Observation of coherent oscillation in single-passage Landau-Zener transitions

Landau-Zener transition (LZT) has been explored in a variety of physical systems for coherent population transfer between different quantum states. In recent years, there have been various proposals for applying LZT to quantum information processing because when compared to the methods using ac pulse for coherent population transfer, protocols based on LZT are less sensitive to timing errors. However, the effect of finite range of qubit energy available to LZT based state control operations has not been thoroughly examined. In this work, we show that using the well-known Landau-Zener formula in the vicinity of an avoided energy-level crossing will cause considerable errors due to coherent oscillation of the transition probability in a single-passage LZT experiment. The data agree well with the numerical simulations which take the transient dynamics of LZT into account. These results not only provide a closer view on the issue of finite-time LZT but also shed light on its effects on the quantum state manipulation.

Landau-Zener transition (LZT) has been explored in a variety of physical systems for coherent population transfer between different quantum states. In recent years, there have been various proposals for applying LZT to quantum information processing because when compared to the methods using ac pulse for coherent population transfer, protocols based on LZT are less sensitive to timing errors. However, the effect of finite range of qubit energy available to LZT based state control operations has not been thoroughly examined. In this work, we show that using the well-known Landau-Zener formula in the vicinity of an avoided energy-level crossing will cause considerable errors due to coherent oscillation of the transition probability in a single-passage LZT experiment. The data agree well with the numerical simulations which take the transient dynamics of LZT into account. These results not only provide a closer view on the issue of finite-time LZT but also shed light on its effects on the quantum state manipulation. L andau-Zener transition (LZT) has broad applications in atomic and molecular physics, quantum optics, condensed matter physics, chemical physics, and quantum information science. For example, LZT has been applied to investigating the jump time and quantum Zeno and anti-Zeno effects of cold atoms in accelerated optical lattices 1,2 , the behavior of molecular magnets at low temperature 3,4 , nonequilibrium phase transitions 5 , and it is also exploited as a tunable beam splitter of wave functions to generate entangled multipartite states 6,7 . LZT also plays a key role in determining whether random optimization problems can be solved using the quantum adiabatic algorithm 8 . Recently, LZT's potential for robust manipulation of coherent quantum states has attracted much attention in the context of quantum information processing 6,7,[9][10][11][12][13][14][15][16][17][18] because LZT may provide a simple and effective solution to the realization of high fidelity quantum state control without the need for precise timing.
The time-dependent Hamiltonian describing LZT in quantum two-level systems can be written in the generic form as where s x,z are Pauli matrices, e(t) 5 vt is the energy difference between the two diabatic (crossing) basis states (i.e., the eigenstates j "ae and j #ae of the s z operator) controlled by an external parameter which depends linearly on time t, and D is the constant gap between the two instantaneous eigenenergy states j1ae and j2ae at the center of the avoided crossing e 5 0, as depicted in Fig. 1(a). In such systems, when e(t) is swept through the avoided crossing, transitions between j6ae with energies E + t ð Þ~+ 1 2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi e t ð Þ 2 zD 2 q can occur and the transition probability is given by the well-known Landau-Zener (LZ) formula P LZ~e where v 5 jde/dtj is the Landau-Zener speed and we have set the reduced Planck constant h~1. Equation (2) gives the probability of finding the system in the excited (ground) state at e f 5 e(t R 1') when it is started in the ground (excited) state at e i 5 e(t R 2'). By defining a 5 D 2 /4v as the adiabaticity parameter the LZ formula can be simplified to P LZ 5 exp(22pa). Although analytical solution to the problem cannot be obtained when e i and/or e f are finite, it is well known that for e i,f ?D, the LZ formula provides an excellent approximation to the actual transition probability and P # < P LZ . However, when e i,f ?D is not satisfied, the LZ formula may become quantitatively inaccurate or even qualitatively incorrect. In spite of some theoretical studies on the effects of finite je i,f j/D on P # , there is an acute lack of adequate experimental evidence.
On the other hand, understanding LZT with moderate values of je i,f j/D is in urgent need because this region of parameter space is important to quantum information processing. For instance, in superconducting qubits the tuning range of energy level spacing is usually limited to a couple of GHz or even as narrow as a few hundreds of MHz while D/2p could be as large as 10 2 MHz 6,7,19-26 . For quantum state control based on sweeping e through avoided crossings, understanding LZT probability's dependence on je i,f j/D and the sweeping time is essential to high fidelity operation. The fidelity of various techniques based on LZT relies critically on the accuracy of the LZ formula which predicts a simple exponential dependence of P LZ on the adiabaticity parameter a only. Therefore, for a qubit starting from the ground (excited) state the probability of finding it in the excited (ground) state after a single passage through the avoided crossing is assumed to be determined entirely by a (i.e., D 2 /4v) but not the detail of the process such as je i,f j/D. Here, using an artificial atom -a superconducting phase qubit -coupled to a microscopic two-level system (TLS), we test the accuracy of the LZ formula in the region of je i,f j/D , 4.3. We show that in contrast to conventional wisdom, in the region of parameter space most relevant to superconducting qubits, P # could deviate significantly from P LZ determined by the LZ formula. Our experiment and numerical simulation demonstrate P # can oscillate coherently as a function of e f for constant a when je i,f j is comparable to D, which is named as coherent Landau-Zener oscillation (LZO).

Results
In our experiment we use a superconducting phase qubit. However, since a single phase qubit does not have an intrinsic avoided energylevel crossing, we utilize an avoided level crossing arising from interaction between the qubit and a microscopic TLS 27,28 . As discussed below in more detail, when the transition frequency of the qubit v 10 is close to that of the TLS v TLS , which is fixed, the first and second excited states of the coupled qubit-TLS system form an effective quantum two-level system described by the LZ Hamiltonian (1). Note that to make quantitative comparisons between the theory/ numerical simulation and the experiment without free parameters, all relevant system parameters, including the energy relaxation and dephasing time of the qubit and the energy gap D, are obtained from direct measurements.
A microscopic picture of the superconducting phase qubit is shown in Fig. 1(b). The qubit consists of an L < 770 pH superconducting loop intersected by a Al/AlO x /Al Josephson tunnel junction with a critical current I 0 < 1.4 mA and a junction capacitance C < 240 fF. By varying the magnetic flux applied to the superconducting loop the potential energy of the qubit becomes asymmetrical. The ground state and the first excited state in the upper potential well, represented by j0ae and j1ae respectively, can be used as the computational basis states of the qubit. For an isolated qubit, the transition frequency between j0ae and j1ae, v 10 , is a single-valued function of the external flux bias W x which is coupled inductively to the superconducting loop through an on-chip flux bias line.
As shown in Fig. 2(a), however, the microwave spectrum of the qubit v 10 (W x ) has a rather large avoided energy-level crossing at W x < 22.8 mW 0 (with respect to the flux bias point at which v 10 /2p < 16.348 GHz) indicating significant interaction between the qubit and a microscopic TLS 30 . The transition frequency between the TLS' ground state jgae and excited state jeae and the qubit-TLS coupling strength are v TLS /2p 5 16.450 6 0.002 GHz and D/2p 5 70.0 6 0.5 MHz from the spectrum and vacuum Rabi oscillation, respectively. Note that in this coupled qubit-TLS system the time-dependent energy difference between the two diabatic states involved in LZT is e(t) 5 v 10 (t) 2 v TLS which depends linearly on the flux bias to a good approximation. The relationship between the flux bias and e can be found from Fig. 2(a). Fig. 1(c) illustrates the experimental procedure used to observe coherent LZO. We begin by setting the initial diabatic energy of the effective quantum two-level system e i at about 100 MHz below v TLS with a static flux bias. The qubit is prepared in its ground state by waiting for much longer than the energy relaxation time T 1 < 70 ns of the qubit. A microwave pulse is then applied to the qubit when it is biased at a fixed value e i /2p < 2100 MHz. The microwave pulse coherently transfers the population of the qubit-TLS from j0gae to one of the system's eigenstates j2ae through a process that is discussed in detail in Methods. The lack of oscillation in T 1 measurement taken at e i as shown in Fig. 2(b) confirms that the initial state of the qubit-TLS system at t 5 0 is indeed the eigenstate j2ae. As illustrated in Fig. 1(c), a time-dependent flux W(t) 5 W LZ t/t sp is then superimposed between t 5 0 and t 5 t sp onto the static flux bias to sweep e linearly from , 2100 MHz to its maximum value e f . The corresponding LZ speed v is thus (e f 2 e i )/t sp . This is followed immediately by a 5-ns readout pulse which performs a projective measurement of the probability P # of finding the qubit in state j1ae (i.e., the coupled system is in state j1gae corresponding to j #ae in Fig. 1(a)). We first measure P # vs. t sp at a constant value of e f by keeping W LZ fixed while increasing t sp from almost 0 ns to 45 ns. The maximum t sp is selected to avoid too much influence of the qubit's energy relaxation. By stepping W LZ from 0 to 211 mW 0 the value of corresponding e f is then varied from about 21.4D to , 4D, in which the condition e f ?D is no longer satisfied. This procedure is repeated at each e f to obtain P # (e f , t sp ). Fig. 3(a) shows the dependence of P # on e f and t sp . It can be seen that P # vs. t sp decays exponentially for e f , 0 (W LZ g [22.8 mW 0 , 0]) with a characteristic time T 1 due to energy relaxation. As e f becomes positive, P # vs. t sp becomes oscillatory.
Since the avoided crossing is traversed only once, the observed oscillation in P # vs. t sp with constant e f must be a consequence of the moderate value of e f /D and is not caused by the Landau-Zener-Stückelberg interference which requires multiple passages through the avoided crossing. It is worth noting that the observed oscillation is not a consequence of ill-prepared initial states with non-negligible probability amplitude in the excited state j1ae of the effective Hamiltonian (1) because the microwave pulse used to initialize the system resonantly couples j0gae to j2ae and has negligible coupling to j1ae due to large frequency detuning. Furthermore, this process of transferring the system to the desired initial state j2ae via a resonant microwave pulse is robust in the sense that it does not depend sensitively on the accuracy of the pulse duration t MW . Deviation in pulse duration simply leaves some probability amplitude in j0gae which has no effect on LZT other than reducing the visibility of the oscillation (see Methods for detail on the initial state preparation). Therefore, we are confident that the oscillation observed in the non-adiabatic region of the parameter space arises neither from Landau-Zener-Stückelberg interference nor unwanted probability amplitude of j1ae in the initial state. This is also supported by the good agreement between the results of experiment and numerical simulation shown in Fig. 3(b), which uses j2ae as the initial state at the start of the single passage sweep.

Discussion
By replacing e f with e i 1 vt, solving the problem of sweeping e in a finite range is transformed to finding P # at finite time. Previous studies have discovered that LZ transition probability reaches the asymptotic value given by equation (2)    Numerically calculated P # vs. e f and t sp with all input parameters obtained from the experiment. The white dashed lines correspond to the value of modified adiabaticity parameter a9 5 10. Notice that in the region below (above) the lower (upper) white dashed line, one has a9 ? 1 thus the system evolves adiabatically and no oscillation in P # is expected as confirmed experimentally. The LZ speed v equals the slope of any straight lines originated from the lower-left corner of the t sp -e f plane. For example, the yellow dashed line in (a) has v 5 400/19.5 < 20.5 MHz/ns. For the sake of clarity, the temporal evolution of the system along the yellow dashed line (constant v) is presented separately in Fig. 4(b). Such coherent LZO has little effect on the adiabatic evolution. Because in the true adiabatic regime, by definition the system always stays in the instantaneous ground state and no LZT could occur. In order to find the region of approximate adiabatic evolution in our experiment, it is necessary to modify the definition of the adiabaticity parameter to a'~v TLS {v 10 ð Þ 2 zD 2 4v . For the adiabatic theorem to hold a'?1 is required. As shown in Fig. 3(a), the white dashed lines represent a9 5 10. It is clear that there is no coherent LZO observed in the region a'?1.
In a popular analogy to optics an avoided crossing acts as an effective beam splitter, with a transmission coefficient corresponding to P LZ in the LZ formula, for quantum wave functions. This beam splitter analogy has been applied successfully to the visualization and explanation of the behavior of superconducting and semiconductor qubits 6,9,19,20,[33][34][35][36] . In this analogy, a single sweep through the avoided crossing is equivalent to passing a beam of light through the beam splitter only once. When e i,f ?D, P # < P LZ and thus a greater LZ speed corresponds to a higher transmission coefficient of the beam splitter according to the LZ formula. But when e i,f ?D is not satisfied, P # differs greatly from P LZ . As an example, P # vs. t sp , and thus the LZ speed v, with e f /2p 5 200 MHz is shown in Fig. 4(a). The maximum in the difference dP # between the experimental P # and those obtained from the LZ formula (2), shown in the inset of Fig. 4(a), can reach 0.21. The observation of coherent LZO strongly suggests that when e i,f ?D is not met corrections to the LZ formula should be considered to avoid conceptual difficulties. Coherent LZO also has significant consequences on the coherent manipulation of quantum states of single qubits and coupled twoqubit systems based on LZT 10,12 . For this approach of quantum state control, the LZ transition probability P LZ plays a central role since each single passage through the avoided crossing results in a unitary operation U LZ given by Ref. 12 where Q s is the Stokes phase 37 , which has no effect on the singlepassage LZT process discussed here and thus can be set to zero for the sake of convenience. As mentioned above, the transition frequency v 10 of most artificial atoms, in particular the superconducting qubits, is limited to a couple of GHz. Because the speed of two-qubit operations is proportional to the inter-qubit coupling strength D, increasing je i,f j/D by reducing D is undesirable. Hence, evaluating U LZ according to the LZ formula (2) could result in significant errors when e f ?D is not satisfied. In order to conduct a quantitative analysis, the experimental data along the yellow dashed line corresponding to constant v, in the Fig. 3(a) are extracted and shown in Fig. 4(b). Oscillation in P # is clearly observed and it is qualitatively different from the exponential decay predicted by equation (2), when decoherence is taken into account. Suppose the initial quantum state is j #ae. Then after a single passage through the avoided crossing, if one replaces P LZ with P # as in the asymptotic situation, the deviation dP # 5 P # 2 P LZ would be quite large. For example, when t sp 5 12.8 ns the deviation dP # 5 0.229, which is unacceptably large for coherent quantum state transformation.
In conclusion, we have investigated the effect of finite energy (e) sweep (or equivalently finite time) on LZT probability P # experimentally. Single-passage technique is used to isolate the effect of finite e f on P # from that of interference caused by passing the avoided crossing multiple times. We find that P # (e f /D, a 5 const) oscillates when e f is comparable to D and a , 1. The good agreement between the experiment and numerical calculation strongly supports the notion that coherent LZO is caused by the underlying transient dynamics of the finite time LZT which cannot be described by the LZ asymptotic formula. In this region of the LZT parameter space, corrections to the LZ formula must be taken into account, otherwise it will lead to substantial errors in quantum state operations based on LZT. The result also shows that when applying the simple beam splitter analogy one should not automatically assume that greater a (i.e., faster sweep) corresponds to larger transmission coefficient (i.e., greater P LZ ) as implied by the asymptotic LZ formula.

Methods
Initial state preparation. We first derive an analytical result explaining the lack of oscillation at the very beginning of LZT. The Hamiltonian of the qubit-TLS system coupled to a microwave field is given as: (in the basis {j0gae, j1gae, j0eae, j1eae}) . This is in stark contrast to the smooth exponential decay expected from the LZ formula (the dashed line). The inset is the difference dP # 5 P # 2 P LZ . (b) The measured (the red squares) and numerically calculated (the solid line) P # vs. t sp with constant LZ speed v < 20.5 MHz/ns corresponding to evolving along the yellow dashed line in Fig. 3(a). Again, P # oscillates in the region where the adiabatic condition a . 1 is not satisfied which is not expected from the LZ formula (the dashed line). where V m is the Rabi frequency, v is the microwave frequency, v TLS is the energy difference between the ground state jgae and the excited state jeae of TLS, D is the coupling strength between the qubit and TLS, d 5 v 10 2 v and d r 5 v TLS 2 v 10 are detunings. By rotating the frame, Hamiltonian (3) can be transformed to the following time-independent form 29,30 H 1~1 2 Next, we rewrite H 1 in which the subspace spanned by {j1gae, j0eae} is diagonalized: where N +~ffi ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi . The basis for H 2 is now {j0gae, j2ae, j1ae, j1eae}. Note that in our experiment, before turning on the microwave the state is at Y(t 5 0) 5 j0gae. By turning on the microwave (V m ? 0), j0gae is coupled to both j2ae and j1ae. The resonance between j0gae and j6ae occurs when 2, from which we obtain the resonant condition: In the limit of v 10 {v TLS ?D, we have v 5 v 10 , which corresponds to the usual twostate Rabi oscillation. Note that in this limit there is also a solution v 5 v TLS .
However, in this case the coupling strength is The reason is that although the microwave frequency could match that of TLS, coupling between the microwave and TLS is negligible which is confirmed by the absence of Rabi oscillation between the two states of the TLS in a separate experiment. In the other limit of v 10 2 v TLS 5 0, we have v~v 10 zv TLS 2 +D, and the dynamics have been thoroughly studied in Ref. 30. In our experiment, we have (v TLS 2 v 10 )/2p < 100 MHz and D/2p < 70 MHz, which means LZT occurs in the region where (v TLS 2 v 10 ) , D. Because the frequency of the applied microwave is v 5 l 2 , which can be determined from the measured energy spectrum shown in Fig. 2(a), j0gae is resonantly coupled to j2ae, which is the eigenstate of H b~v10 1g Although there is in principle also a coupling between j0gae and j1ae, the effective coupling is much smaller because of the large detuning, as discussed below. For D/2p < 70 MHz, (v TLS 2 v 10 )/2p < 100 MHz, the resonance between j0gae and j2ae occurs at d=2p^11 MHz, we obtain N { =2p^36:7 MHz and N z =2p^116:4 MHz. In our experiments, the coupling strength between j0gae and j2ae is about 20 MHz and that between j0gae and j1ae would be N { N z |20 MHz^6:3 MHz. Because 6.3 MHz is comparable with 20 MHz, one may think that coupling between j0gae and j1ae cannot be neglected. However, there is also a large detuning of about 122 MHz between j0gae and j1ae. Therefore, the effective coupling between j0gae and j1ae is reduced to (6.3 2 /122) < 0.33 MHz and thus can be safely neglected.
To be more precise, we calculated the population P 6 (where P 6 is the population of state j6ae) after the application of a p pulse numerically, and it is found that P z =P {^5 |10 {5 . Based on this analysis, when v 5 l 2 , the dynamics can be described by the Hamiltonian in the subspace {j0gae, j2ae}: where I is a 2 3 2 identity matrix.
For initial state Y(t 5 0) 5 j0gae, the amplitude of j2ae is Considering Therefore, with a microwave pulse of duration t MW which is used to prepare the initial state, we have This is the reason why in the experiment we observe a usual Rabi oscillation instead of  Rabi beating 30 which is indicated by the red circles, as shown in Fig. 2(b). In addition, when microwave is turned off, the subspace {j1ae, j2ae} is isolated from j0gae and j1eae. Projected into the subspace {j1ae, j2ae}, the system stays in the eigenstate j2ae. This explains why we observe a monotone decay of P # with no oscillations, as indicated by the blue triangles in Fig. 2(b).
Effect of t MW and e f /D on LZT. In this section, we discuss two factors that may affect the LZT probability, i.e., the width of the microwave pulse t MW used to prepare the initial state at e i and the end of the normalized diabatic energy sweeping e f /D, respectively. After a microwave pulse, by projecting into the subspace {j1gae, j0eae}, the system is in the eigenstate j2ae. Then the dynamics of LZT can be described by H b with a timedependent v 10 (t), i.e., To investigate the Landau-Zener diffraction effect, we sweep v 10 (t) across v TLS , i.e., v 10 t ð Þ~v 10 t~0 ð Þzvt, 0ƒtƒt sp À Á : ð12Þ When v 10 t~t sp À Á {v TLS ?D, we expect that the Landau-Zener asymptotic formula holds and P # < P LZ . Thus no oscillations should occur in P # . This is confirmed by the result of numerical simulation shown in Fig. 5(c), where e f /2p 5 1450 MHz < 20.7D, reproducing the exponential decay behavior described by the asymptotic LZ formula independent of the initial state of the qubit-TLS system. In this case, the population in the qubit state j1ae can be expressed as where the first term reflects the effect of microwave duration t MW in preparing the initial state, the second term corresponds to the LZT probability, and the third term represents the relaxation effect. However, as v 10 (t 5 t sp ) moves towards v TLS , the situation v 10 t~t sp À Á {v TLS ?D does not hold any more, and we observe oscillation features in the t sp direction, as shown in Fig. 5(a) (experiment) and 5(b) (numerical simulation). Notice that Fig. 5(a) and 5(b) also confirm that the effect of imprecise p pulse is an incomplete transfer of system from j0gae to j2ae, which reduces the probability amplitude of j2ae from the maximum value, instead of resulting in nonnegligible probability amplitude in the unwanted j1ae state.