Qubit devices and the issue of quantum decoherence

Howard E. Brandt

A plethora of qubit devices is under consideration for possible development ranging from quantum telereporters to quantum computers. The primary obstacle to the success of these efforts is the phenomenon of quantum decoherence, the rapid vanishing of the off-diagonal components of the reduced density matrix representing the computational degrees of freedom of the device. Following a review of the physics of quantum decoherence, several instructive examples are exposited. Decoherence issues associated with the various possible approaches to quantum computing are addressed. Also, possible generic methods are reviewed for surmounting the decoherence obstacle.

Eddy Collin, Grégoire Ithier, P. Joyez +2 more

Howard E. Brandt

We first review the mathematical physics of quantum decoherence in generic single-qubit devices, including environmental interactions. Next, we formulate the persistence probability for a qubit device as the probability of measuring the qubits in their unperturbed state without the decoherence arising from environmental interactions. A decoherence time can be obtained from the persistence probability. Drawing on recent work of Garg, and also Palma, Suomine, and Ekert, we apply the persistence probability formalism to a generic single-qubit device coupled to a thermal environment, and also apply it to a trapped-ion quantum register coupled to its ion vibrational modes.

F. Chiarello, E Paladino, Maria Gabriella Castellano +4 more

A particular superconducting quantum interference device (SQUID) qubit, namely the double SQUID qubit, can be manipulated by rapidly modifying its potential with the application of fast flux pulses.In this system we observe coherent oscillations exhibiting non-exponential decay, indicating an unconventional decoherence mechanism.Moreover, via tuning the qubit in different conditions (different oscillation frequencies) by changing the pulse height, we observe a crossover between two distinct decoherence regimes and the existence of an 'optimal' point where the qubit is only weakly sensitive to intrinsic noise.We find that this behavior is in agreement with a model considering the decoherence caused essentially by low-frequency noise contributions, and we discuss the experimental results and possible issues.

Pablo Paz Juan