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Magnetically tunable singlet-triplet spin qubit in a four-electron InGaAs coupled quantum dot.

Weiss KM, Miguel-Sanchez J, Elzerman JM - Sci Rep (2013)

Bottom Line: However, at this fixed operating point the ground-state splitting can no longer be tuned into resonance with e.g. another qubit, limiting the options for coupling multiple qubits.Here, we propose using a four-electron coupled quantum dot to implement a singlet-triplet qubit that features a magnetically tunable level splitting.As a first step towards full experimental realization of this qubit design, we use optical spectroscopy to demonstrate the tunability of the four-electron singlet-triplet splitting in a moderate magnetic field.

View Article: PubMed Central - PubMed

Affiliation: Institute for Quantum Electronics, ETH Zurich, CH-8093 Zurich, Switzerland.

ABSTRACT
A pair of self-assembled InGaAs quantum dots filled with two electrons can act as a singlet-triplet spin qubit that is robust against nuclear spin fluctuations as well as charge noise. This results in a T2* coherence time two orders of magnitude longer than that of a single electron, provided the qubit is operated at a particular "sweet spot" in gate voltage. However, at this fixed operating point the ground-state splitting can no longer be tuned into resonance with e.g. another qubit, limiting the options for coupling multiple qubits. Here, we propose using a four-electron coupled quantum dot to implement a singlet-triplet qubit that features a magnetically tunable level splitting. As a first step towards full experimental realization of this qubit design, we use optical spectroscopy to demonstrate the tunability of the four-electron singlet-triplet splitting in a moderate magnetic field.

No MeSH data available.


Related in: MedlinePlus

Locating the four-electron regime at B = 0 T.(a) PL from QD-B (in colorscale) as a function of V. Dotted vertical lines indicate the gate voltages where the charge configuration of the optically excited states changes. XB0 (XB1−) indicates emission from the neutral exciton (negative trion) in QD-B. PL involving the four-electron S and T ground states (highlighted in the orange box) exhibits a characteristic curvature. The larger signal of the T transition is due to the threefold degeneracy of the spin triplets. Inset: schematic energy diagram illustrating XB1− emission in the (3,1) regime. (b) PL from QD-R (in colorscale), which is weaker than that from QD-B because holes can tunnel from QD-R to QD-B before recombination. XR0 (XR1−) indicates emission from the neutral exciton (negative trion) in QD-R. The significant overlap between the XR0 and XR1− transition below −220 mV suggests that the tunnelling rate between the s-orbital and the back contact is slower than the radiative recombination rate of ~1 GHz. In contrast, the sharp transitions between plateaus above −140 mV indicate that the tunnelling rate from the p-orbitals to the back contact is larger than ~1 GHz. The multiple “satellite lines” that are especially strong for XR1− are most likely due to fluctuations in the charge of QD-B, leading to a shift in PL due to charge sensing33. Inset: schematic energy diagram illustrating emission in the (3,1) regime, which involves the s-orbital in QD-R, and therefore does not reflect the anti-crossings involving the p-orbitals.
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f2: Locating the four-electron regime at B = 0 T.(a) PL from QD-B (in colorscale) as a function of V. Dotted vertical lines indicate the gate voltages where the charge configuration of the optically excited states changes. XB0 (XB1−) indicates emission from the neutral exciton (negative trion) in QD-B. PL involving the four-electron S and T ground states (highlighted in the orange box) exhibits a characteristic curvature. The larger signal of the T transition is due to the threefold degeneracy of the spin triplets. Inset: schematic energy diagram illustrating XB1− emission in the (3,1) regime. (b) PL from QD-R (in colorscale), which is weaker than that from QD-B because holes can tunnel from QD-R to QD-B before recombination. XR0 (XR1−) indicates emission from the neutral exciton (negative trion) in QD-R. The significant overlap between the XR0 and XR1− transition below −220 mV suggests that the tunnelling rate between the s-orbital and the back contact is slower than the radiative recombination rate of ~1 GHz. In contrast, the sharp transitions between plateaus above −140 mV indicate that the tunnelling rate from the p-orbitals to the back contact is larger than ~1 GHz. The multiple “satellite lines” that are especially strong for XR1− are most likely due to fluctuations in the charge of QD-B, leading to a shift in PL due to charge sensing33. Inset: schematic energy diagram illustrating emission in the (3,1) regime, which involves the s-orbital in QD-R, and therefore does not reflect the anti-crossings involving the p-orbitals.

Mentions: To implement the four-electron qubit experimentally, we select a pair of tunnel-coupled self-assembled InGaAs QDs2122 where the lower dot is ~6 nm red-detuned from the upper one. This unusual configuration ensures that QD-R is charged with three electrons before the first electron enters QD-B. From the voltage-dependent photoluminescence (PL) at B = 0 T, we identify the region in V where the CQD contains four electrons. The PL from QD-B clearly reflects the anti-crossings for both singlet and triplet states (highlighted in the orange box in Fig. 2a). The S transition can be identified by its ~3 times weaker intensity compared to the transition involving the threefold degenerate T states. From the shape of the anti-crossing, we find the inter-dot tunnelling rate between the s-orbital in QD-R and the p-orbitals in QD-B to be ~60 GHz. There are no signs of the anti-crossings in the PL from QD-R (orange box in Fig. 2b), since this involves recombination from its low-lying s-orbital, which does not tunnel-couple to QD-B due to the large energy difference (see the inset to Fig. 2b).


Magnetically tunable singlet-triplet spin qubit in a four-electron InGaAs coupled quantum dot.

Weiss KM, Miguel-Sanchez J, Elzerman JM - Sci Rep (2013)

Locating the four-electron regime at B = 0 T.(a) PL from QD-B (in colorscale) as a function of V. Dotted vertical lines indicate the gate voltages where the charge configuration of the optically excited states changes. XB0 (XB1−) indicates emission from the neutral exciton (negative trion) in QD-B. PL involving the four-electron S and T ground states (highlighted in the orange box) exhibits a characteristic curvature. The larger signal of the T transition is due to the threefold degeneracy of the spin triplets. Inset: schematic energy diagram illustrating XB1− emission in the (3,1) regime. (b) PL from QD-R (in colorscale), which is weaker than that from QD-B because holes can tunnel from QD-R to QD-B before recombination. XR0 (XR1−) indicates emission from the neutral exciton (negative trion) in QD-R. The significant overlap between the XR0 and XR1− transition below −220 mV suggests that the tunnelling rate between the s-orbital and the back contact is slower than the radiative recombination rate of ~1 GHz. In contrast, the sharp transitions between plateaus above −140 mV indicate that the tunnelling rate from the p-orbitals to the back contact is larger than ~1 GHz. The multiple “satellite lines” that are especially strong for XR1− are most likely due to fluctuations in the charge of QD-B, leading to a shift in PL due to charge sensing33. Inset: schematic energy diagram illustrating emission in the (3,1) regime, which involves the s-orbital in QD-R, and therefore does not reflect the anti-crossings involving the p-orbitals.
© Copyright Policy - open-access
Related In: Results  -  Collection

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f2: Locating the four-electron regime at B = 0 T.(a) PL from QD-B (in colorscale) as a function of V. Dotted vertical lines indicate the gate voltages where the charge configuration of the optically excited states changes. XB0 (XB1−) indicates emission from the neutral exciton (negative trion) in QD-B. PL involving the four-electron S and T ground states (highlighted in the orange box) exhibits a characteristic curvature. The larger signal of the T transition is due to the threefold degeneracy of the spin triplets. Inset: schematic energy diagram illustrating XB1− emission in the (3,1) regime. (b) PL from QD-R (in colorscale), which is weaker than that from QD-B because holes can tunnel from QD-R to QD-B before recombination. XR0 (XR1−) indicates emission from the neutral exciton (negative trion) in QD-R. The significant overlap between the XR0 and XR1− transition below −220 mV suggests that the tunnelling rate between the s-orbital and the back contact is slower than the radiative recombination rate of ~1 GHz. In contrast, the sharp transitions between plateaus above −140 mV indicate that the tunnelling rate from the p-orbitals to the back contact is larger than ~1 GHz. The multiple “satellite lines” that are especially strong for XR1− are most likely due to fluctuations in the charge of QD-B, leading to a shift in PL due to charge sensing33. Inset: schematic energy diagram illustrating emission in the (3,1) regime, which involves the s-orbital in QD-R, and therefore does not reflect the anti-crossings involving the p-orbitals.
Mentions: To implement the four-electron qubit experimentally, we select a pair of tunnel-coupled self-assembled InGaAs QDs2122 where the lower dot is ~6 nm red-detuned from the upper one. This unusual configuration ensures that QD-R is charged with three electrons before the first electron enters QD-B. From the voltage-dependent photoluminescence (PL) at B = 0 T, we identify the region in V where the CQD contains four electrons. The PL from QD-B clearly reflects the anti-crossings for both singlet and triplet states (highlighted in the orange box in Fig. 2a). The S transition can be identified by its ~3 times weaker intensity compared to the transition involving the threefold degenerate T states. From the shape of the anti-crossing, we find the inter-dot tunnelling rate between the s-orbital in QD-R and the p-orbitals in QD-B to be ~60 GHz. There are no signs of the anti-crossings in the PL from QD-R (orange box in Fig. 2b), since this involves recombination from its low-lying s-orbital, which does not tunnel-couple to QD-B due to the large energy difference (see the inset to Fig. 2b).

Bottom Line: However, at this fixed operating point the ground-state splitting can no longer be tuned into resonance with e.g. another qubit, limiting the options for coupling multiple qubits.Here, we propose using a four-electron coupled quantum dot to implement a singlet-triplet qubit that features a magnetically tunable level splitting.As a first step towards full experimental realization of this qubit design, we use optical spectroscopy to demonstrate the tunability of the four-electron singlet-triplet splitting in a moderate magnetic field.

View Article: PubMed Central - PubMed

Affiliation: Institute for Quantum Electronics, ETH Zurich, CH-8093 Zurich, Switzerland.

ABSTRACT
A pair of self-assembled InGaAs quantum dots filled with two electrons can act as a singlet-triplet spin qubit that is robust against nuclear spin fluctuations as well as charge noise. This results in a T2* coherence time two orders of magnitude longer than that of a single electron, provided the qubit is operated at a particular "sweet spot" in gate voltage. However, at this fixed operating point the ground-state splitting can no longer be tuned into resonance with e.g. another qubit, limiting the options for coupling multiple qubits. Here, we propose using a four-electron coupled quantum dot to implement a singlet-triplet qubit that features a magnetically tunable level splitting. As a first step towards full experimental realization of this qubit design, we use optical spectroscopy to demonstrate the tunability of the four-electron singlet-triplet splitting in a moderate magnetic field.

No MeSH data available.


Related in: MedlinePlus