Single spin quantum dot (Loss-DiVincenzo) qubit

Description

This qubit consists of single electron spin trapped in a quantum dot (QD) in semiconductor system such as Si/SiGe or Si MOS heterostructures. The spin states of the electron defines the qubit |0> and |1> states. Since spin is a magnetic property, spin qubit does not directly interact with the electrical environment, which leads to long coherence times. Entangling gate between neighboring spins can be achieved by electrical control of the exchange coupling via tuning the tunneling between coupled QDs, and single qubit gates can be efficiently implemented using electric-dipole spin resonance (EDSR).

Figure

Motivation

Individual spin with S=1/2 is probably the simplest realization of a qubit. The electron spin qubit in semiconductor QD provides easy electrical control, potentially good scalability, and compatibility with current semiconductor technology.

References

• https://journals.aps.org/pra/abstract/10.1103/PhysRevA.57.120
• http://science.sciencemag.org/content/359/6374/439
• https://www.nature.com/articles/nature25766

Linked Papers

• loss-divincenzo-1998-quantum-dots

Related Entries

• kane-qubit
• singlet-triplet-qubit
• exchange-only-qubit

Seed Metadata

• date_published: 1998-01-01

Physics

Qubit = spin-1/2 of a single excess electron in a quantum dot ($|\uparrow ⟩$, $|\downarrow ⟩$). Two-qubit gate via Heisenberg exchange when the tunnel barrier is pulsed low:

$H_s(t)=J(t){\vec{S}}_1\cdot {\vec{S}}_2,J(t)=\frac{4t_0^2(t)}{u}$

where $t_0$ is the tunneling matrix element and $u$ is the charging energy. Pulsing $\int Jdt=\pi /2$ gives $\sqrt{\text{SWAP}}$, which combined with single-qubit Z-rotations (via local magnetic fields) forms a universal gate set. Spin coherence times are inherently longer than charge coherence times since spin is insensitive to electric field fluctuations.

Related Qubits

• singlet-triplet-qubit — two-spin encoding, no local fields needed
• exchange-only-qubit — three-spin encoding, exchange-only control
• aeon-qubit — always-on exchange variant
• semiconductor-charge-qubit — charge-based encoding (shorter coherence)
• kane-qubit — donor spin in silicon

Key Metrics

Metric	Value	Notes	Fidelity reference
Qubit coherence $T_1$	1–45 s	Electron spin in Si/SiGe (2024)	Elzerman et al. 2004
Qubit coherence $T_2$	0.5–28 ms	Hahn echo in ${}^{28}$Si	Veldhorst et al. 2014
Gate fidelity (1Q)	99.6–99.95%	ESR or EDSR driven	Yoneda et al. 2018
Gate fidelity (2Q)	99–99.8%	Exchange-based $\sqrt{\text{SWAP}}$	Noiri et al. 2022
Gate time (1Q)	1–100 μs	Depends on drive mechanism	—
Gate time (2Q)	1–100 ns	Exchange pulse	—
Readout fidelity	98–99.5%	Spin-to-charge conversion + SET	Noiri et al. 2022
Qubit footprint	~50–100 nm pitch	Very small; CMOS-compatible	—
Operating temperature	20 mK–1 K	Silicon: some operation at >1 K	—
Connectivity	Nearest-neighbor (1D/2D)	Exchange range ~100 nm	—