Coherent spin control of s-, p-, d- and f-electrons in a silicon quantum dot

Abstract

Once the periodic properties of elements were unveiled, chemical bonds could be understood in terms of the valence of atoms. Ideally, this rationale would extend to quantum dots, often termed artificial atoms, and quantum computation could be performed by merely controlling the outer-shell electrons of dot-based qubits. Imperfections in the semiconductor material, including at the atomic scale, disrupt this analogy between atoms and quantum dots, so that real devices seldom display such a systematic many-electron arrangement. We demonstrate here an electrostaticallydefined quantum dot that is robust to disorder, revealing a well defined shell structure. We observe four shells (31 electrons) with multiplicities given by spin and valley degrees of freedom. We explore various fillings consisting of a single valence electron -namely 1, 5, 13 and 25 electrons -as potential qubits, and we identify fillings that yield a total spin-1 on the dot. An integrated micromagnet allows us to perform electrically-driven spin resonance (EDSR). Higher shell states are shown to be more susceptible to the driving field, leading to faster Rabi rotations of the qubit. We investigate the impact of orbital excitations of the p-and d-shell electrons on single qubits as a function of the dot deformation. This allows us to tune the dot excitation spectrum and exploit it for faster qubit control. Furthermore, hotspots arising from this tunable energy level structure provide a pathway towards fast spin initialisation. The observation of spin-1 states may be exploited in the future to study symmetry-protected topological states in antiferromagnetic spin chains and their application to quantum computing.Qubit architectures based on electron spins in gatedefined silicon quantum dots benefit from a high level of controllability, where single and multi-qubit coherent operations are realised solely with electrical and magnetic manipulation. Furthermore, their direct compatibility with silicon microelectronics fabrication offers unique scale-up opportunities 1 . However, fabrication reproducibility and disorder pose challenges for single electron quantum dots. Even when the single-electron regime is achievable, the last electron often is confined in a very small region, limiting the effectiveness of electrical control and interdot tunnel coupling. Many-electron quantum dots were proposed as a qubit platform decades ago 2 , with the potential of resilience to charge noise 3,4 and a more tunable tunnel coupling strength to other qubits 5 . In the multielectron regime, the operation of a quantum dot qubit is more sensitive to its shape. If it is axially symmetric, the orbital energy levels will be quasi-degenerate 6-8 , which is detrimental for quantum computing. On the contrary, if the quantum dot is very elongated, a regular shell structure will not form, and the valence electron will not operate as a simple spin-1/2 system 2,9 . FILLING S-, P-, D-AND F-ORBITALS IN A SILICON QUANTUM DOTThe scanning electron microscope (SEM) image in Fig. 1a shows a s...

Operation of a silicon quantum processor unit cell above one kelvin

“…We achieve this by isolating the QDs from the electron reservoir, initialising and reading the qubits solely via tunnelling of electrons between the two QDs [7][8][9]. We coherently control the qubits using electrically-driven spin resonance (EDSR) [10,11] in isotopically enriched silicon 28 Si [12], attaining single-qubit gate fidelities of 98.6% and coherence time T * 2 = 2 µs during 'hot' operation, comparable to those of spin qubits in natural silicon at millikelvin temperatures [13][14][15][16]. Furthermore, we show that the unit cell can be operated at magnetic fields as low as 0.1 T, corresponding to a qubit control frequency of 3.5 GHz, where the qubit energy is well below the thermal energy.…”

“…Figure 1a shows a scanning electron microscope (SEM) image of a silicon metal-oxide-semiconductor (Si-MOS) double QD device nominally identical to the one measured. The device is designed with a cobalt micromagnet to facilitate EDSR, whereby an AC voltage at frequency f qubit is applied to the micromagnet electrode to drive spin resonance [10], and a single electron transistor (SET) charge sensor is used to detect changes in the electron occupation of the two QDs [11]. The experimental setup is described in Extended Data Figure 1.…”

“…The ability to operate the unit cell without any changes in electron occupation throughout initialization, control and readout is the prerequisite for scaling it up to large 2D arrays (see Figure 1h), where qubit control can be achieved by global magnetic resonance or via an array of micromagnets that allow local Figure 1 | An isolated spin qubit processor unit cell. a, SEM image of an identical two-qubit device with Co micromagnet for EDSR control [11]. b, Schematic of the Al gate stack.…”

“…c, Starting with a single dot under G1 as in Ref. [11], we load 6 electrons from the reservoir onto QD1. d, We lower VG2 just enough to deplete all electrons under G2.…”

“…Here, the lower two electrons in each dot form a spin-zero closed shell in the lower conduction-band valley state, and we use the spins of the unpaired electrons in the upper valley states of the silicon QDs as our qubits [27]. It is also possible to operate the qubits in the (1,1) and (1,3) charge configurations (see Extended Data Figure 3), but (3,3) is chosen for better EDSR driving strength and J-gate control [11].…”