Consistency of ground state and spectroscopic measurements on flux qubits

We compare the results of ground state and spectroscopic measurements carried out on superconducting flux qubits which are effective two-level quantum systems. For a single qubit and for two coupled qubits we show excellent agreement between the parameters of the pseudospin Hamiltonian found using both methods. We argue that by making use of the ground state measurements the Hamiltonian of N coupled flux qubits can be reconstructed as well at temperatures smaller than the energy level separation. Such a reconstruction of a many-qubit Hamiltonian can be useful for future quantum information processing devices.     

Vacuum Rabi oscillations in a macroscopic superconducting qubit oscillator system

We have observed the coherent exchange of a single energy quantum between a flux qubit and a superconducting LC circuit acting as a quantum harmonic oscillator. The exchange of an energy quantum is known as the vacuum Rabi oscillation: the qubit is oscillating between the excited state and the ground state and the oscillator between the vacuum state and the first excited state. We also show that we can detect the state of the oscillator with the qubit and thereby obtained evidence of level quantization of the LC circuit. Our results support the idea of using oscillators as couplers of solid-state qubits.     

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.     

Continuous monitoring of Rabi oscillations in a Josephson flux qubit

Under resonant irradiation, a quantum system can undergo coherent (Rabi) oscillations in time. We report evidence for such oscillations in a continuously observed three-Josephson-junction flux qubit, coupled to a high-quality tank circuit tuned to the Rabi frequency. In addition to simplicity, this method of Rabi spectroscopy enabled a long coherence time of about 2.5 micros, corresponding to an effective qubit quality factor approximately 7000.     

Josephson-junction qubits: entanglement and coherence

We review briefly the problems that are driving the search for a quantum computer. These include, primarily, methods for encryption and decryption based on Shor’s algorithm for factoring large integers and the use of Pell’s equation for encryption. We also outline some of the approaches that have been suggested for implementing a quantum computer and then focus on Josephson-junction systems as qubits. We have been investigating the current-biased Josephson junction for this application, a suggestion we made about 2 years ago. We have studied macroscopic quantum tunneling and energy level spectroscopy, using microwaves, in single junctions and recently we have begun measurements of the two-quantum bit (qubit) system, i.e. two capacitively coupled junctions. Theoretical studies of energy levels and their dynamic evolution are also in progress. In the present report we discuss the basics of single Josephson junctions and compare their potential as qubits with the potentials of other systems. We also discuss our future plans to obtain greater isolation of the junctions from sources of decoherence and to develop realistic qubits. An important first step must be to exhibit quantum entanglement and measure coherence times. Then it must be shown that the states of the qubits can be initialized, that gate operations can be performed, and that the results can be read out.     

Superconducting phase qubits with shadow-evaporated Josephson junctions

We develop a fabrication process for the superconducting phase qubits in which Josephson junctions for both the qubit and superconducting quantum interference device(SQUID) detector are prepared by shadow evaporation with a suspended bridge. Al junctions with areas as small as 0.05 μm~2 are fabricated for the qubit, in which the number of the decoherencecausing two-level systems(TLS) residing in the tunnel barrier and proportional to the junction area are greatly reduced. The measured energy spectrum shows no avoided crossing arising from coherent TLS in the experimentally reachable flux bias range of the phase qubit, which demonstrates the energy relaxation time T_1 and dephasing time T_φ on the order of 100 ns and 50 ns, respectively. We discuss several possible origins of decoherence from incoherent or weakly-coupled coherent TLS and further improvements of the qubit performance.     

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.     

Temperature dependence of coherent oscillations in Josephson phase qubits

We experimentally investigate the temperature dependence of Rabi oscillations and Ramsey fringes in superconducting phase qubits. In a wide range of temperatures, we find that both the decay time and the amplitude of these coherent oscillations remain nearly unaffected by thermal fluctuations. In the two-level limit, coherent qubit response rapidly vanishes as soon as the energy of thermal fluctuations k(B)T becomes larger than the energy level spacing variant Planck's over h omega of the qubit. In contrast, a sample of much shorter coherence times displayed semiclassical oscillations very similar to Rabi oscillation, but showing a qualitatively different temperature dependence. Our observations shed new light on the origin of decoherence in superconducting qubits. The experimental data suggest that, without degrading already achieved coherence times, phase qubits can be operated at temperatures much higher than those reported till now.     

ac Stark shift and dephasing of a superconducting qubit strongly coupled to a cavity field

We have performed spectroscopy of a superconducting charge qubit coupled nonresonantly to a single mode of an on-chip resonator. The strong coupling induces a large ac Stark shift in the energy levels of both the qubit and the resonator. The dispersive shift of the resonator frequency is used to nondestructively determine the qubit state. Photon shot noise in the measurement field induces qubit level fluctuations leading to dephasing which is characteristic for the measurement backaction. A crossover in line shape with measurement power is observed and theoretically explained. For weak measurement a long intrinsic dephasing time of T2>200 ns of the qubit is found.     

Long-lived qubit memory using atomic ions

We demonstrate experimentally a robust quantum memory using a magnetic-field-independent hyperfine transition in 9Be+ atomic ion qubits at a magnetic field B approximately = 0.01194 T. We observe that the single physical qubit memory coherence time is greater than 10 s, an improvement of approximately 5 orders of magnitude from previous experiments with 9Be+. We also observe long coherence times of decoherence-free subspace logical qubits comprising two entangled physical qubits and discuss the merits of each type of qubit.     

Quantum information,oscillations and the psyche

In this paper, taking the theory of quantum information as a model, we consider the human unconscious, pre-consciousness and consciousness as sets of quantum bits (qubits). We view how there can be communication between these various qubit sets. In doing this we are inspired by the theory of nuclear magnetic resonance. In this way we build a model of handling a mental qubit with the help of pulses of a mental field. Starting with an elementary interaction between two qubits we build two-qubit quantum logic gates that allow information to be transferred from one qubit to the other. In this manner we build a quantum process that permits consciousness to “read” the unconscious and vice versa. The elementary interaction, e.g. between a pre-consciousness qubit and a consciousness one, allows us to predict the time evolution of the pre-consciousness + consciousness system in which pre-consciousness and consciousness are quantum entangled. This time evolution exhibits Rabi oscillations that we name mental Rabi oscillations. This time evolution shows how for example the unconscious can influence consciousness. In a process like mourning the influence of the unconscious on consciousness, as the influence of consciousness on the unconscious, are in agreement with what is observed in psychiatry.     

Preparation of multi-photon Fock states and quantum entanglement properties in circuit QED

We demonstrate the controllable generation of multi-photon Fock states in circuit quantum electrodynamics （circuit QED）. The external bias flux regulated by a counter can effectively adjust the bias time on each superconducting flux qubit so that each flux qubit can pass in turn through the circuit cavity and thereby avoid the effect of decoherence. We further investigate the quantum correlation dynamics of coupling superconducting qubits in a Fock state. The results reveal that the lower the photon number of the light field in the number state, the stronger the interaction between qubits is, then the more beneficial to maintaining entanglement between qubits it will be.     

Ballistic Aharonov–Bohm quantum bits and quantum gates

We propose an architecture to perform quantum computation, using ballistic electrons as qubits and coupled quantum rings as quantum gates. In the proposed architecture two adjacent one-dimensional wires, creating a single qubit, are connected to two coupled quantum rings, where the required magnetic flux is provided by enclosed nano-sized magnets. The phase modulation of the wave function of the ballistic electrons under the Aharonov–Bohm effect is carefully designed to facilitate reprogrammable and dynamically controllable quantum gates. Arbitrary single-qubit quantum gates with high fidelity can be constructed on the basis of this architecture.     

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.     

Fabrication and characterization of superconducting multiqubit device with niobium base layer

Superconducting transmon qubits are the leading platform in solid-state quantum computing and quantum simulation applications. In this work, we develop a fabrication process for the transmon multiqubit device with a niobium base layer,shadow-evaporated Josephson junctions, and airbridges across the qubit control lines to suppress crosstalk. Our results show that these multiqubit devices have well-characterized readout resonators, and that the energy relaxation and Ramsey(spin-echo) dephasing times are up to ～40 μs and 14(47) μs, respectively. We perform single-qubit gate operations that demonstrate a maximum gate fidelity of 99.97%. In addition, two-qubit vacuum Rabi oscillations are measured to evaluate the coupling strength between qubits, and the crosstalk among qubits is found to be less than 1% with the fabricated airbridges. Further improvements in qubit coherence performance using this fabrication process are also discussed.     

A trilayer process for the fabrication of Al phase qubits

Herein we develop an Al/AlOx/Al trilayer process,feasible to fabricate complex circuits with wiring crossovers,for the preparation of Al junctions and phase qubits.The AlOx layer is obtained by in situ thermal oxidation,which provides high-quality junction tunnel barriers.The Al junctions show a considerably low leakage current and the Josephson critical current density can be conveniently controlled in the range of a few to above 100 A/cm2,which is favorable in the phase qubit application.Macroscopic quantum tunneling,energy spectrum,energy relaxation time,Rabi oscillation,and Ramsey interference of the Al phase qubits are measured,demonstrating clearly quantum coherent dynamics with a timescale of 10 ns.Further improvements of the coherent dynamic properties of the device are discussed.     

Josephson结开关电流分布的测量方案探讨

Josephson结的开关电流存在着一定的分布.利用开关电流的分布,我们可以推算出Josephson结的逸出率.进一步结合合适的微波辐照,还可以获得结的诸如能级、拉比振荡等许多相关的量子特性.Josephson结的开关电流分布的获得,对于研究超导量子比特,包括相位量子比特、电荷量子比特、磁通量子比特和涡流量子比特以及他们的组合量子比特都有着重要意义.我们提出了三种测量方案,对这三种方案进行了比较,并初步的对自制的NbN/AlNx/NbN Josephson结的开关电流进行了多次(104次)测量,得到一定温度下的开关电流分布的直方图.针对三种方案各自的优缺点及已有的结果,我们提出了需要进一步改进的措施,对于下一步开展在极低温下(mK)Josephson结的开关电流分布的测量有着重要的意义.     

Topological quantum buses: coherent quantum information transfer between topological and conventional qubits

We propose computing bus devices that enable quantum information to be coherently transferred between topological and conventional qubits. We describe a concrete realization of such a topological quantum bus acting between a topological qubit in a Majorana wire network and a conventional semiconductor double quantum dot qubit. Specifically, this device measures the joint (fermion) parity of these two different qubits by using the Aharonov-Casher effect in conjunction with an ancilliary superconducting flux qubit that facilitates the measurement. Such a parity measurement, together with the ability to apply Hadamard gates to the two qubits, allows one to produce states in which the topological and conventional qubits are maximally entangled and to teleport quantum states between the topological and conventional quantum systems.     

Dynamics of entanglement protection of two qubits using a driven laser field and detunings:Independent and common,Markovian and/or non-Markovian regimes

Preventing quantum entanglement from decoherence effect is of theoretical and practical importance in the quantum information processing technologies.In this regard,we consider the entanglement dynamics of two identical qubits where the qubits which are coupled to two independent(Markovian and/or non-Markovian) as well as a common reservoir at zero temperature are further interacted with a classical driving laser field.Then,we study the preservation of generated two-qubit entanglement in various situations using the concurrence measure.It is shown that by applying the classical driving field and so the possibility of controlling the Rabi frequency,the amount of entanglement of the two-qubit system is improved in the off-resonance condition between the qubit and the central cavity frequencies(central detuning) in both non-Markovian and Markovian reservoirs.While the central detuning has a constructive role,the detuning between the qubit and the classical field(laser detuning) affects negatively on the entanglement protection.The obtained results show that long-living entanglement in the non-Markovian reservoir is more accessible than in the Markovian reservoir.We demonstrate that,in a common reservoir non-zero stationary entanglement is achievable whenever the two-qubit system is coupled to the reservoir with appropriate values of relative coupling strengths.     

Gate control of quantum dot-based electron spin–orbit qubits

We investigate theoretically the coherent spin dynamics of gate control of quantum dot-based electron spin–orbit qubits subjected to a tilted magnetic field under electric-dipole spin resonance (EDSR). Our results reveal that Rabi oscillation of qubit states can be manipulated electrically based on rapid gate control of SOC strength. The Rabi frequency is strongly dependent on the gate-induced electric field, the strength and orientation of the applied magnetic field. There are two major EDSR mechanisms. One arises from electric field-induced spin–orbit hybridization, and the other arises from magnetic field-induced energy-level crossing. The SOC introduced by the gate-induced electric field allows AC electric fields to drive coherent Rabi oscillations between spin-up and -down states. After the crossing of the energy-levels with the magnetic field, the spin-transfer crossing results in Rabi oscillation irrespective of whether or not the external electric field is present. The spin–orbit qubit is transferred into the orbit qubit. Rabi oscillation is anisotropic and periodic with respect to the tilted and in-plane orientation of the magnetic field originating from the interplay of the SOC, orbital, and Zeeman effects. The strong electrically-controlled SOC strength suggests the possibility for scalable applications of gate-controllable spin–orbit qubits.