The US companies ColdQuanta and Atom Computing are both building devices using neutral atoms. By using two lasers facing in opposite positions the atoms are cooled to a very low temperature, explaining why the qubits are sometimes called “cold atoms”. An advantage of this technique is that because the neutral atoms have no charge, there is no electrical repulsion, and they can be placed closer together, making it easier to scale devices. In neutral atom qubits, individual atoms are trapped and shuffled around by lasers using “optical tweezers”. In 2021 workers at Quantinuum used this complex technique to demonstrate real time error correcting. This may seem cumbersome, but a similar technique, CCD, is used in the state of the art James Webb telescope. In QCCD the trapped ion qubits are shuffled between different processing zones using dynamic electric fields. A new technology for trapped-ion is the quantum charge-coupled device (QCCD). Companies that build trapped ion devices include Quantinuum, a combination of Cambridge Quantum with Honeywell Quantum Solutions, and IonQ.ĭevices with trapped ion qubits have relatively low error rates, but it is not clear how scalable these devices are. The details are covered in an excellent tutorial by Pennylane. Once trapped the ions can be cooled using lasers, and then manipulated into different quantum states using lasers of different frequencies. Although a static electric field can’t trap an ion on its own, the ions can be trapped by an oscillating electric field in a Paul trap. Trapped ion qubitsĪtoms which have lost or gained an electron are called ions, and are electrically charged. In trapped ion and neutral atom qubits the two quantum states of the qubit are represented by two different energy levels of an electron in an atom. Beware, by necessity this blog contains some technical details which some readers may wish to skip over. In this blog I will also look at three competing paradigms for quantum computing. It is possible to overcome limited connectivity by swapping quantum states from one qubit to another, but there is an overhead to this process, which requires extra two-qubit gates. Some quantum algorithms need each qubit to be connected to every other qubit. The “quantum volume” of a quantum device is a figure of merit reflecting both the number of qubits and their error rates.Īs well as resilience against errors, and scalability, it is important that a qubit can connect to as many other qubits as possible. Some quantum algorithms will require thousands, or even millions of qubits, so all contemporary devices need a plan to scale the number of qubits. Two-qubit gate operations, where one qubit controls another qubit, are particularly prone to error because of cross talk with other qubits. A qubit also needs to have fast operations, and low error rates, so it is possible to carry out many gate operations with minimal errors. A qubit needs to be resilient against noise, and maintain its superposition, and entanglement with other qubits, for a long time in the face of environmental interactions. We are in the era of Noisy Intermediate Scale Quantum devices, where noise and scale both limit the computations that can be carried out. However, different types of qubit have different strengths and weaknesses, which you may need to be aware of. The qubits inside quantum devices can be regarded as black boxes.
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