is the second largest rate , after leakage and describes a rapid relaxation process , which can be seen in the short time scale . 
   @xmath248 corresponds the longest relaxation process which leads to total mixing of two doublet subspaces in the stationary limit . 
   the dynamics in the doublet subspace including spin - flip processes is presented in fig . 
   [ spinflip]a . 
   the time evolution of @xmath249 is similar to the case without spin - flip ( compare with fig . 
   [ singdynamic ] ) , but now its reduction in the short time scale is faster due to @xmath250 . 
   at the same time the @xmath251 is built - up with the rate @xmath250 , and goes to the stationary limit @xmath252 with the rate @xmath253 . 
   the qubit dynamics can be also seen in the spin current flowing through the left @xmath254 and right junction @xmath255 
   see fig.[spinflip ]  b. the shape of the total current @xmath256 is very similar as in the case without spin - flip . to get 
   more information one needs to measure the spin dependent currents @xmath257 . 
   the dashed and dotted blue curves in fig . 
   [ spinflip ]  b show @xmath258 and @xmath259 with the characteristic times @xmath260 in the short and long time scale . 
   the fast increase of the total current in the right electrode @xmath180 for the very short time scale is related with @xmath178 which now is two times shorter . 
   ( blue dotted curve ) , @xmath261 ( blue dashed curve ) and right junction with spin @xmath262 ( black dashed curve ) . 
   notice that @xmath263 . 
   the total currents @xmath264 are plotted as a solid lines . 
   the parameters taken in the calculations are the same as in fig . 
   [ singdynamic ] . 
   the relaxation rates are @xmath265 , @xmath150 , @xmath151 , @xmath266 , and @xmath267 . ] 
   let us estimate the characteristic times calculated in the paper . 
   in the first order of approximation the relaxation times are proportional to @xmath268 defined in equation ( [ transferrates ] ) . 
   the tunnel rate @xmath66 in ( [ transferrates ] ) is the order of nev for sequential transport @xcite 
   . however it can be much larger in the coherent regime , 
   @xmath269 @xmath26ev @xcite . 
   if we assume @xmath270 @xmath26ev for our system then the relaxation and decoherence time is @xmath271 ns and @xmath272 ns for the case with singlet . 
   for triplet we have @xmath273 ns and @xmath274 
   ns . the leakage to the singlet and triplet states are @xmath275 ns and @xmath276 ns respectively . 
   these relaxation times are the same order as the decoherence time @xmath277 ns due to hyperfine interaction in gaas - based quantum dots @xcite . for the spin - flip 
   processes the relaxation times are : @xmath278 ns and @xmath279 ns . 
   one can see that due to the long relaxation time @xmath280 the qubit conserves its spin coherence for a time needed for a read - out process . 
   summarizing we have proposed the qubit controlled by a symmetry breaking effect in a triangular tqd system . 
   the main result of the paper is the new method for read - out of the qubit state by the current measurement in the doublet blockade regime , and the analysis of the qubit dynamics in the presence of decoherence processes caused by interaction with the electrodes . 
   we assumed that each dot contains one spin and the qubit was encoded in the doublet subspace . 
   the qubit states has been controlled by the applied gate potentials which break the triangular symmetry . 
   the calculations have been performed in the the heisenberg model where the exchange couplings are modified by the orientation @xmath34 of the electric field with respect to the triangular axes . for a specific @xmath34 one of the doublets 
   is occupied and can be taken as an initial qubit state for further manipulations . by quick impulses of the electric field 
   one can perform the pauli x - gate and z - gate operations . 
   a composition of these two operations gives full unitary control of the single qubit . 
   moreover we have demonstrated the new method to read - out of the qubit states using the electric transport through tqd and the doublet blockade effect . 
   the method is compatible with pure electrical manipulations and the spin - to - charge conversion is not necessary . the doublet blockade effect is related with an asymmetry of tunnel rates between the doublet states and the electrodes . for some specific symmetry of tqd one of the doublet states is a dark one and the electron transport is blocked . 
   we have considered two cases with the singlet and the triplet as a ground state for two electrons . for the singlet case 
   the current is blocked due to the doublet @xmath48 , whereas for transport from the triplet the dark state is the doublet @xmath47 . 
   the doublet blockade can be also used to detect the qubit states in the linear tqd . 
   however to satisfy the blockade condition @xmath56 one of the electrodes must be connected to the central dot . 
   moreover the blockade can be applied to dynamical initialization of the qubit state as well as to perform landau - zener passages @xcite . 
   we have also considered the time dependent electron transport in the doublet blockade regime . 
   our research gives information about dynamics of the qubit , the coherent oscillations and the relaxation processes due to presence of the electrodes . 
   a role of the leakage processes from the doublet to two electron states has been studied as well . for the triplet case 
   the leakage is larger than for singlet due to activation of the quadruplet state . 
   we have also presented the driven case where the mixing parameter between the doublet state is time dependent @xmath281 . in the resonance condition 
   @xmath196 the doublet blockade is partially removed and one can observe strong rabi oscillations . 
   moreover we have investigated mixing of the doublet subspaces with @xmath11 and @xmath239 caused by the spin - flip processes in the electrodes . 
   the total mixing time @xmath280 is very long what is promising for manipulation and read - out of the qubit . 
   z. shi , c. b. simmons , d. r. ward , j. r. prance , r. t. mohr , t. s. koh , j. k. gamble , x. wu , d. e. savage , m. g. lagally , m. friesen , s. n. coppersmith , and m. a. eriksson , phys . rev . 
   b * 88 * , 075416 ( 2013 ) .
summary: we present a model of a qubit built of a three coherently coupled quantum dots with three spins in a triangular geometry . 
   the qubit states are encoded in the doublet subspace and they are controlled by a gate voltage , which breaks the triangular symmetry of the system . 
   we show how to prepare the qubit and to perform one qubit operations . 
   a new type of the current blockade effect will be discussed . 
   the blockade is related with an asymmetry of transfer rates from the electrodes to different doublet states and is used to read - out of the dynamics of the qubit state . 
   our research also presents analysis of the rabi oscillations , decoherence and leakage processes in the doublets subspace .