Loss DiVincenzo Spin Qubit: First Quantum Computing Proposal
Spin qubits
Loss DiVincenzo spin qubits, inspired by transistors, are a possible quantum computer development path. Spin qubit quantum computers can aggregate millions of quantum bits on a chip, like classical circuits. This approach manipulates electron and electron hole spins in semiconductor devices to define Qubit.
Revolutionary DiVincenzo Spin Qubit Loss
In 1997, Daniel Loss and David P. DiVincenzo proposed the spin qubit quantum computer. They proposed qubits made of electrons trapped in Quantum dots, which had spin-1/2 degree of freedom. This must be distinguished from nuclear spin-based proposals like the Kane quantum computer.
The Loss-DiVincenzo approach addresses DiVincenzo's scalable quantum computer requirements:
Strong quantum measurements.
Well-defined qubit identification.
Reliable state preparedness.
Low decoherence.
Correct quantum gate operations.
Lateral quantum dot systems are promise for this type of quantum computer.
Key Operations: Control, Readout, Gates
QuTech team leader Menno Veldhorst explains spin qubit initialisation, reading, and control. For complex quantum applications, connecting two qubits to build logic gates is essential.
Using local magnetic fields or spin manipulation techniques, the Loss–DiVincenzo quantum computer uses the controlled NOT (CNOT) gate and an inter-dot gate voltage for swap operations.
A pulsed inter-dot gate voltage is used to switch the exchange constant in the Heisenberg Hamiltonian, making it time-dependent. This operation is valid under certain conditions:
Quantum dots require a level separation (ΔE) greater than kT (Boltzmann constant x temperature).
To avoid transitions to higher orbital levels, the pulse time scale (τs) must exceed ħ/ΔE.
The decoherence time (Γ⁻¹) must exceed τs.
The swap operator and swap gate square root can be obtained with a pulse duration and half that period, respectively. The conditional phase shift “XOR” gate is created by combining the square root of swap operations with spin rotation operations. By enclosing the target qubit with Hadamard gates, this conditional phase shift becomes a CNOT gate.
Building Blocks: Quantum Dot Electron Trapping
Spin qubits require trapping electrons and using their spin as the qubit. Understanding and exploiting charging energy and Coulomb blockade is necessary to construct the qubit. Charge detection can confirm a single electron in a quantum dot. Spin qubits require quantum dots.
Experimental Results and Future Prospects
Experimental spin qubits have been generated by locally depleting two-dimensional electron vapours in germanium and gallium arsenide (GaAs) semiconductors. They've been made in graphene and other materials.
Intel is aggressively developing silicon spin qubits, a recent achievement. Most importantly, the silicon platform can be employed with modern semiconductor device fabrication methods, making qubit scaling easier. Some silicon devices, called “hot qubits,” operate at a few kelvins, which helps quantum processors have more qubits.
Recently, Watson et al. (2018) shown that a two-spin qubit quantum processor can experimentally realise two quantum algorithms. The online QuantumInspire platform from Delft University of Technology can build and run quantum algorithms on a two-silicon spin qubit processor called “Spin-2”. Progress is being made to scale up these technologies for large-scale quantum integrated circuits.