X. K. Zhou

Quantum computation with untunable couplings.

Most quantum computer realizations require the ability to apply local fields and tune the couplings between qubits, in order to realize single bit and two bit gates which are necessary for universal quantum computation. We present a scheme to remove the necessity of switching the couplings between qubits for two bit gates, which are more costly in many cases. Our strategy is to compute with encoded qubits in and out of carefully designed interaction free subspaces analogous to decoherence free subspaces. We give two examples to show how universal quantum computation is realized in our scheme with local manipulations to physical qubits only, for both diagonal and off diagonal interactions.

Quantum simulation of 2D topological physics in a 1D array of optical cavities

Orbital angular momentum (OAM) of light is a fundamental optical degree of freedom that has recently motivated much exciting research in diverse fields ranging from optical communication to quantum information. We show for the first time that it is also a unique and valuable resource for quantum simulation, by demonstrating theoretically how 2d topological physics can be simulated in a 1d array of optical cavities using OAM-carrying photons. Remarkably, this newly discovered application of OAM states not only reduces required physical resources but also increases feasible scale of simulation. By showing how important topics such as edge-state transport and topological phase transition can be studied in a small simulator with just a few cavities ready for immediate experimental exploration, we demonstrate the prospect of photonic OAM for quantum simulation which can have a significant impact on the research of topological physics.Orbital angular momentum of light is a fundamental optical degree of freedom characterized by unlimited number of available angular momentum states. Although this unique property has proved invaluable in diverse recent studies ranging from optical communication to quantum information, it has not been considered useful or even relevant for simulating nontrivial physics problems such as topological phenomena. Contrary to this misconception, we demonstrate the incredible value of orbital angular momentum of light for quantum simulation by showing theoretically how it allows to study a variety of important 2D topological physics in a 1D array of optical cavities. This application for orbital angular momentum of light not only reduces required physical resources but also increases feasible scale of simulation, and thus makes it possible to investigate important topics such as edge-state transport and topological phase transition in a small simulator ready for immediate experimental exploration.

Quantum simulation of Heisenberg spin chains with next-nearest-neighbor interactions in coupled cavities

We propose a scheme to simulate one-dimensional

Dressed States of Josephson phase qubit coupled to an LC circuit

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An RSFQ variable duty cycle oscillator for driving a superconductive qubit

-type Heisenberg spin models with competing interactions between nearest neighbors (NNs) and next NNs in photon-coupled microcavities. Our scheme exploits the rich resources and flexible controls available in such a system to realize arbitrarily adjustable ratios between the effective NN and next-NN coupling strengths. Such a powerful capability allows us to simulate frustration phenomena and disorder behaviors in one-dimensional systems arising from next-NN interactions, a large class of problems of great importance in condensed-matter physics. Our scheme is robust due to the lack of atomic excitations, which suppresses spontaneous emission and cavity decay strongly.

Permutation-invariant monotones for multipartite entanglement characterization

We study the dynamics of a current biased Josephson phase qubit capacitively coupled to an LC circuit. We find that the eigenstates of this system are dressed states that are entangled states between the phase qubit and the LC resonator. We demonstrate that these dressed states can be probed by measuring the avoided crossing in the spectrum of the system. We present our experimental setup to investigate them. This system is interesting not only in demonstrating entanglement, the essential element for quantum information processing (QIP), but also in serving as a first step toward a solid-state analog of cavity QED.

An unshunted comparator as a device for quantum measurements

We design an RSFQ oscillator with a variable duty cycle to drive a superconductive qubit. This design has been optimized to minimize the decoherence when coupled to the superconductive persistent current qubit. A continuous RSFQ ring oscillator reads the contents of a Non-Destructive Read Out memory cell. By using two out-of-phase counters to Set and Reset the cell, we can vary the duty cycle of the pulses read from the memory cell. This train of flux quanta is filtered, then used to drive the persistent current qubit. The precision is sufficient to measure relaxation time and possibly Rabi oscillations.

Integrated photonic qubit quantum computing on a superconducting chip

In this work we consider the permutational properties of multipartite entanglement monotones. Based on the fact that genuine multipartite entanglement is a property of the entire multi-qubit system, we argue that ideal definitions for its characterizing quantities must be permutation-invariant. Using this criterion, we examine the three 4-qubit entanglement monotones introduced by Osterloh and Siewert [Phys. Rev. A. 72, 012337]. By expressing them in terms of quantities whose permutational properties can be easily derived, we find that one of these monotones is not permutation-invariant. We propose a permutation-invariant entanglement monotone to replace it, and show that our new monotone properly measures the genuine 4-qubit entanglement in 4-qubit cluster-class states. Our results provide some useful insights in understanding multipartite entanglement.

Dynamically Manipulating Topological Physics and Edge Modes in a Single Degenerate Optical Cavity

The unshunted single-flux-quantum SFQ comparator is described for the first time. Its dynamic behavior is surprisingly similar to the familiar resistively-shunted SFQ comparator. For certain parameter ranges both junctions of the comparator may pulse at the same time to create a reflected anti-pulse. This phenomenon is utilized in a new SFQ comparator design with better coherence properties for qubit readout. Considerations of quantum noise for the unshunted SFQ comparator are discussed.

Scalable Fault-Tolerant Quantum Computation in Decoherence-Free Subspaces

We study a quantum computing system using microwave photons in transmission line resonators on a superconducting chip as qubits. We show that linear optics and other controls necessary for quantum computing can be implemented by coupling to Josephson devices on the same chip. By taking advantage of the strong nonlinearities in Josephson junctions, photonic qubit interactions can be realized. We analyze the gate error rate to demonstrate that our scheme is realistic even for Josephson devices with limited decoherence times. As a conceptually innovative solution based on existing technologies, our scheme provides an integrated and scalable approach to the next key milestone for photonic qubit quantum computing.