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Marc J. Feldman

Publications and source records attributed to Marc J. Feldman.

10 recordsLinked to original sources
arXiv2004

Scalable fault-tolerant quantum computation in DFS blocks

We investigate how to concatenate different decoherence-free subspaces (DFSs) to realize scalable universal fault-tolerant quantum computation. Based on tunable $XXZ$ interactions, we present an architecture for scalable quantum computers which can fault-tolerantly perform universal quantum computation by manipulating only single type of parameter. By using the concept of interaction-free subspaces we eliminate the need to tune the couplings between logical qubits, which further reduces the technical difficulties for implementing quantum computation.

quant-ph↗
arXiv2003

Dispersive Manipulation of Paired Superconducting Qubits

We combine the ideas of qubit encoding and dispersive dynamics to enable robust and easy quantum information processing (QIP) on paired superconducting charge boxes sharing a common bias lead. We establish a decoherence free subspace on these and introduce universal gates by dispersive interaction with a LC resonator and inductive couplings between the encoded qubits. These gates preserve the code space and only require the established local symmetry and the control of the voltage bias.

cond-mat.supr-con↗
arXiv2002

Conditions for Nondistortion Interrogation of Quantum System

Under some physical considerations, we present a universal formulation to study the possibility of localizing a quantum object in a given region without disturbing its unknown internal state. When the interaction between the object and probe wave function takes place only once, we prove the necessary and sufficient condition that the object's presence can be detected in an initial state preserving way. Meanwhile, a conditioned optimal interrogation probability is obtained.

quant-ph↗
arXiv2002

A general framework of nondistortion quantum interrogation

We present a general framework to study nondistortion quantum interrogation which preserves the internal state of the quantum object being detected. We obtain the necessary and sufficient condition for successful performing nondistortion interrogation for unknown quantum object when the interaction between the probe system and the detected system takes place only once. When the probe system and interrogation process have been limited we develop a mathematical frame to determine whether it is possible to realize NQI processes only relying on the choice of the original probe state. We also consider NQI process in iterative cases. A sufficient criterion for NQI is obtained.

quant-ph↗
arXiv2002

Quantum computation with un-tunable 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 in and out of carefully designed interaction free subspaces analogous to decoherence free subspaces, which allows us to effectively turn off and turn on the interactions between the encoded qubits. 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.

quant-ph↗
arXiv2001

High efficiency Nondistortion Quantum Interrogation of atoms in quantum superpositions

We consider the nondistortion quantum interrogation (NQI) of an atom prepared in a quantum superposition. By manipulating the polarization of the probe photon and making connections to interaction free measurements of opaque objects, we show that nondistortion interrogation of an atom in a quantum superposition can be done with efficiency approaching unity. However, if any component of the atom's superposition is completely transparent to the probe wave function, a nondistortion interrogation of the atom is impossible.

quant-ph↗
arXiv2001

Nondistortion Quantum Interrogation using EPR entangled Photons

We propose a novel scheme for nondistortion quantum interrogation (NQI), defined as an interaction-free measurement which preserves the internal state of the object being detected. In our scheme, two EPR entangled photons are used as the probe and polarization sensitive measurements are performed at the four ports of the Mach-Zehnder interferometer. In comparison with the previous single photon scheme, it is shown that the two photon approach has a higher probability of initial state preserving interrogation of an atom prepared in a quantum superposition. In the case that the presence of the atom is not successfully detected, the experiment can be repeated since the initial state of the atom is unperturbed.

quant-ph↗
arXiv2001

Design of an RSFQ Control Circuit to Observe MQC on an rf-SQUID

We believe that the best chance to observe macroscopic quantum coherence (MQC) in a rf-SQUID qubit is to use on-chip RSFQ digital circuits for preparing, evolving and reading out the qubit's quantum state. This approach allows experiments to be conducted on a very short time scale (sub-nanosecond) without the use of large bandwidth control lines that would couple environmental degrees of freedom to the qubit thus contributing to its decoherence. In this paper we present our design of a RSFQ digital control circuit for demonstrating MQC in a rf-SQUID. We assess some of the key practical issues in the circuit design including the achievement of the necessary flux bias stability. We present an "active" isolation structure to be used to increase coherence times. The structure decouples the SQUID from external degrees of freedom, and then couples it to the output measurement circuitry when required, all under the active control of RSFQ circuits. Research supported in part by ARO grant # DAAG55-98-1-0367.

quant-ph↗
arXiv2001

A "Tipping Pulse" Scheme for rf-SQUID Qubits

We introduce a technique to control the macroscopic quantum state of an rf SQUID qubit. We propose to employ a stream of single flux quantum (SFQ) pulses magnetically coupled to the qubit junction to momentarily suppresses its critical current. This effectively lowers the barrier in the double-well rf-SQUID potential thereby increasing the tunneling oscillation frequency between the wells. By carefully choosing the time interval between SFQ pulses one may accelerate the interwell tunneling rate. Thus it is possible to place the qubit into a chosen superposition of flux states and then effectively to freeze the qubit state. We present both numerical simulations and analytical time-dependent perturbation theory calculations that demonstrate the technique. Using this strategy one may control the quantum state of the rf-SQUID in a way analogous to the πpulses in other qubit schemes. Research supported in part by ARO grant # DAAG55-98-1-0367.

quant-ph↗