Spin-valley lifetimes in a silicon quantum dot with tunable valley splitting

Yang, C. H.; Rossi, Alessandro; Ruskov, Rusko; Lai, Nai Shyan; Mohiyaddin, Fahd A.; Lee, S.; Tahan, Charles; Klimeck, Gerhard; Morello, Andrea; Dzurak, A. S.

doi:10.1038/ncomms3069

Cited by 295 publications

(550 citation statements)

References 48 publications

(69 reference statements)

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“…Single qubit gate fidelities estimated for current parameters with valley splitting control 20 , reach the fault-tolerance threshold of the recently developed surface codes 9 . The use of a micromagnet facilitates selective addressing of neighbouring spins and provides a coupling mechanism of quantum dot spins to stripline resonators that can form the basis for two-qubit gates and a scalable architecture 30 .…”

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confidence: 80%

Electrical control of a long-lived spin qubit in a Si/SiGe quantum dot

et al. 2014

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Nanofabricated quantum bits permit large-scale integration but usually suffer from short coherence times due to interactions with their solid-state environment 1 . The outstanding challenge is to engineer the environment so that it minimally affects the qubit, but still allows qubit control and scalability. Here we demonstrate a long-lived single-electron spin qubit in a Si/SiGe quantum dot with all-electrical two-axis control. The spin is driven by resonant microwave electric fields in a transverse magnetic field gradient from a local micromagnet 2,3 , and the spin state is read out in single-shot mode 4 . Electron spin resonance occurs at two closely spaced frequencies, which we attribute to two valley states. Thanks to the weak hyperfine coupling in silicon, Ramsey and Hahn echo decay timescales of s µ 1 and s µ 40 , respectively, are observed. This is almost two orders of magnitude longer than the intrinsic timescales in III-V quantum dots 5,6 , while gate operation times are comparable to those achieved in GaAs 3,7,8 . This places the single-qubit rotations in the fault-tolerant regime 9 and strongly raises the prospects of quantum information processing based on quantum dots.The proposal by Loss and DiVincenzo 10 to define quantum bits by the state of a single electron spin in a gate-defined semiconductor quantum dot has guided research for the past 15 years 7 . Most progress was made in well-controlled III-V quantum dots, where spin manipulation with two 6,11 , three 12 and four 13 dots has been realized, but gate fidelities and spin coherence times are limited by the unavoidable interaction with the fluctuating nuclear spins in the host substrate 5,6 . While the randomness of the nuclear spin bath could be mitigated to some extent by feedback techniques 14 , eliminating the nuclear spins by using group IV host materials offers the potential for extremely long electron spin coherence times that exceed one second in P impurities in bulk 28 Si 15,16 .Much effort has been made to develop stable spin qubits in quantum dots defined in carbon nanotubes 17,18 , Ge/Si core/shell nanowires 19 , Si MOSFETs 20,21 and Si/SiGe 2D electron gases 16,22,23 . However, coherent control in these group IV quantum dots is so far limited to a Si/SiGe singlet-triplet qubit with only single-axis control 23 and a carbon nanotube single-electron spin qubit, with a Hahn echo decay time of 65 ns 17 .Our device is based on an undoped Si/SiGe heterostructure with two layers of electrostatic gates (Fig. 1a). Compared to conventional, doped heterostructures, this technology strongly improves charge stability 23 . First, accumulation gates ( mV 150 a + V ) are used to induce a twodimensional electron gas (2DEG) in a 12 nm wide Si quantum well 37 nm below the surface. Second, a set of depletion gates, labelled 1-12 in Fig. 1a, is used to form a single or double quantum dot in the 2DEG, flanked by a quantum point contact and another dot intended as charge sensors. Two μm 1 -wide, 200 nm-thick, and μm 5 . 1 -long Co magnets are placed...

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confidence: 80%

Electrical control of a long-lived spin qubit in a Si/SiGe quantum dot

et al. 2014

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“…For example, gates C1 and C2 are employed to control the confinement of the dot with minimal effect on the tunnel barrier potential profiles. Devices of this kind have already been proven to be very effective in precisely controlling the quantum dot energy level spectrum 32 . As we discuss below, important advantages in the context of charge pumping can also be obtained.…”

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confidence: 99%

An Accurate Single-Electron Pump Based on a Highly Tunable Silicon Quantum Dot

et al. 2014

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Nanoscale single-electron pumps can be used to generate accurate currents, and can potentially serve to realize a new standard of electrical current based on elementary charge. Here, we use a silicon-based quantum dot with tunable tunnel barriers as an accurate source of quantized current. The charge transfer accuracy of our pump can be dramatically enhanced by controlling the electrostatic confinement of the dot using purposely engineered gate electrodes. Improvements in the operational robustness, as well as suppression of non-adiabatic transitions that reduce pumping accuracy, are achieved via small adjustments of the gate voltages. We can produce an output current in excess of 80 pA with experimentally determined relative uncertainty below 50 parts per million.As early as one and a half centuries ago, J. C. Maxwell envisaged the need for a system of standards based on phenomena at the atomic scale and directly related to invariant constants of nature. 1 However, Maxwell could not anticipate that, in order to harness the behaviour of the world at the nanometer scale, a completely new physical interpretation was needed, namely, quantum mechanics. At first, the laws of quantum mechanics seemed to reveal fundamental limits to the accuracy of physical measurements. Concepts like the Heisenberg uncertainty principle, which imposes intrinsic fluctuations on the values of non-commuting observables, and the wavefunction collapse, responsible for the randomization of a system configuration after performing a measurement, appeared to be at odds with the requirement of deterministic consistency that is paramount for metrological purposes. Nevertheless, quantum-based systems are today acknowledged as the most stable and reliable metrological tools, as they can be strongly intertwined with fundamental constants. Exquisitely quantum-mechanical phenomena such as the ac Josephson effect 2 and the quantum Hall effect 3 have paved the way towards new and more reliable reference standards for the units of voltage and resistance, respectively.Major efforts are currently ongoing to re-define the unit of electrical current, the ampere (A), in terms of the elementary charge, e, by means of quantum technologies 4,5 . A practical implementation of this standard may be the electron pump, a device in which a quantum phenomenon, namely tunnelling, and classical Coulomb repulsion, are combined to control the transfer of an integer number of elementary charges. This device ideally generates a quantized output current, I P = nef , where n is an integer and f is the frequency of an external periodic drive. Several enabling technologies have already been developed including metal/oxide tunnel barrier devices 6,7 , normal-metal/superconductor turnstiles 8,9 , graphene double quantum dots 10 , donor-based pumps 11-13 , silicon-based quantum dot pumps 14-18 and GaAs-based quantum dot pumps [19][20][21][22][23][24][25][26][27] . To date, the latter scheme provides the lowest uncertainty of 1.2 parts per million (ppm) yielding current in excess o...

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“…[15][16][17][18][19] While interface confinement and scattering can lift this degeneracy, details at the interface, whether it is surface roughness or steps, play important roles in determining the magnitude of the valley splitting E VS , [20][21][22][23][24][25][26][27][28][29] so that device variability is large. Experimentally measured E VS ranges from vanishingly small, to several hundreds of µeV, 9,30,31 to possibly a few meV. 32 Furthermore, to achieve controllability, spin qubits are generally located near or at the interface between the host and the barrier materials.…”

Section: Introductionmentioning

confidence: 99%

“…[3][4][5][6][7][8][9][10] Specifically, the low abundance of isotopes with finite nuclear spins ( 29 Si) in natural Si significantly reduces the hyperfine interaction strength 11 and the spin dephasing. 7 Isotopic purification further suppresses this decoherence channel, so that Si behaves as if it is a "semiconductor vacuum" for a spin qubit.…”

Section: Introductionmentioning

confidence: 99%

Spin relaxation in a Si quantum dot due to spin-valley mixing

Huang

2014

Phys. Rev. B

103

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We study the relaxation of an electron spin qubit in a Si quantum dot due to electrical noise. In particular, we clarify how the presence of conduction-band valleys influences spin relaxation. In single-valley semiconductor quantum dots, spin relaxation is through the mixing of spin and envelope orbital states via spin-orbit interaction. In Si, the relaxation could also be through the mixing of spin and valley states. We find that the additional spin relaxation channel, via spinvalley mixing and electrical noise, is indeed important for an electron spin in a Si quantum dot. By considering both spin-valley and intra-valley spin-orbit mixings and Johnson noise in a Si device, we find that the spin relaxation rate peaks at the hot spot, where the Zeeman splitting matches the valley splitting. Furthermore, because of a weaker field dependence, the spin relaxation rate due to Johnson noise could dominate over phonon noise at low magnetic fields, which fits well with recent experiments.

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Spin-valley lifetimes in a silicon quantum dot with tunable valley splitting

Cited by 295 publications

References 48 publications

Electrical control of a long-lived spin qubit in a Si/SiGe quantum dot

Electrical control of a long-lived spin qubit in a Si/SiGe quantum dot

An Accurate Single-Electron Pump Based on a Highly Tunable Silicon Quantum Dot

Spin relaxation in a Si quantum dot due to spin-valley mixing

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