Trapped-ion quantum computer
Ions, or charged atomic particles, can be confined and suspended in free space using electromagnetic fields.
[1] The fundamental operations of a quantum computer have been demonstrated experimentally with the currently highest accuracy in trapped-ion systems.
As of December 2023, the largest number of particles to be controllably entangled is 32 trapped ions.
[2] The first implementation scheme for a controlled-NOT quantum gate was proposed by Ignacio Cirac and Peter Zoller in 1995,[3] specifically for the trapped-ion system.
The same year, a key step in the controlled-NOT gate was experimentally realized at NIST Ion Storage Group, and research in quantum computing began to take off worldwide.
[citation needed] In 2021, researchers from the University of Innsbruck presented a quantum computing demonstrator that fits inside two 19-inch server racks, the world's first quality standards-meeting compact trapped-ion quantum computer.
[5][4] The electrodynamic quadrupole ion trap currently used in trapped-ion quantum computing research was invented in the 1950s by Wolfgang Paul (who received the Nobel Prize for his work in 1989[6]).
[1][10] Furthermore, the additional vibrations of the added ions greatly complicate the quantum system, which makes initialization and computation more difficult.
These quantum states occur when the trapped ions vibrate together and are completely isolated from the external environment.
This initialization process is standard in many physics experiments and can be performed with extremely high fidelity (>99.9%).
After decay, the ion is continually excited by the laser and repeatedly emits photons.
These photons can be collected by a photomultiplier tube (PMT) or a charge-coupled device (CCD) camera.
If the ion collapses into the other qubit state, then it does not interact with the laser and no photon is emitted.
By counting the number of collected photons, the state of the ion may be determined with a very high accuracy (>99.99%).
[13] One of the requirements of universal quantum computing is to coherently change the state of a single qubit.
The term "rotation" alludes to the Bloch sphere representation of a qubit pure state.
These rotations are the universal building blocks for single-qubit gates in quantum computing.
Once the Hamiltonian is found, the formula for the unitary operation performed on the qubit can be derived using the principles of quantum time evolution.
Although this model utilizes the rotating wave approximation, it proves to be effective for the purposes of trapped-ion quantum computing.
However, as previously discussed, a finite number of qubits can be stored in each trap while still maintaining their computational abilities.
Ions can also be made to turn corners at a "T" junction, allowing a two dimensional trap array design.
An example is the quantum charge-coupled device (QCCD) designed by D. Kielpinski, Christopher Monroe and David J.
[15] QCCDs resemble mazes of electrodes with designated areas for storing and manipulating qubits.
Ions in the QCCD's memory region are isolated from any operations and therefore the information contained in their states is kept for later use.
Gates, including those that entangle two ion states, are applied to qubits in the interaction region by the method already described in this article.
[15] When an ion is being transported between regions in an interconnected trap and is subjected to a nonuniform magnetic field, decoherence can occur in the form of the equation below (see Zeeman effect).
Additional relative phases could arise from physical movements of the trap or the presence of unintended electric fields.
[1] However, since α from the interaction with the magnetic field is path-dependent, the problem is highly complex.
[1] Decoherence also proves to be challenging to eliminate, and is caused when the qubits interact with the external environment undesirably.
[11] It is therefore important that a trapped-ion quantum computer can perform this operation by meeting the following three requirements.
