Superconducting quantum bits

Superconducting quantum bits(qubits) and circuits are the leading candidate for the implementation of solid-state quantum computation. They have also been widely used in a variety of studies of quantum physics, atomic physics, quantum optics, and quantum simulation. In this article, we will present an overview of the basic principles of the superconducting qubits, including the phase, flux, charge, and transmon(Xmon) qubits, and the progress achieved so far concerning the improvements of the device design and quantum coherence property. Experimental studies in various research fields using the superconducting qubits and circuits will be briefly reviewed.     

Superconducting tetrahedral quantum bits

We propose a design for a qubit with four superconducting islands in the topology of a symmetric tetrahedron, uniformly frustrated with one-half flux quantum per loop and one-half Cooper pair per island. This structure emulates a noise-resistant spin-1/2 system in a vanishing magnetic field. The flux frustration boosts quantum fluctuations and relieves the constraints on junction fabrication. Variability of manipulation and optimized readout are additional benefits of this design.     

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.     

Long-lived memory for mesoscopic quantum bits

We describe a technique to create long-lived quantum memory for quantum bits in mesoscopic systems. Specifically we show that electronic spin coherence can be reversibly mapped onto the collective state of the surrounding nuclei. The coherent transfer can be efficient and fast and it can be used, when combined with standard resonance techniques, to reversibly store coherent superpositions on the time scale of seconds. This method can also allow for "engineering" entangled states of nuclear ensembles and efficiently manipulating the stored states. We investigate the feasibility of this method through a detailed analysis of the coherence properties of the system.     

Decoherence in superconducting quantum bits by phonon radiation

We discuss a fundamental limitation for the coherent operation of superconducting quantum bits originating from phonon radiation generated in the Josephson junctions of the device. The time dependent superconducting phase across the junction produces an electric field that couples to the underlying crystal lattice via the piezoelectric effect. We determine the radiation resistance of the junction due to phonon emission and derive substantial decoherence rates for the quantum bits, which are compatible with quality factors measured in recent experiments.     

Collective quantum jumps of Rydberg atoms

We study an open quantum system of atoms with a long-range Rydberg interaction, laser driving, and spontaneous emission. Over time, the system occasionally jumps between a state of low Rydberg population and a state of high Rydberg population. The jumps are inherently collective, and in fact, exist only for a large number of atoms. We explain how entanglement and quantum measurement enable the jumps, which are otherwise classically forbidden.     

Quantifying quantum correlation via quantum coherence

Resource theory is applied to quantify the quantum correlation of a bipartite state and a computable measure is proposed. Since this measure is based on quantum coherence, we present another possible physical meaning for quantum correlation, i.e., the minimum quantum coherence achieved under local unitary transformations. This measure satisfies the basic requirements for quantifying quantum correlation and coincides with concurrence for pure states. Since no optimization is involved in the final definition, this measure is easy to compute irrespective of the Hilbert space dimension of the bipartite state.     

Laser-induced quantum coherence in a semiconductor quantum well

The phenomenon of electromagnetically induced quantum coherence is demonstrated between three confined electron subband levels in a quantum well which are almost equally spaced in energy. Applying a strong coupling field, two-photon resonant with the 1-3 intersubband transition, produces a pronounced narrow transparency feature in the 1-2 absorption line. This result can be understood in terms of all three states being simultaneously driven into "phase-locked" quantum coherence by a single coupling field. We describe the effect theoretically with a density matrix method and an adapted linear response theory.     

Biexciton quantum coherence in a single quantum dot

Nondegenerate (two-wavelength) two-photon absorption using coherent optical fields is used to show that there are two different quantum mechanical pathways leading to formation of the biexciton in a single quantum dot. Of specific importance to quantum information applications is the resulting coherent dynamics between the ground state and the biexciton from the pathway involving only optically induced exciton/biexciton quantum coherence. The data provide a direct measure of the biexciton decoherence rate which is equivalent to the decoherence of the Bell state in this system, as well as other critical optical parameters.     

Macroscopic quantum coherence in ferritin

In a breakthrough experiment, Awschalom et al. [Phys. Rev. Lett. 68, 3092 (1992)] have demonstrated that the antiferromagnetic core in ferritin resonates between two states with oppositely directed Néel vectors, making it the first observation of MQC. A theory has been developed for this resonance including the effect of the 100 or so⁵⁷Fe nuclear spins expected in each ferritin core. Since the hyperfine coupling is known to be ～68 MHz and the MQC frequency is ～1 MHz, the degeneracy of the Néel states, and with it, the MQC resonance, is destroyed in all ferritin particles except those with zero total staggered nuclear spin. From the measured size of λ″(ω), the energy being absorbed by the ferritin is at least 4000 times larger than the maximum permissible. Hence, the true importance of these experiments lies not in the narrow issue of MQC, but in the disproof of long cherished theoretical conservation laws.     

Controlling ultracold Rydberg atoms in the quantum regime

We discuss the properties of Rydberg atoms in a magnetic Ioffe-Pritchard trap being commonly used in ultracold atomic physics experiments. The Hamiltonian is derived, and it is demonstrated how tight traps alter the coupling of the atom to the magnetic field. We solve the underlying Schr?dinger equation of the system within a given n manifold and show that for a sufficiently large Ioffe field strength the 2n;{2}-dimensional system of coupled Schr?dinger equations decays into several decoupled multicomponent equations governing the center of mass motion. An analysis of the fully quantized center of mass and electronic states is undertaken. In particular, we discuss the situation of tight center of mass confinement outlining the procedure to generate a low-dimensional ultracold Rydberg gas.     

Duplicating classical bits with universal quantum cloning machine

It seems there is a large gap between quantum cloning and classical duplication since quantum mechanics forbid perfect copies of unknown quantum states. In this paper, we prove that a classical duplication process can be realized by using a universal quantum cloning machine(QCM). A classical bit is encoded not on a single quantum state, but on a large number of single identical quantum states. Errors are inevitable when copying these identical quantum states due to the quantum no-cloning theorem. When a small part of errors are ignored, i.e., errors as the minority are automatically corrected by the majority, the fidelity of duplicated copies of classical information will approach unity infinitely. In this way, the classical bits can be duplicated precisely with a universal QCM, which presents a natural transition from quantum cloning to classical duplication. The implement of classical duplication by using QCM shines new lights on the universality of quantum mechanics.     

Spectroscopy of spin-orbit quantum bits in indium antimonide nanowires

A double quantum dot in the few-electron regime is achieved using local gating in an InSb nanowire. The spectrum of two-electron eigenstates is investigated using electric dipole spin resonance. Singlet-triplet level repulsion caused by spin-orbit interaction is observed. The size and the anisotropy of singlet-triplet repulsion are used to determine the magnitude and the orientation of the spin-orbit effective field in an InSb nanowire double dot. The obtained results are confirmed using spin blockade leakage current anisotropy and transport spectroscopy of individual quantum dots.     

N@C60 and P@C60 as quantum bits

Due to their long electron spin relaxation times, the endohedral fullerenes N@C₆₀ and P@C₆₀ are good candidates for the implementation of qubits in an electron spin quantum computer. A central operation in this context is the rotation of the spin direction by an arbitrary angle. In the present experiment, this nutation behavior was studied in pulsed electron spin resonance measurements. We show that, even at room temperature, about 50 Rabi oscillations (about 100 qubit operations) can be performed without refocusing the spin system, although inhomogeneities are present. A special feature of the group V endohedral fullerenes is the electron spinS=3/2, which complicates the nutation behavior. The zero-field splitting at low temperature gives rise to different nutation frequencies for the (1/2,?1/2) transition and the (±3/2, ±1/2) transitions. The frequency ratio is 2/3^1/2.     

Probing electronic coherence in a gas of dipole-dipole coupled Rydberg atoms

We demonstrate a novel time-domain method to probe electronic coherence in ensembles of cold Rydberg atoms coupled via nearly resonant dipole-dipole interactions. Short laser pulses create coherent superpositions of few-electron eigenstates which evolve under the influence of pulsed electric fields. The pulses steer the dynamics, enhancing the probability for finding atoms in np, rather than initially excited ns states. The enhancement reflects the underlying electronic coherence which persists for >10 μs, 2?orders of magnitude longer than previously measured dephasing times in the same system. Simulations suggest that atom motion is responsible for the eventual decoherence.     

Entangling quantum gate in trapped ions via Rydberg blockade

We present a theoretical analysis of the implementation of an entangling quantum gate between two trapped Ca⁺ ions which is based on the dipolar interaction among ionic Rydberg states. In trapped ions, the Rydberg excitation dynamics is usually strongly affected by mechanical forces due to the strong couplings between electronic and vibrational degrees of freedom in inhomogeneous electric fields. We demonstrate that this harmful effect can be overcome using dressed states that emerge from the microwave coupling of nearby Rydberg states. At the same time. these dressed states exhibit long-range dipolar interactions which we use to implement a controlled adiabatic phase gate. Our study highlights a route toward a trapped ion quantum processor in which quantum gates are realized independently of the vibrational modes.     

Dwell-time-limited coherence in open quantum dots

We present measurements of the electron phase coherence time tau(varphi) on a wide range of open ballistic quantum dots (QDs) made from InGaAs heterostructures. The observed saturation of tau(varphi) below temperatures 0.5 K相似文献