arXiv subjects

J. Q. You

Publications and source records attributed to J. Q. You.

178 records · Page 10Linked to original sources

arXiv2007

Switchable coupling between charge and flux qubits

We propose a hybrid quantum circuit with both charge and flux qubits connected to a large Josephson junction that gives rise to an effective inter-qubit coupling controlled by the external magnetic flux. This switchable inter-qubit coupling can be used to transfer back and forth an arbitrary superposition state between the charge qubit and the flux qubit working at the optimal point. The proposed hybrid circuit provides a promising quantum memory because the flux qubit at the optimal point can store the tranferred quantum state for a relatively long time.

cond-mat.supr-con↗

arXiv2007

Variable-frequency-controlled coupling in charge qubit circuits: Effects of microwave field on qubit-state readout

To implement quantum information processing, microwave fields are often used to manipulate superconuducting qubits. We study how the coupling between superconducting charge qubits can be controlled by variable-frequency magnetic fields. We also study the effects of the microwave fields on the readout of the charge-qubit states. The measurement of the charge-qubit states can be used to demonstrate the statistical properties of photons.

quant-ph↗

arXiv2007

Persistent single-photon production by tunable on-chip micromaser with a superconducting quantum circuit

We propose a tunable on-chip micromaser using a superconducting quantum circuit (SQC). By taking advantage of externally controllable state transitions, a state population inversion can be achieved and preserved for the two working levels of the SQC and, when needed, the SQC can generate a single photon. We can regularly repeat these processes in each cycle when the previously generated photon in the cavity is decaying, so that a periodic sequence of single photons can be produced persistently. This provides a controllable way for implementing a persistent single-photon source on a microelectronic chip.

quant-ph↗

arXiv2006

Dissipative dynamics of coupled quantum dots under quantum measurement

We have studied the dissipative dynamics of a solid-state qubit with an extra electron confined to either one of two coupled quantum dots. Previous theoretical work based on Bloch-type rate equations gave an unphysical uniform occupation probability of the electron in the quantum dots even for non-identical dots. We show that this is due to neglecting higher order interactions in the analysis. By including higher order terms, we obtain expected asymmetric occupation probabilities for non-identical dots. Our work demonstrates that the high order interaction terms can lead to important qualitative impacts on the operation of the qubit.

cond-mat.mes-hall↗

arXiv2006

Efficient one-step generation of large cluster states with solid-state circuits

Highly entangled states called cluster states are a universal resource for measurement-based quantum computing (QC). Here we propose an efficient method for producing large cluster states using superconducting quantum circuits. We show that a large cluster state can be efficiently generated in just one step by turning on the inter-qubit coupling for a short time. Because the inter-qubit coupling is only switched on during the time interval for generating the cluster state, our approach is also convenient for preparing the initial state for each qubit and for implementing one-way QC via single-qubit measurements. Moreover, the cluster state is robust against parameter variations.

quant-ph↗

arXiv2006

Measurement-based quantum computation with superconducting charge qubits

We present a robust method, based only on measurements, to produce superconducting cluster states. The measurement of the current of a few parallel Josephson-junction qubits realizes a novel type of quantum-state selector. Using this selector, one can produce various quantum entangled states and also realize a controlled-NOT gate without requiring an exact control of the interqubit interactions. In particular, cluster states for quantum computation could be produced with only single-qubit measurements.

quant-ph↗

arXiv2006

Superconducting Circuits and Quantum Information

Superconducting circuits can behave like atoms making transitions between two levels. Such circuits can test quantum mechanics at macroscopic scales and be used to conduct atomic-physics experiments on a silicon chip.

quant-ph↗

arXiv2005

Hybridized solid-state qubit in the charge-flux regime

Most superconducting qubits operate in a regime dominated by either the electrical charge or the magnetic flux. Here we study an intermediate case: a hybridized charge-flux qubit with a third Josephson junction (JJ) added into the SQUID loop of the Cooper-pair box. This additional JJ allows the optimal design of a low-decoherence qubit. Both charge and flux $1/f$ noises are considered. Moreover, we show that an efficient quantum measurement of either the current or the charge can be achieved by using different area sizes for the third JJ.

cond-mat.supr-con↗

arXiv2005

Optical selection rules and phase-dependent adiabatic state control in a superconducting quantum circuit

We analyze the optical selection rules of the microwave-assisted transitions in a flux qubit superconducting quantum circuit (SQC). We show that the parities of the states relevant to the superconducting phase in the SQC are well-defined when the external magnetic flux $Φ_{e}=Φ_{0}/2$, then the selection rules are same as the ones for the electric-dipole transitions in usual atoms. When $Φ_{e}\neq Φ_{0}/2$, the symmetry of the potential of the artificial "atom'' is broken, a so-called $Δ$-type "cyclic" three-level atom is formed, where one- and two-photon processes can coexist. We study how the population of these three states can be selectively transferred by adiabatically controlling the electromagnetic field pulses. Different from $Λ$-type atoms, the adiabatic population transfer in our three-level $Δ$-atom can be controlled not only by the amplitudes but also by the phases of the pulses.

quant-ph↗

arXiv2005

Correlation-induced suppression of decoherence in capacitively coupled Cooper-pair boxes

Charge fluctuations from gate bias and background traps severely limit the performance of a charge qubit in a Cooper-pair box (CPB). Here we present an experimentally realizable method to control the decoherence effects of these charge fluctuations using two strongly capacitively coupled CPBs. This coupled-box system has a low-decoherence subspace of two states. Our results show that the inter-box Coulomb correlation can help significantly suppress decoherence of this two-level system, making it a promising candidate as a logical qubit, encoded using two CPBs.

cond-mat.supr-con↗

arXiv2005

Fast two-bit operations in inductively coupled flux qubits

A central problem for implementing efficient quantum computing is how to realize fast operations (both one- and two-bit ones). However, this is difficult to achieve for a collection of qubits, especially for those separated far away, because the interbit coupling is usually much weaker than the intrabit coupling. Here we present an experimentally feasible method to effectively couple two flux qubits via a common inductance and treat both single and coupled flux qubits with more realistic models which include the loop inductance. The main advantage of our proposal is that a strong interbit coupling can be achieved using a small inductance, so that two-bit operations as fast as one-bit ones can be easily realized. We also show the flux dependence of the transitions between states for the coupled flux qubits.

cond-mat.supr-con↗

arXiv2003

Controllable manipulation and entanglement of macroscopic quantum states in coupled charge qubits

We present an experimentally implementable method to couple Josephson charge qubits and to generate and detect macroscopic entangled states. A large-junction superconducting quantum interference device is used in the qubit circuit for both coupling qubits and implementing the readout. Also, we explicitly show how to achieve a microwave-assisted macroscopic entanglement in the coupled-qubit system.

cond-mat.supr-con↗

arXiv2003

Quantum information processing with superconducting qubits in a microwave field

We investigate the quantum dynamics of a Cooper-pair box with a superconducting loop in the presence of a nonclassical microwave field. We demonstrate the existence of Rabi oscillations for both single- and multi-photon processes and, moreover, we propose a new quantum computing scheme (including one-bit and conditional two-bit gates) based on Josephson qubits coupled through microwaves.

cond-mat.supr-con↗

arXiv2003

Quantum computing with many superconducting qubits

Two of the major obstacles to achieve quantum computing (QC) are (i) scalability to many qubits and (ii) controlled connectivity between any selected qubits. Using Josephson charge qubits, here we propose an experimentally realizable method to efficiently solve these two central problems. Since any two charge qubits can be effectively coupled by an experimentally accessible inductance, the proposed QC architecture is scalable. In addition, we formulate an efficient and realizable QC scheme that requires only one (instead of two or more) two-bit operation to implement conditional gates.

cond-mat.supr-con↗

arXiv2003

Scalable quantum computing with Josephson charge qubits

A goal of quantum information technology is to control the quantum state of a system, including its preparation, manipulation, and measurement. However, scalability to many qubits and controlled connectivity between any selected qubits are two of the major stumbling blocks to achieve quantum computing (QC). Here we propose an experimental method, using Josephson charge qubits, to efficiently solve these two central problems. The proposed QC architecture is scalable since any two charge qubits can be effectively coupled by an experimentally accessible inductance. More importantly, we formulate an efficient and realizable QC scheme that requires only one (instead of two or more) two-bit operation to implement conditional gates.

cond-mat.supr-con↗

arXiv2001

Resonant peak splitting for ballistic conductance in magnetic superlattices

We investigate theoretically the resonant splitting of ballistic conductance peaks in magnetic superlattices. It is found that, for magnetic superlattices with periodically arranged $n$ identical magnetic-barriers, there exists a general $(n-1)$-fold resonant peak splitting rule for ballistic conductance, which is the analogy of the $(n-1)$-fold resonant splitting for transmission in $n$-barrier electric superlattices (R. Tsu and L. Esaki, Appl. Phys. Lett. {\bf 22}, 562 (1973)).

cond-mat.mes-hall↗