Abstract
Silicon quantum dot spin qubits provide a promising platform for largescale quantum computation because of their compatibility with conventional CMOS manufacturing and the long coherence times accessible using ^{28}Si enriched material. A scalable errorcorrected quantum processor, however, will require control of many qubits in parallel, while performing error detection across the constituent qubits. Spin resonance techniques are a convenient path to parallel twoaxis control, while Pauli spin blockade can be used to realize local parity measurements for error detection. Despite this, silicon qubit implementations have so far focused on either singlespin resonance control, or control and measurement via voltagepulse detuning in the twospin singlet–triplet basis, but not both simultaneously. Here, we demonstrate an integrated device platform incorporating a silicon metaloxidesemiconductor double quantum dot that is capable of singlespin addressing and control via electron spin resonance, combined with highfidelity spin readout in the singlettriplet basis.
Introduction
The manipulation of singlespin qubits in silicon, using either ac magnetic^{1,2} or electric^{3,4,5,6} fields at microwave frequencies, has been a powerful driver of progress in the field of solid state qubit development, in part due to the sophistication of microwave technology which allows convenient twoaxis control of the qubit via simple phase adjustment, and the generation of complex pulse sequences for dynamical decoupling. This has resulted in highfidelity singlequbit gates^{2,4,5,6,7} and initial twoqubit gates now realised in a variety of structures^{8,9,10}. To date, all demonstrations of singleshot readout in silicon systems employing spin resonance^{1,2,3,4,6} have utilized singlespin selective tunnelling to a reservoir^{11}. While convenient, this reservoirbased readout approach is not well suited to gatebased dispersive sensing^{12}, which has significant advantages in terms of minimizing electrode overheads for largescale qubit architectures. In contrast, readout based on Pauli spin blockade^{13} in the singlet–triplet basis of a double QD^{14} is compatible with dispersive sensing and, when combined with an ancilla qubit, can be used for parity readout in quantum error detection and correction codes^{15,16,17}. Moreover, because singlet–triplet readout can provide highfidelity spin readout at much lower magnetic fields than singlespin reservoirbased readout^{11}, it allows spinresonance control to be performed at lower microwave frequencies, which will benefit scalability.
Qubits based on singlet–triplet spin states were first demonstrated in GaAs heterostructures^{14,18} and have now been operated in a variety of siliconbased structures^{19,20,21,22,23}. Highfidelity singleshot singlettriplet readout has also recently been demonstrated in various silicon systems^{22,24,25}.
Here, in order to combine the ability to address individual spin qubits using ESR with the voltagepulsebased detuning control and highfidelity readout of pairs of spins in the singlettriplet basis, we employ a ^{28}Si metaloxidesemiconductor (SiMOS) double quantum dot device^{26,27} (Fig. 1a, b) with a microwave transmission line that can be used to supply ESR pulses, similar to one previously used for demonstration of a twoqubit logic gate^{8}. The device also includes an integrated singleelectrontransistor (SET) sensor to achieve the singlecharge sensitivity required for singlet–triplet readout. Electrons are populated into the two quantum dots (QD1 and QD2) with occupancy (N_{1}, N_{2}) using positive voltages on gates G1 and G2. An electron reservoir is induced beneath the Si–SiO_{2} interface via a positive bias on gate ST, which also serves as the SET top gate. The reservoir is isolated from QD1 and QD2 by a barrier gate B (see Fig. 1a, b).
Results
Singleshot singlet–triplet readout
Figure 1c shows the stability diagram of the double QD system in the charge regions (N_{1}, N_{2}) where we operate the device. When two electrons occupy a double quantum dot, the exchange interaction results in an energy splitting between the singlet (S) and triplet (T_{−},T_{0}, T_{+}) spin states. The exchange interaction can be controlled by electrical pulsing on nearby gates, providing a means to initialize, control and read out the singlet and triplet states^{14}. At the core of singlet–triplet spin readout is the observation of Pauli spinblockade (PSB)^{19,28,29,30,31}. When pulsing from the (1, 1)→ to (0, 2) charge configurations, the QD1 electron tunnels to QD2 only when the two spatially separated electrons were initially in the singlet spin configuration. The triplet states are blockaded from tunnelling due to the large exchange interaction in the (0, 2) charge configuration. The blockade is made observable on the stability diagram by applying a pulse sequence^{19,28} to gates G1 and G2 as depicted in Fig. 1c. After first flushing the system of a QD1 electron to create the (0, 1) state at A, a (1, 1) state at B loads a randomly configured mixture of singlet and triplet states (solid arrow in Fig. 1c). The current through the nearby singleelectrontransistor (SET) is recorded at this position, tuned to be at the halfmaximum point of a Coulomb peak. The system is then ramped to a variable measurement point (dashed arrows in Fig. 1c, d) where the SET current is measured again. A map of the comparison current ΔI_{SET} between these two points is created, where the derivative in sweep direction d(ΔI_{SET})/d(ΔV_{G1}) (Fig. 1c) decorrelates the capacitive coupling of the control gates to the SET island. A change in the charge configuration marks a shift in the SET current, clearly observed as bright/dark bands. The bright band in the centre of the (1,1)–(0,2) anticrossing of Fig. 1c is consistent with PSB, where the blockade triangle is restricted to a narrow trapezoidal area, bounded by state cotunnelling via the reservoir and the first available excited triplet state^{19}.
The charge sensor design used (Fig. 1a) is relatively insensitive to interdot charge transitions, due to the symmetry of the QD1 and QD2 locations with respect to the SET island^{32}. In order to enhance the blockade signal for this layout, we employ statelatching using the nearby electron reservoir^{33}. Recent studies of reservoir charge state latching^{22,24} and intermediate excited states^{34} in semiconductor quantum dot devices have led to methods to reduce readout error by almost an order of magnitude^{22}. A variant of this state latching is observed and utilized here.
The latching is produced via asymmetric couplings of the two dots to the common electron reservoir^{33}, where a (1, 1)(1, 2) dotreservoir metastable charge state is produced via a combination of the low tunnel rate between QD2 and the reservoir (shown as Γ_{Slow} in Fig. 1b) and cotunnelling between QD1, QD2 and the reservoir (Γ_{Fast} in Fig. 1b). The latching results in the prominent feature observed at the (1, 1)(1, 2) transition in Fig. 1c. In contrast, when the system is initialized in the (0, 2) charge configuration, the singlet state is prepared robustly due to large energy splitting, and the resulting map in Fig. 1d has no latched PSB region, as expected. The energy splitting between the (0, 2) singlet ground state and first available triplet state is measured to be (1.7 ± 0.2)% of the charging energy E_{C} (see Supplementary Notes 1 and 2, Supplementary Figs. 1, 2). Typically for this device design E_{C} ~ 10–20 meV^{35}, indicating that this splitting exceeds electron thermal energies by two orders of magnitude. The first available triplet here is likely the first excited valley state^{35,36,37}. To compare the visibility of the standard PSB and latched PSB, histograms of ΔI_{SET} are shown in Fig. 1f, g respectively. We find that state latching increases our measurement visibility from around 70 to 98%, reducing the misidentification error by more than 16fold for this SiMOS device layout (see Supplementary Note 1 and Supplementary Fig. 3). We note that this measurement fidelity of F_{M} = 99% does not include errors that occur during the evolution from a separated (1, 1) charge state to the blockade region, which we discuss in more detail below.
Singlet–triplet Hamiltonian for SiliconMOS qubits
The large valley splitting in SiMOS devices^{8,35} allows us to restrict ourselves to the lowest valley state when considering spin dynamics near the (0, 2)–(1, 1) anticrossing, which we now address. These dynamics are governed by a Hamiltonian in which singlespin distinguishability and exchange are in competition. Singlespin distinguishability arises from the varying Zeeman energy between each dot, interpreted as a sitespecific effective gfactor and resulting in an energy difference δE_{Z} = \(g_2\mu _{\mathrm{B}}B_2^z  g_1\mu _{\mathrm{B}}B_1^z\). For high inplane magnetic field, the varying effective gfactors result from a combination of interface spinorbit terms, which depend on local strain, electric fields, and the atomistic details of the oxide interface^{35,38,39}. Further, recent studies have shown that this Hamiltonian parameter can be modulated by the direction of the applied field with respect to the crystallographic axis^{23}. In previous devices we have observed gfactor differences between QDs as large as 0.5%^{8}; at highfield, Overhauser contributions to δE_{Z} are negligible in isotopically purified samples. At lower magnetic fields, magnetic screening from the superconducting aluminum gates may also contribute significantly to δE_{Z}^{40}. For these experiments, the scale of δE_{Z} is predominantly set by choice in the magnitude of the external magnetic field; however, inevitable microscopic dottodot variation in this parameter for future qubit arrays would best be handled by calibration and refocusing methods^{15}.
The Hamiltonian term in competition with the Zeeman gradient is kinetic exchange, which lowers the energy of the spin singlet energy by an amount J(ε) due to interdot tunnelling. In the standard FermiHubbard model, J(ε) is proportional to \(2t_{\mathrm{c}}^2(\varepsilon ){\mathrm{/}}\left \varepsilon \right\) for large ε, where t_{c}(ε) is the interdot tunnel coupling and ε combines the onsite charging energy and electrochemical potential difference between the two dots^{41} (Supplementary Note 3). In previous experiments^{8} on a similar SiMOS twoqubit device the tunnel coupling at the anticrossing was estimated as \(900\sqrt 2 \) MHz. For both devices, t_{c} at the anticrossing is fixed, set by the device geometry. This parameter can be made tunable via the incorporation of exchange gates^{42} into the SiMOS architecture. In this model, the groundstate singlet is hybridized between (0, 2) and (1, 1) charge states as \(\left {S_{\mathrm{H}}} \right\rangle \) = \({\mathrm{cos}}(\theta {\mathrm{/}}2)\left {{\mathrm{(1}},{\mathrm{1)}}S} \right\rangle \) + \({\mathrm{sin}}(\theta {\mathrm{/}}2)\left {{\mathrm{(0}},{\mathrm{2)}}S} \right\rangle \), where \(\theta =  {\mathrm{tan}}^{  1}(2t_{\mathrm{c}}{\mathrm{/}}\varepsilon )\). Again neglecting higher energy valley or orbital states, the spintriplet states \(\left {T_m} \right\rangle \) with twospin angular momentum projection m = 0, ±1 are fully separated in the (1, 1) charge state. Besides being split from the m = 0 states by the mean Zeeman energy, \(\bar E_{\mathrm{Z}}\), the \(\left {T_ \pm } \right\rangle \) states may couple to the hybridized singlet states by local magnetic fields which are orthogonal to the average applied field, as well as by spin orbit coupling. We summarize such terms by a spinflipping term Δ(θ).
Hence in the basis \(\left\{ {\left {T_ + } \right\rangle ,\left {T_0} \right\rangle ,\left {T_  } \right\rangle ,\left {S_{\mathrm{H}}} \right\rangle } \right\}\), the approximate effective Hamiltonian is written (Supplementary Note 3)
A typical energy spectrum of this Hamiltonian as a function of detuning ε is shown in Fig. 2a for small magnetic fields, B^{z} ~ J(ε)/gμ_{B}.
Characterizing the singlettriplet Hamiltonian
The anticrossing between the \(\left {S_{\mathrm{H}}} \right\rangle \) and \(\left {T_ \pm } \right\rangle \) states due to Δ(θ) can be used to map out the energy separation \(\left {E_{S_{\mathrm{H}}}(\varepsilon )  E_{T_  }(\varepsilon )} \right\) as a function of small detuning ε by performing a spin funnel experiment^{14}. Here, we initialize in a (0, 2) singlet ground state, \(\left {(0,2)S} \right\rangle \), and pulse toward the spatially separated \(\left {(1,1)S} \right\rangle \), as shown in Fig. 2b, c. By varying the applied magnetic field \(B_0^z\) while dwelling at various values of detuning ε, the location of the anticrossing can be mapped out via the increased triplet probability P_{T} (Fig. 2d) due to mixing under Δ(θ). Ramping across the anticrossing causes a coherent population transfer between \(\left {S_{\mathrm{H}}} \right\rangle \) and \(\left {T_  } \right\rangle \) due to Landau–Zener tunnelling^{43} proportional to \({\mathrm{exp}}\left( {  2\pi {\mathrm{\Delta }}(\theta )^2{\mathrm{/}}\left. {\hbar \nu } \right)} \right)\), characterized by the ratio of Δ(θ) to the energy level velocity ν = \(\left {d\left( {E_{S_{\mathrm{H}}}  E_{T_  }} \right)} \right{\mathrm{/}}dt\). As the ramp rate rises the singlet state \(\left {S_{\mathrm{H}}} \right\rangle \) is increasingly maintained (see Fig. 2f) and so the triplet return probability P_{T} falls, as we observe in Fig. 2e. By fitting this data (Supplementary Note 4) we estimate \(\left {{\mathrm{\Delta }}(\theta )} \right\) at the location of the minimum energy gap is (196 ± 6) kHz at \(B_0^z\) = 0 (an offset field of \(B_{OS}^z\) = −1.04 ± 0.06 mT is estimated from spin funnel asymmetry). Further \(\left {{\mathrm{\Delta }}(\theta )} \right\) = 16.72 ± 1.64 MHz at \(B_0^z\) = 155 mT, where the uncertainty here (and elsewhere) corresponds to 95% confidence intervals.
There are a number of possible processes that can contribute to Δ in the siliconMOS platform. For 800 ppm nuclearspin1/2 ^{29}Si in the isotopically enriched ^{28}Si epilayer^{44}, we expect random hyperfine fields in all vector directions with rootmeansquare of order 50 kHz^{20} for unpolarized nuclei, so this may contribute to Δ. However, other effects may have a comparable contribution. At low \(B_0^z\)(≲50 mT), Meissner effects from the superconducting aluminum gates can provide transverse local magnetic fields at the location of the QDs; contributions to δB^{z} from this effect of up to a few MHz have been reported^{40}. Further, offdiagonal terms in the difference between the electron gtensors can contribute to coupling between (1, 1) states. Finally, in the presence of interdot tunnelling, the interface spin–orbit interaction provides a separate contribution to Δ, leading to estimated couplings of tens of kHz at low\(B_0^z\). Detailed studies on magnetic field magnitude and angle dependence, such as those performed to isolate hyperfine from spin–orbit contributions in nuclearrich materials such as GaAs^{45,46}, are required to separate and explore each of these individual effects.
We can further characterize the Hamiltonian in Eq. (1) at much larger detuning ε than is accessible via the spin funnel by performing a Landau–Zener–Stueckelberg (LZS) interference experiment^{18,43} (Fig. 2g, h). This is performed at \(B_0^z\) = 0, but the residual magnetic field present, which may include some nuclear polarization^{47} is sufficient to split the \(\left {T_0} \right\rangle \) and \(\left {T_ \pm } \right\rangle \) states. By setting the ramp rate across the \(\left {S_{\mathrm{H}}} \right\rangle \)/\(\left {T_  } \right\rangle \) anticrossing to \(2\pi \nu \approx \left {\mathrm{\Delta }} \right^2{\mathrm{/}}\hbar \), an approximately equal superposition of both states is created. Dwelling for varying times τ_{D} and detunings ε results in a Stueckelberg phase accumulation ϕ = \({\int} \left( {E_{S_{\mathrm{H}}}\left( {\varepsilon [t]} \right)  E_{T_  }\left( {\varepsilon [t]} \right)} \right)dt{\mathrm{/}}\hbar \), with \(E_{S_{\mathrm{H}}}\left( {E_{T_  }} \right)\) the energy of the \(\left {S_{\mathrm{H}}} \right\rangle \left( {\left {T_  } \right\rangle } \right)\) state. Depending on the accumulated phase, the returning passage through the anticrossing either constructively interferes, resulting in the blockaded \(\left {T_  } \right\rangle \), or destructively interferes, bringing the system back to \(\left {S_{\mathrm{H}}} \right\rangle \). By keeping ν constant throughout the experiment the Fourier transform (Fig. 2g) of the interference pattern (Fig. 2h) directly extracts the energy separation \(\left {E_{S_{\mathrm{H}}}(\varepsilon )  E_{T_  }(\varepsilon )} \right\) as a function of detuning. In future experiments, this Fourier transform could also be spectroscopically resolved via microwave excitation across the S_{H}/T_{−} transitions, either using our integrated ESR control or via photonassisted tunnelling^{48}.
We now investigate exchange between the hybridised singlet \(\left {S_{\mathrm{H}}} \right\rangle \) and unpolarized triplet \(\left {T_0} \right\rangle \) by applying an external magnetic field \(B_0^z\) = 200 mT to strongly split away the ∣T_{±}⟩ triplet states. At these fields the Zeeman energy difference δE_{Z} dominates exchange J(ε) deep in the (1, 1) region, and the eigenstates there become \(\left { \downarrow \uparrow } \right\rangle \) and \(\left { \uparrow \downarrow } \right\rangle \), as depicted in Fig. 3a. Maintaining a ramp rate ν fast enough to be diabatic with respect to Δ, but slow enough to be adiabatic with respect to t_{c}(ε), ensures adiabatic preparation of a ground state \(\left { \downarrow \uparrow } \right\rangle \) or \(\left { \uparrow \downarrow } \right\rangle \), depending upon the sign of δE_{Z} = \(g_2\mu _{\mathrm{B}}B_2^z  g_1\mu _{\mathrm{B}}B_1^z\). At \(B_0^z\) = 200 mT we expect the Meissner effect to be quenched, so that δE_{Z} is dominated by the effective gfactor difference between the dots.
For simplicity we henceforth assume δE_{Z} > 0, so that we adiabatically prepare \(\left { \downarrow \uparrow } \right\rangle \) for large ε. Following the pulse sequence illustrated in Fig. 3c, coherentexchangedriven oscillations can then be observed between \(\left { \downarrow \uparrow } \right\rangle \) and \(\left { \uparrow \downarrow } \right\rangle \) by rapidly plunging the prepared state \(\left { \downarrow \uparrow } \right\rangle \) back towards the (1, 1)(0, 2) anticrossing where J(ε) is no longer negligible. Variable dwell time τ_{D} results in coherent exchange oscillations, and the reversal of the rapid plunge leaves the state in a superposition of \(\left { \downarrow \uparrow } \right\rangle \) and \(\left { \uparrow \downarrow } \right\rangle \). The semiadiabatic ramp back to (0, 2) maps the final state \(\left { \downarrow \uparrow } \right\rangle \) to the \(\left {(0,2)S} \right\rangle \) singlet, while \(\left { \uparrow \downarrow } \right\rangle \) is mapped to a blockaded state via the ∣T_{0}⟩ triplet^{14,19}. The resulting data is shown in Fig. 3d, e.
Individual qubit addressability via electron spin resonance
We note that previous experiments performed at \(B_0^z\) = 1.4 T on another SiMOS device exploited the gfactor difference between two QDs in the lowJ(ε) region to perform a twoqubit controlledphase operation^{8}. Utilizing the highJ(ε) region as above, the \(\left { \downarrow \uparrow } \right\rangle \leftrightarrow \left { \uparrow \downarrow } \right\rangle \) operation can extend the twoqubit toolbox to include a SWAP gate, with a potentially shorter operation time, in this case with τ_{SWAP}~0.25 μs, limited by exchange pulse rise times.
Having characterized the system in the singlet–triplet basis, we now investigate the compatibility of spin blockade readout with individual QD (i.e., single spin) addressability via electron spin resonance (ESR)^{2}, a combination desirable for scalable spin qubit architectures incorporating error correction^{15,16}. Using the pulse sequence illustrated in Fig. 3f, we again adiabatically prepare the largeε ground state \(\left { \downarrow \uparrow } \right\rangle \), as discussed above. We now apply an ac magnetic field to perform ESR with pulse duration 25 μs, supplied by the onchip microwave transmission line^{49} (Fig. 1a), to drive transitions that correspond to \(\left { \downarrow \uparrow } \right\rangle \leftrightarrow \left { \downarrow \downarrow } \right\rangle \) and \(\left { \downarrow \uparrow } \right\rangle \leftrightarrow \left { \uparrow \uparrow } \right\rangle \) at large detuning, when exchange is small (see Fig. 3a). Any excitation from the ground state will now map to the blockaded triplet state population. Figure 3g shows the measured ESR spectrum as a function of detuning ε. The higher frequency f_{ESR2} branch corresponds to a coherent rotation of the electron spin in QD2, while the lower frequency f_{ESR1} rotates the QD1 spin. At large detuning f_{ESR1}~4.2 GHz, consistent with the applied magnetic field \(B_0^z\) = 150 mT for this experiment. As ε decreases (and J(ε) increases), the ground state is better described as \(\left {S_{\mathrm{H}}} \right\rangle \), so the transitions become \(\left {S_{\mathrm{H}}} \right\rangle \leftrightarrow \left { \downarrow \downarrow } \right\rangle \) and \(\left {S_{\mathrm{H}}} \right\rangle \leftrightarrow \left { \uparrow \uparrow } \right\rangle \) and exchange now competes with ESR, resulting in a lower visibility. For large detuning, the reduction in visibility can be produced from a number of sources. Here, the tuning of the semiadiabatic ramp rate is critical in this region to prevent excitation (discussed further in Supplementary Note 7). The reduction in visibility seen here is due to an increase in preparation and readout error. This could be mitigated in future experiments though pulse optimisation of the semiadiabatic ramps.
Exchange coupling between qubits
Each of the experiments described above probes the Hamiltonian in Eq. (1) for different ranges of detuning. Figure 4 collates the results of all experiments and plots the energy splitting between the hybridised singlet \(\left {S_{\mathrm{H}}} \right\rangle \) and unpolarised triplet \(\left {T_0} \right\rangle \) across all detuning values. Close to the (0, 2)(1, 1) anticrossing, for low ε, the splitting is dominated by exchange coupling J, while for large ε, δE_{Z} dominates. As expected, the energy differences obtained from the LZS interferometry (for \(B_0^z \approx 0\)) diverge from those obtained via ESR (where \(B_0^z\) = 150 mT), since when \(B_0^z \approx 0\) there remains only a small residual δE_{Z} due to combined Meissner screening and weak Overhauser fields. Figure 4 also shows a fit to the data employing the Hamiltonian of Eq. (1) as documented elsewhere. A constant t_{c} fits poorly; instead a model for a dependence of the tunnel coupling on ε is employed (see Supplementary Note 6). At the anticrossing (ε = 0), the curve fit to this model indicates t_{c}(ε = 0) = 1.864 ± 0.033 GHz and δg = (0.43 ± 0.02) × 10^{−3}. We note that this tunnel coupling is comparable to that observed for a separate twoqubit device^{8} for which t_{c}(ε) = \(900\sqrt 2 \) MHz.
Discussion
By analyzing the error processes present in these experiments, we can identify where improvements will be required before these mechanisms can be integrated into a parity readout tool useful for future multiqubit architectures. We can discriminate between the effect of various error processes by comparing blockade probability observations under different operating regimes in the exchange oscillation data of Fig. 3e. The histograms shown in Fig. 1g each reveal state preparation and measurement (SPAM)related errors, leading to a visibility maximum of 98% (orange data) and an error of 0.8% associated with \(\left {(0,2)S} \right\rangle \) preparation and the transfer process to a latched readout position (red data). Additional to these SPAM errors are the transfer and mapping error processes present when converting states semiadiabatically from the (0, 2)→(1, 1) or (1, 1)→(0, 2) charge transitions respectively. The combined error from SPAM, state transfer and mapping can be observed from the background visibility at a detuning where exchange is minimal. Here, the prepared \(\left {(0,2)S} \right\rangle \) state is ideally transferred to and from the (1, 1) region without loss, resulting in zero triplet probability. In contrast, the average blockaded return probability from Fig. 3e (and therefore the combined transfer and mapping errors) saturates to around 30%. From the decay in the oscillations of Fig. 3e as a function of operation time τ_{D}, we find a maximum control fidelity of F_{π} = 0.95 ± 0.04 at ε = 0.6 meV. We find that the decay time is proportional to the Rabi period, suggesting that exchange noise limits our control fidelity. Further, comparisons with the 61% visibility of the first fringe in Fig. 3e suggests that diabaticity errors due to each fast plunge to/from the exchange position are also present. With respect to our system’s utility for providing a parity readout tool, the main error source in the present work appears to occur during adiabatic transfer into and out of the (1, 1) region. Timedependent simulations (Supplementary Note 7 and Supplementary Table 1) of the model Hamiltonian Eq. (1) show that this error can be well explained by diabaticity with respect to t_{c}(ε) near the anticrossing. We expect that this error can be significantly reduced by optimizing the shape of the ramp as a function of detuning, to remain diabatic with respect to Δ near the \(\left {S_{\mathrm{H}}} \right\rangle {\mathrm{/}}\left {T_  } \right\rangle \) crossing, while staying adiabatic elsewhere. Of relevance to the fidelity of exchangebased twoqubit gates, we note that charge and voltage noise will couple via detuning ε to produce noise in exchange. Our simulations (Supplementary Note 7 and Supplementary Fig. 5) indicate that the level of charge noise expected^{7,20} in our system results in a \(\left {S_{\mathrm{H}}} \right\rangle {\mathrm{/}}\left {T_0} \right\rangle \) oscillation decay consistent with our measurements. The effect of charge noise could be minimized by symmetric biasing^{50}, with the use of an additional exchange gate.
To conclude, we have for the first time in a silicon device experimentally combined singlespin control using electron spin resonance, with highfidelity singleshot readout in the singlet–triplet basis. By characterising the relevant energy scales Δ, δE_{Z} and t_{c}(ε) of the twospin Hamiltonian, we found that we could coherently manipulate both the S/T_{−} and S/T_{0} states, the latter of which provides potential for a fast twoqubit SWAP gate at high exchange. The integration of lowfrequency ESR of individual spins with singlettriplet based initialisation and readout holds promise for qubit architectures operating at significantly lower magnetic fields and higher temperatures. Future experiments will focus on improvements in operational fidelities, as well as further characterisation of lowfrequency ESR operation. The presented initialisation and readout of singlet–triplet states attests to the compatibility of the SiMOS quantum dot platform with parity readout based on spinblockade, key for the realisation of a future largescale siliconbased quantum processor^{15,16}.
Methods
Device fabrication
The device is fabricated on an epitaxially grown, isotopically purified ^{28}Si epilayer with residual ^{29}Si concentration of 800 ppm^{44}. Following the multilevel gatestack silicon MOS technology^{26}, four layers of Algates are fabricated on top of a SiO_{2} dielectric with a thickness of 5.9 nm. Gate layers have a thickness of 25, 60, 80, and 80 nm, with three layers used to form the device and the fourth layer attributed to the ESR transmission line. Overlapping layers are separated by thermally grown aluminum oxide.
Experimental setup
The measurements were conducted in a dilution refrigerator with base temperature T_{bath} = 30 mK. DC voltages were applied using batterypowered voltage sources and are combined with voltage pulses using an arbitrary waveform generator (LeCroy ArbStudio 1104) through resistive voltage divider network. Filters were included for DC, slowpulse and fastpulse lines (10 Hz to 80 MHz). Microwave pulses were delivered by an Agilent E8257D analogue signal generator, passing signal through a 10 dBm attenuator at the 4 K plate and 3 dBm attenuator at the mixing chamber plate.
All the measured qubit statistics are based on counting the blockade signal in the latched region as described in the main text. The operating region within the experiments involves a system of two tunnel coupled quantum dots with a total of two electrons shared between them. The latched readout procedure involves conditional loading of a third electron from tunnelcoupled reservoir onto one of these dots. Each data point represents the average of between 100 and 1200 single shot blockade events, including experiment trace repetition. Stability maps generated from three level pulsing could be produced with less averaging, with figure data being the average of 40 shots per point.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Acknowledgements
We thank Mark Gyure for helpful discussions. We acknowledge support from the Australian Research Council (CE11E0001017 and CE170100039), the US Army Research Office (W911NF1310024 and W911NF1710198) and the NSW Node of the Australian National Fabrication Facility. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Office or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein. M.V. and B.H. acknowledges support from the Netherlands Organization for Scientific Research (NWO) through a Rubicon Grant. K.M.I. acknowledges support from a GrantinAid for Scientific Research by MEXT, NanoQuine, FIRST, and the JSPS CoretoCore Program.
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M.A.F. and B.H. performed experiments. M.V. designed the device, fabricated by K.W.C. and F.E.H. with A.S.D’s supervision. K.M.I. prepared and supplied the ^{28}Si epilayer. W.H., T.T., C.H.Y., and A.L. contributed to the preparation of experiments. M.A.F., B.H., and A.S.D. designed the experiments, with T.D.L., W.H., D.C., K.W.C., T.T., C.H.Y., A.L., A.M. contributing to results discussion and interpretation. M.A.F., B.H., and A.S.D. wrote the manuscript with input from all coauthors.
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Fogarty, M., Chan, K., Hensen, B. et al. Integrated silicon qubit platform with singlespin addressability, exchange control and singleshot singlettriplet readout. Nat Commun 9, 4370 (2018). https://doi.org/10.1038/s4146701806039x
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