Nuclear spin effects in semiconductor quantum dots

Chekhovich, E. A.; Makhonin, M. N.; Tartakovskii, A. I.; Yacoby, Amir; Bluhm, Hendrik; Nowack, Katja C.; Vandersypen, L. M. K.

doi:10.1038/nmat3652

Cited by 225 publications

(233 citation statements)

References 112 publications

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“…This formula is expected from the classical "folded down" perturbation approach [1][2][3][4][5][6][7][11][12][13][14]16 . On the other hand, all data points of strained In(Ga)As/GaAs QDs exhibit a denominator ∆ HL that is effectively reduced by δ = 78.6 meV with respect to the class of unstrained GaAs QDs and fall then close to another curve:…”

Section: K · P Analysis Of Atomistic Microscopic Resultsmentioning

confidence: 99%

“…The perturbation potential δV C 2v lowers the QD symmetry to C 2v and hence introduces HH-LH mixing, which modifies the ground hole state HH0 [1][2][3][4][5][6][7][11][12][13][14]16,19,22 ,…”

Section: Discussion Of the Supercoupling Effectmentioning

confidence: 99%

“…(ii) providing an efficient channel for spin decoherence [5][6][7][8][9][10] , (iii) creating a polarization anisotropy of light emission which is important for quantum information schemes [11][12][13][14][15] , and (iv) giving an additional efficient mechanism for optical initialisation of hole spin qubit 16,17 . The origin of this mixing has fascinated physicists and chemists ever since low-dimensional semiconductor nanostructures such as 2D quantum wells, 1D quantum wires, and 0D QDs have been made [11][12][13]19 , yet, the controlling mechanisms are rather vague.…”

Section: Introductionmentioning

confidence: 99%

“…The most popular theoretical approach used in nanostructures is to fold the Luttinger-Kohn k · p or PikusBir strained Hamiltonian of bulk zinc-blende (ZB) semiconductors down to an effective 2 × 2 HH Hamiltonian and taking the admixture of neighboring bands such as LH band into account perturbatively [5][6][7][8][9]18 . In the early days of nanostructures research, the HH-LH mixing was depicted as a result of spatial quantum confinement 5,14,[20][21][22][23][24] , which leads to finite off-diagonal matrix elements within the Luttinger-Kohn k · p Hamiltonian.…”

Section: Introductionmentioning

confidence: 99%

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Supercoupling between heavy-hole and light-hole states in nanostructures

2015

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The heavy-hole (HH) and light-hole (LH) components of the valence states in 3D bulk semiconductors can mix quantum mechanically as the dimensionality is reduced in forming 2D, 1D nanostructures and 0D quantum dots (QDs). This coupling controls the tuning of the excitonic fine structure splitting, provides an efficient channel for the spin coherence, and leads to polarization anisotropy of light emission, central to several quantum information schemes. Current understanding is that the mixing scales with the square of δV HL /∆ HL , where δV HL and ∆ HL are the coupling matrix elements of the crystal potential and the energy separation between the primary HH0 and LH0 states, respectively. We discuss two classes of HH-LH coupling mechanisms.First, coupling factors occurring through the numerator δV HL , referred to as "direct coupling", including the well-known (1) quantum confinement, (2) built-in strain, and (3) shape elongation, as well as three additional direct coupling mechanisms discussed here: (4) the intrinsic C 2v crystal field effect, (5) the local symmetry of the interface, and (6) the alloy disorder. We quantify these 6 direct HH-LH coupling effects by performing atomistic pseudopotential calculations on a range of strained and unstrained QDs of different morphologies. We find that in unstrained self-assembled QDs such as GaAs/AlGaAs effects (1)-(6) contribute 0%, 0%, 0%, 0%, 40%, and 60%, respectively, whereas in strained self-assembled QDs such as InGaAs/GaAs they contribute 0%, 0%, 78%, 0%, 8%, and 14%, respectively, to the direct HH-LH coupling δV HL . These relative contributions to direct HH-LH coupling differ significantly from what was previously believed.Second, we discover an unexpected HH-LH coupling that effectively reduces the denominator ∆ HL by the presence of a dense ladder of intermediate states between the HH0 and LH0 states (analogous to super-exchange in magnetism). Supercoupling amplifies and propagates the HH-LH interaction and is the dominant source of HH-LH mixing in strained nanostructures where ∆ HL is fairly large, so by the direct coupling mechanism alone δV HL /∆ HL would be expected to be rather small. Supercoupling explains a number of outstanding puzzles including the surprising fact that in strained (InAs/GaAs) QDs the mixing is very strong despite the fact that ∆ HL is large, and offers a new way to manipulate HH-LH mixing and hence associated properties in nanostructures.

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Section: K · P Analysis Of Atomistic Microscopic Resultsmentioning

confidence: 99%

Section: Discussion Of the Supercoupling Effectmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Supercoupling between heavy-hole and light-hole states in nanostructures

2015

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“…We showed that this feedback-based protection algorithm can protect the qubit coherence far beyond the dephasing time of the qubit, even if no active control is applied to decouple it from the noise. The algorithm can be extended to applications in quantum information processing and quantum sensing, and it could be implemented in many other hybrid spin systems, such as phosphorus [27] or antimony [28] donors in silicon, defects in silicon carbide [29] or quantum dots [30]. As we applied a coherent feedback protocol, thus avoiding measuring the state of the ancilla, the decoherent effects of the bath are effectively stored in the ancilla.…”

Section: Figmentioning

confidence: 99%

Coherent feedback control of a single qubit in diamond

Hirose

Cappellaro

2016

Nature

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Engineering desired operations on qubits subjected to the deleterious effects of their environment is a critical task in quantum information processing, quantum simulation and sensing. The most common approach is to rely on open-loop quantum control techniques, including optimal control algorithms, based on analytical [1] or numerical [2] solutions, Lyapunov design [3] and Hamiltonian engineering [4]. An alternative strategy, inspired by the success of classical control, is feedback control [5]. Because of the complications introduced by quantum measurement [6], closed-loop control is less pervasive in the quantum settings and, with exceptions [7,8], its experimental implementations have been mainly limited to quantum optics experiments. Here we implement a feedback control algorithm with a solid-state spin qubit system associated with the Nitrogen Vacancy (NV) centre in diamond, using coherent feedback [9] to overcome limitations of measurement-based feedback, and show that it can protect the qubit against intrinsic dephasing noise for milliseconds. In coherent feedback, the quantum system is connected to an auxiliary quantum controller (ancilla) that acquires information about the system's output state (by an entangling operation) and performs an appropriate feedback action (by a conditional gate). In contrast to open-loop dynamical decoupling (DD) techniques [10], feedback control can protect the qubit even against Markovian noise and for an arbitrary period of time (limited only by the ancilla coherence time), while allowing gate operations. It is thus more closely related to Quantum Error Correction schemes [11][12][13][14], which however require larger and increasing qubit overheads. Increasing the number of fresh ancillas allows protection even beyond their coherence time. We can further evaluate the robustness of the feedback protocol, which could be applied to quantum computation and sensing, by exploring an interesting tradeoff between information gain and decoherence protection, as measurement of the ancilla-qubit correlation after the feedback algorithm voids the protection, even if the rest of the dynamics is unchanged.To demonstrate coherent feedback with spin qubits, we choose two of the most common tasks for qubits, implementing the no-operation (NOOP) and NOT gates, while cancelling the effects of noise. A simple, measurementbased feedback scheme, exploiting one ancillary qubit, was proposed in [16]. The correction protocol (Fig. 1a) works by entangling the qubit-ancilla system before the desired gate operation. By selecting an entangling operation U c appropriate for the type of bath acting on the system, information about the noise action is encoded in the ancilla state. After undoing the entangling operation, the qubit coherence can be restored by a feedback action, Hadamard gates prepare and read out a superposition state of the qubit, |φ q = 1 √ 2 (|0 + |1 ). Amid entangling gates between qubit and ancilla, the qubit is subjected to noise (and possibly unitary gates U ). We assume the ancil...

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