Observing different quantum trajectories in cavity QED

The experimental observation of quantum jumps is an example of single open quantum systems that, when monitored, evolve in terms of stochastic trajectories conditioned on measurements results. Here we present a proposal that allows the experimental observation of a much larger class of quantum trajectories in cavity QED systems. In particular, our scheme allows for the monitoring of engineered thermal baths that are crucial for recent proposals for probing entanglement decay and also for entanglement protection. The scheme relies on the interaction of a three-level atom and a cavity mode that interchangeably play the roles of system and probe. If the atom is detected the evolution of the cavity fields follows quantum trajectories and vice-versa.Comment: 5 pages, 2 figure

A 10-bit 40MS/s Pipelined ADC in a 0.13μm CMOS Process

This paper presents a 10-bit analogue to digital converter (ADC) that will be integrated in a general purpose charge readout ASIC that is the new generation of mixed-mode integrated circuits for Time Projection Chamber (TPC) readout. It is based on a pipelined structure with double sampling and was implemented with switched capacitor circuits in eight 1.5-bit stages followed by a 2-bit stage. The power consumption is adjustable with the conversion rate and varies between 15 and 34mW for a 15 to 40MS/s conversion speed. The ADC occupies a silicon area of 0.7mm2 in a 0.13μm CMOS process and operates from a single 1.5V supply

Cooperativity of a few quantum emitters in a single-mode cavity

We theoretically investigate the emission properties of a single-mode cavity coupled to a mesoscopic number of incoherently pumped quantum emitters. We propose an operational measure for the medium cooperativity, valid both in the bad and in the good cavity regimes. We show that the opposite regimes of subradiance and superradiance correspond to negative and positive cooperativity, respectively. The lasing regime is shown to be characterized by nonnegative cooperativity. In the bad cavity regime we show that the cooperativity defines the transitions from subradiance to superradiance. In the good cavity regime it helps to define the lasing threshold, also providing distinguishable signatures indicating the lasing regime. Increasing the quality of the cavity mode induces a crossover from the solely superradiant to the lasing regime. Furthermore, we verify that lasing is manifested in a wide range of positive cooperative behavior, showing that stimulated emission and superradiance can coexist. The robustness of the cooperativity is studied with respect to experimental imperfections, such as inhomogeneous broadening and pure dephasing

Continuous Quantum Error Correction Through Local Operations

We propose local strategies to protect global quantum information. The protocols, which are quantum error correcting codes for dissipative systems, are based on environment measurements, direct feedback control and simple encoding of the logical qubits into physical qutrits whose decaying transitions are indistinguishable and equally probable. The simple addition of one extra level in the description of the subsystems allows for local actions to fully and deterministically protect global resources, such as entanglement. We present codes for both quantum jump and quantum state diffusion measurement strategies and test them against several sources of inefficiency. The use of qutrits in information protocols suggests further characterization of qutrit-qutrit disentanglement dynamics, which we also give together with simple local environment measurement schemes able to prevent distillability sudden death and even enhance entanglement in situations in which our feedback error correction is not possible.Comment: Accepted for publication in Phys. Rev.

Universal and deterministic manipulation of the quantum state of harmonic oscillators: a route to unitary gates for Fock State qubits

We present a simple quantum circuit that allows for the universal and deterministic manipulation of the quantum state of confined harmonic oscillators. The scheme is based on the selective interactions of the referred oscillator with an auxiliary three-level system and a classical external driving source, and enables any unitary operations on Fock states, two-by-two. One circuit is equivalent to a single qubit unitary logical gate on Fock states qubits. Sequences of similar protocols allow for complete, deterministic and state-independent manipulation of the harmonic oscillator quantum state.Comment: 4 pages, 4 figure