Superconducting Qubits

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Quantum computing deals with the processing of information according to the laws of quantum mechanics. Within the last few years, it has attracted considerable attention because quantum computers are expected to be capable of performing certain tasks which no classical computers can do in practical time scales. 

1. Description

2. Why

3. How

4. Future Trends

5. Related Links

Description

Early proposals for quantum computers were mainly based on quantum optical systems, such as those utilizing . 

  • Laser-cooled trapped ions,
  • Photon or atoms in quantum electrodynamical ~QED! cavities,and nuclear magnetic resonance.

These systems are well isolated from their environment and satisfy the low-decoherence criterion for implementing quantum computing.Physicists in the US have taken another step towards the dream of a quantum computer by entangling two superconducting quantum bits (or qubits) for the first time. Circuits made from superconducting elements are promising candidates for a real quantum computer because they are compatible with conventional methods for making integrated circuits.

Why 

Superconducting electronics has not been able to compete with Si- and GaAs-technology in the field of computers, not even for special supercomputers.Electrical circuits made from superconductors are promising candidates for a working quantum computer because they can be made from thin films using conventional microchip fabrication technology. Coupling can be achieved simply by electrical connections between qubits – far easier than the trapped ion approach, where ions need to be shuttled about so they can interact.Moreover, due to quantum error correction algorithms,5 now decoherence6 is not regarded as an insurmountable barrier to quantum computing.Scalability of quantum computer architectures to many qubits is of central importance for realizing quantum computers of practical use, considerable efforts have recently been devoted to solid-state qubits.However, in the emerging field of Quantum Computing the situation is completely different. Now ``quantum coherence'' is the key issue and superconductivity has great advantages due to its built-in principle of ``macroscopic quantum coherence''. An important feature of superconducting junctions is a possibility to reach long decoherence times. 

How 

Superconducting qubits are quantum counterparts of the 1s and 0s used in today’s computers. They have been measured one at a time to avoid unwanted effects on neighboring qubits. The advance shows that the properties of artificial atoms can be coordinated in a way that is consistent with a quantum phenomenon called “entanglement” observed in real atoms. Entanglement is the “quantum magic” allowing the construction of logic gates in a quantum computer, a means of ensuring that the value of one qubit can be determined by the value of another in a predictable way. 

  • Two photons can be created in an experiment such that if one is polarized in the vertical direction, then the other is always polarized horizontally.
  • By measuring the polarization of one of the pair, we immediately know the state of the other, no matter how far apart they are.
  • This "spooky action at a distance", which has no classical analogue, could allow multiple bits of information to be processed at the same time in a quantum computer.
  • Such a device could therefore outperform a classical computer by many orders of magnitude.
  • There are currently many rival ways of entangling particles, for example by trapping ions at ultra-low temperatures and manipulating their internal energy states with lasers. 

However, demonstrating entanglement is hard. In particular, the particles, or qubits, have to be sufficiently isolated from the environment so that the fragile entangled state exists for long enough to allow a calculation to be carried out. Various other conditions also have to be met -- together known as the "DiVincenzo criteria" -- such as being able to measure both qubits at the same time.The researchers used a delicate method known as "quantum state tomography" to confirm the entanglement, whereby a series of different parameters are measured for the two particles and used to reconstruct the quantum state, much as image “slices” are captured and combined into a three-dimensional picture in tomographic medical imaging.

Future Trends 

Matthias Steffen and colleagues at Santa Barbara were able to entangle two qubits, each made from a Josephson tunnel junction, that meet the DiVincenzo criteria completely with a precision of 87% of theoretical values.Although physicists have been able to entangle up to eight ions at the same time -- whereas the present work entangles just two quibits -- Steffen insists superconducting qubits are a viable approach towards quantum computing. "Substituting some of the materials in the fabrication process should translate to a straightforward improvement of our results and in the long run, continued materials research should also help improve qubit performance," he says.

Keywords

Quantum state tomography, qubit, quantum computing, Flux-qubit

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