Decoupling a hole spin qubit from the nuclear spins

Abstract

A huge effort is underway to develop semiconductor nanostructures as low noise hosts for qubits. The main source of dephasing of an electron spin qubit in a GaAs-based system is the nuclear spin bath 1-3 . A hole spin may circumvent the nuclear spin noise 4 . In principle, the nuclear spins can be switched off for a pure heavy-hole spin 4-6 . In practice, it is unknown to what extent this ideal limit can be achieved. A major hindrance is that p-type devices are often far too noisy. We investigate here a single hole spin in an InGaAs quantum dot embedded in a new generation of low-noise p-type device. We measure the hole Zeeman energy in a transverse magnetic field with 10 neV resolution by dark state spectroscopy as we create a large transverse nuclear spin polarization. The hole hyperfine interaction is highly anisotropic: the transverse coupling is < 1% of the longitudinal coupling. For unpolarized, randomly fluctuating nuclei, the ideal heavy-hole limit is achieved down to neV energies; equivalently dephasing times up to a µs. The combination of large T * 2 and strong optical dipole 3,7-11 make the single hole spin in a GaAs-based device an attractive quantum platform.A localized, single spin is a small and fast qubit. The exchange interaction between neighbouring spins facilitates two-qubit operations 12 . Implementation in a semiconductor benefits from advanced semiconductor heterostructures and nano-fabrication; implementation in a GaAs-based heterostructure, the cleanest and most versatile semiconductor system, by trapping the spin to a quantum dot also facilitates the creation of a spin-photon interface. A stumbling block is that the nuclear spins in the quantum dot lead to a rapid loss of electron spin coherence (both T 2 and T * 2 processes). This has motivated interest in isotopically-pure silicon, a nuclear spin-free host 13 . However, the large effective mass of a conduction electron in Si demands much smaller structures and lower operating temperatures; the valley degeneracy is an additional complication 14 ; the strong

Phase-tuned entangled state generation between distant spin qubits

“…Using current state-ofthe-art light-collection strategies [43,44], the entanglement rate could be improved to 130 kHz. This rate approaches the inverse of electron-spin coherence times in self-assembled QDs [21,23,45], a crucial benchmark for fault tolerant scalability [46]. With further improvement of this rate, the second spin of a QD molecule [47] could be utilized as a memory qubit.…”

Generation of Non‐Classical Light Using Semiconductor Quantum Dots

“…For applications in quantum technologies, it is important to quantify the decoherence timescales and to understand the dephasing mechanisms in order to extend achievable coherence times. To these ends a lot of research has been carried out and the main dephasing mechanisms have been identified to be coupling to the nuclear spins as well as charge noise . The nuclear spins of the atoms forming the QD are seen by the central spin as a magnetic Overhauser field.…”

“…However, in a temporal or spatial ensemble, the Overhauser field is randomly distributed leading to fast dephasing of electron spins with . Due to their p‐like central cell wavefunction holes have a weaker hyperfine contact interaction and consequently more than one order of magnitude longer have been reported in time domain experiments and in frequency domain measurements (coherent population trapping) for holes times approaching one microsecond were reported . On intermediate timescales of hundreds of ns, the Overhauser field coherently evolves in time due to the quadrupolar moments resulting from the strained nuclei .…”

Hyperfine interaction for holes in quantum dots: k·p model

“…Hyperfine coupling between the spin of a hole localized in a self-assembled quantum dot (QD) and the nuclear spins of the atoms of the host materials has been a subject of intense experimental [1][2][3][4][5][6][7][8] and theoretical [6,9,10] investigations in recent years. The original reason for resurgence of interest in this topic was the prospect of using hole spins as qubits with long coherence times [1,2,8,[11][12][13][14]. This was motivated by the fact that dephasing of electron spins in QDs, being an obstacle to their application as qubits for quantum information processing purposes, is dominated by their hyperfine (hf) interaction with the nuclear spins of the host material [15][16][17][18][19][20], and the hole-nucleus coupling was expected to be much weaker than the electron-nucleus one [9,10].…”