Observation of high coherence in Josephson junction qubits measured in a three-dimensional circuit QED architecture

Superconducting quantum circuits based on Josephson junctions have made rapid progress in demonstrating quantum behavior and scalability. However, the future prospects ultimately depend upon the intrinsic coherence of Josephson junctions, and whether superconducting qubits can be adequately isolated from their environment. We introduce a new architecture for superconducting quantum circuits employing a three-dimensional resonator that suppresses qubit decoherence while maintaining sufficient coupling to the control signal. With the new architecture, we demonstrate that Josephson junction qubits are highly coherent, with T2 ～ 10 to 20 μs without the use of spin echo, and highly stable, showing no evidence for 1/f critical current noise. These results suggest that the overall quality of Josephson junctions in these qubits will allow error rates of a few 10(-4), approaching the error correction threshold.     

Robust Quantum Gates in Decoherence-Free Subspaces with Josephson Charge Qubits

We propose a new and feasible scheme to implement quantum gates in decoherence-free subspaces (DFSs) with Josephson charge qubits situated in a circuit QED architecture. Based on the resonator-assisted interaction, the controllable interqubit couplings occur only by tuning the individual flux biases, by which we obtain the DFS-encoded universal quantum gates. Compared with the non-DFS situation, we numerically consider the robustness of the DFS-encoded scheme that can be insensitive to the collective noises. Thus the protocol may perform the fault-tolerant quantum computing with Josephson charge qubits.     

Nanoscale superconducting quantum bits

Various physical systems were proposed for quantum information processing. Among those nanoscale devices appear most promising for integration in electronic circuits and large-scale applications. We discuss Josephson junction circuits in two regimes where they can be used for quantum computing. These systems combine intrinsic coherence of the superconducting state with control possibilities of single-charge circuits. In the regime where the typical charging energy dominates over the Josephson coupling, the low-temperature dynamics is limited to two states differing by a Cooper-pair charge on a superconducting island. In the opposite regime of prevailing Josephson energy, the phase (or flux) degree of freedom can be used to store and process quantum information. Under suitable conditions the system reduces to two states with different flux configurations. Several qubits can be joined together into a register. The quantum state of a qubit register can be manipulated by voltage and magnetic field pulses. The qubits are inevitably coupled to the environment. However, estimates of the phase coherence time show that many elementary quantum logic operations can be performed before the phase coherence is lost. In addition to manipulations, the final state of the qubits has to be read out. This quantum measurement process can be accomplished using a single-electron transistor for charge Josephson qubits, and a d.c.-SQUID for flux qubits. Recent successful experiments with superconducting qubits demonstrate for the first time quantum coherence in macroscopic systems.     

超导量子比特实验的开端

我们对超导量子比特领域的科学背景、历史起源和早期发展做简要评述.莱格特(Anthony J. Leggett)为这个领域打下了理论基础.克拉克(John Clarke)和他的两个学生马丁尼(John Martinis)和德沃雷(Michel H. Devoret)最早通过偏电流约瑟夫森结,首次观察到约瑟夫森结的量子行为.后来德沃雷实现了电荷量子比特叠加态、电荷-磁通混合量子比特的拉比共振和其他演化及投影测量.中村泰信(Yasunobu Nakamura)首先实现电荷量子比特的量子叠加和拉比振荡,还参与莫伊(J. E. Mooij)组实现了磁通量子比特的拉比振荡和读出.     

Quantum two-level systems in Josephson junctions as naturally formed qubits

The two-level systems (TLSs) naturally occurring in Josephson junctions constitute a major obstacle for the operation of superconducting phase qubits. Since these TLSs can possess remarkably long decoherence times, we show that such TLSs can themselves be used as qubits, allowing for a well controlled initialization, universal sets of quantum gates, and readout. Thus, a single current-biased Josephson junction can be considered as a multiqubit register. It can be coupled to other junctions to allow the application of quantum gates to an arbitrary pair of qubits in the system. Our results indicate an alternative way to realize superconducting quantum information processing.     

Nondestructive readout for a superconducting flux qubit

We present a new readout method for a superconducting flux qubit, based on the measurement of the Josephson inductance of a superconducting quantum interference device that is inductively coupled to the qubit. The intrinsic flux detection efficiency and backaction are suitable for a fast and nondestructive determination of the quantum state of the qubit, as needed for readout of multiple qubits in a quantum computer. We performed spectroscopy of a flux qubit and we measured relaxation times of the order of 80 micros.     

State tomography of capacitively shunted phase qubits with high fidelity

We introduce a new design concept for superconducting phase quantum bits (qubits) in which we explicitly separate the capacitive element from the Josephson tunnel junction for improved qubit performance. The number of two-level systems that couple to the qubit is thereby reduced by an order of magnitude and the measurement fidelity improves to 90%. This improved design enables the first demonstration of quantum state tomography with superconducting qubits using single-shot measurements.     

Entangling two uncoupled flux qubits via their sequential interaction with a quantized electromagnetic field

A theoretical scheme for the generation of maximally entangled states of two superconducting flux qubits via their sequential interaction with a monochromatic quantum field is presented. The coupling of the qubits with the quantized field can be tuned on and off resonance by modulating the effective Josephson energy of each qubit via an externally applied magnetic flux. The system operates in such a way as to transfer the entanglement from a bipartite field-qubit subsystem to the two qubits. This scheme is attractive in view of the implementation of practical quantum processing systems.     

基于约瑟夫森器件的超导量子比特

超导量子比特利用了超导约瑟夫森隧道结的非线性效应,采用了半导体集成电路的工艺,以其无能耗,大设计加工自由度,易规模化等优点而倍受注目。本文对超导量子比特的基本原理及发展过程作了简要综述。首先简要回顾了量子计算的历史,然后介绍了超导量子比特的设计及其调控,并对各种超导量子比特的消相干进行了讨论。     

Description of the evolution of the state of “josephson atoms” in the context of the informational interpretation of quantum mechanics

The evolution of a quantum system is informationally interpreted and used to describe decay in the coherent states of Josephson atoms, including qubits based on two- and three-junction superconducting quantum interferometers. The reduced Lindblad equation is employed to examine the measurement procedure of the Josephson qubit state and the influence of a measuring device on its coherent-state decay.     

Decoherence in josephson phase qubits from junction resonators

Although Josephson junction qubits show great promise for quantum computing, the origin of dominant decoherence mechanisms remains unknown. Improving the operation of a Josephson junction based phase qubit has revealed microscopic two-level systems or resonators within the tunnel barrier that cause decoherence. We report spectroscopic data that show a level splitting characteristic of coupling between a two-state qubit and a two-level system. Furthermore, we show Rabi oscillations whose "coherence amplitude" is significantly degraded by the presence of these spurious microwave resonators. The discovery of these resonators impacts the future of Josephson qubits as well as existing Josephson technologies.     

Quantum information transfer with Cooper-pair box qubits in circuit QED

We propose a feasible scheme to transfer quantum information with Cooper-pair box qubits arrayed in a circuit QED. Qubits interact with a quantum data bus generated by a one-dimensional transmission line resonator. Based on the Raman adiabatic passage, the cavity bus-assisted quantum population transfer between any selected pair of qubits can be controlled by addressing the applied gate pulses. Therefore, the scheme provides the possibility for effectively implementing scalable quantum information transfer with Josephson devices.     

Spectroscopy of three-particle entanglement in a macroscopic superconducting circuit

We study the quantum mechanical behavior of a macroscopic, three-body, superconducting circuit. Microwave spectroscopy on our system, a resonator coupling two large Josephson junctions, produced complex energy spectra well explained by quantum theory over a large frequency range. By tuning each junction separately into resonance with the resonator, we first observe strong coupling between each junction and the resonator. Bringing both junctions together into resonance with the resonator, we find spectroscopic evidence for entanglement between all 3 degrees of freedom and suggest a new method for controllable coupling of distant qubits, a key step toward quantum computation.     

Vertical Josephson interferometers for quantum computation

We characterize a niobium-based vertical Josephson interferometer which we propose to include in a superconducting loop for applications to quantum computation using flux qubits. The most interesting feature of this device is that the Josephson current is precisely modulated by a small transversal magnetic field parallel to superconducting loop plane from a maximum to zero, with fine control and precision. This device can be used to independently control the off-diagonal Hamiltonian terms of flux qubits and/or to control the flux transfer function of a superconducting transformer for inter-qubits coupling.     

Scalable geometric quantum computing with Cooper-pair box qubits in circuit QED

We theoretically present a scheme to realize the scalable geometric quantum computing with Cooper-pair box (CPB) qubits in circuit QED. A one-dimensional transmission line resonator in circuit QED acting as quantum data bus generates a common cavity mode and interacts with each CPB. It is found that the interqubit couplings between any pair of qubits are switchable by individually adjusting the gate pulses applied to the selected CPBs. In this proposed scheme, we can both controllably and selectively address logic gates in geometric scenarios, which opens the possibility to implement the scalable fault-tolerant quantum computing with Josephson qubits.     

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 con-nectivity between any selected qubits are two of the major stumbling blocks to achieve quantum com-puting (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.     

Measurement-induced nonlocality and geometric quantum discord in two SC-charge qubits

By using geometric quantum discord and measurement-induced nonlocality, quantum correlations are investigated for two superconducting (SC) charge qubits that share a large Josephson junction where the field is assumed to be prepared initially in a coherent state. It is found that the difference between measure measurement-induced nonlocality and geometric quantum discord, of the final state of the two SC-charge qubits system which is especial case of X-states, is equal to a constant value. It is found that the quantum correlations and entanglement of the qubits are very sensitive to the mean number of the coherent photons. The entanglement exists in small intervals of death quantum discord and measurement-induced nonlocality. This is further evidence in support of the fact that quantum correlation and entanglement are not synonymous.     

An optimization method for the calculation of Hamiltonian matrix elements for Josephson flux qubits

This work aims to describe the method of analytical calculation of an orthonormalized basis of states of the Josephson flux quantum bits (qubits) using a two-level approximation under the condition that the potential energy of the system is a combination of two potential wells separated by a tunnel barrier. For illustration, the calculation results in the case of the well-known three- and four-junction flux qubits, as well as promising silent qubits, are presented.     

Quantum computation and simulation with superconducting qubits

Superconducting circuits based on Josephson junctions are regarded as one of the most promising technologies for the implementation of scalable quantum computers. This review presents the basic principles of superconducting qubits and shows the progress of quantum computing and quantum simulation based on superconducting qubits in recent years.The experimental realization of gate operations, readout, error correction codes, as well as some quantum algorithms are summarized, followed by an introduction of quantum simulation. And then some important applications in fields including condensed matter physics, quantum annealing, and quantum chemistry are discussed.     

Temperature square dependence of the low frequency charge noise in the Josephson junction qubits

To verify the hypothesis about the common origin of the low-frequency 1/f noise and the quantum f noise recently measured in the Josephson charge qubits, we study the temperature dependence of the 1/f noise and decay of coherent oscillations. The T2 dependence of the 1/f noise is experimentally demonstrated, which supports the hypothesis. We also show that dephasing in the Josephson charge qubits off the electrostatic energy degeneracy point is consistently explained by the same low-frequency 1/f noise that is observed in the transport measurements.