Quantum Dot Molecule Devices with Optical Control of Charge Status and Electronic Control of Coupling

Abstract

Tunnel-coupled pairs of optically active quantum dots-quantum dot molecules (QDMs)-offer the possibility to combine excellent optical properties such as strong light-matter coupling with two-spin singlet-triplet (S − T 0 ) qubits having extended coherence times. The S − T 0 basis formed using two spins is inherently protected against electric and magnetic field noise. However, since a single gate voltage is typically used to stabilize the charge occupancy of the dots and control the inter-dot orbital couplings, operation of the S − T 0 qubits under optimal conditions remains challenging. Here, an electric field tunable QDM that can be optically charged with one (1h) or two holes (2h) on demand is presented. A four-phase optical and electric field control sequence facilitates the sequential preparation of the 2h charge state and subsequently allows flexible control of the inter-dot coupling. Charges are loaded via optical pumping and electron tunnel ionization. Oneand two-hole charging efficiencies of (93.5 ± 0.8)% and (80.5 ± 1.3)% are achieved, respectively. Combining efficient charge state preparation and precise setting of inter-dot coupling allows for the control of few-spin qubits, as would be required for the on-demand generation of 2D photonic cluster states or quantum transduction between microwaves and photons.

Controlled Coherent Coupling in a Quantum Dot Molecule Revealed by Ultrafast Four-Wave Mixing Spectroscopy

“…Before the FWM pulse sequence is started, the QDM is typically in its ground states, i.e., in a thermal equilibrium of the hole states. However, recently an experimental scheme has been developed to deterministically initialize the QDM in one of the two hole states . It is thus becoming possible to induce a unidirectional coherence transfer between the two QDs.…”

“…However, recently an experimental scheme has been developed to deterministically initialize the QDM in one of the two hole states. 49 It is thus becoming possible to induce a unidirectional coherence transfer between the two QDs. This would manifest in the 2D FWM spectrum with the appearance of only one off-diagonal peak.…”

Gate‐Based Protocol Simulations for Quantum Repeaters using Quantum‐Dot Molecules in Switchable Electric Fields

“…Similarly, hole spins can be employed in this context, and the initialization of such states has recently been demonstrated. [29,34] For the microscopic description of electrons in the QDM, we employ an effective model using basis states |𝜓 B ⟩ and |𝜓 T ⟩ of the electron residing in the bottom (B) or top (T) dot, respectively. While more sophisticated methods like k ⋅ p calculations are possible, the above effective model has been used successfully to describe experimental spectra in QDMs.…”

“…The holes tunnel out of the QDM, while electron tunneling is prevented by the wider band gap tunneling barrier immediately adjacent to the QDM. [ 29 ] Thereby, the two electrons are prepared in a spin‐singlet state. Optical charging is needed since conventional charging via Coulomb blockade of carrier tunneling from a doped contact requires operation at a certain electric field, which renders the tuning of the electric field for gate operations impossible.…”

Recent advances in hole-spin qubits

“…For self-assembled quantum dots, scaling-up of single qubits through fabrication is challenging. While quantum-dot molecules formed by holes (in two vertically stacked dots) are actively studied [101][102][103] and, in fact, the coherent exchange interaction of two holes was demonstrated early on [92], an attractive alternative is to establish entanglement through the emitted photons. In 2016, an entangled state of two distant hole spins (5 m apart) was generated through a heralded protocol, based on the emission of Raman photons [104].…”