Conservation of Angular Momentum in a Flux Qubit

Oscillations of superconducting current between clockwise and counterclockwise directions in a flux qubit do not conserve the angular momentum of the qubit. To compensate for this effect the solid containing the qubit must oscillate in unison with the current. This requires entanglement of quantum states of the qubit with quantum states of a macroscopic body. The question then arises whether slow decoherence of quantum oscillations of the current is consistent with fast decoherence of quantum states of a macroscopic solid. This problem is analyzed within an exactly solvable quantum model of a qubit embedded in an absolutely rigid solid and for the elastic model that conserves the total angular momentum. We show that while the quantum state of a flux qubit is, in general, a mixture of a large number of rotational states, slow decoherence is permitted if the system is macroscopically large. Practical implications of entanglement of qubit states with mechanical rotations are discussed.     

Entangling a single NV centre with a superconducting qubit via parametric couplings between photons and phonons in a hybrid system

We propose an efficient scheme for generating entangled states between a single nitrogen-vacancy (NV) centre in diamond and a superconducting qubit in a hybrid set-up. In this device, the NV centre and the superconducting qubit couple to a nanomechanical resonator and a superconducting coplanar waveguide cavity, respectively, while the microwave cavity and the mechanical resonator are parametrically coupled with a tunable coupling strength. We show that, highly entangled states between the NV centre and the superconducting qubit can be achieved, by means of the Jaynes–Cummings interactions in the NV-resonator and qubit-cavity subsystems which transfer the entanglement between the vibration phonons and the cavity photons to the NV centre and the superconducting qubit. This work may provide interesting applications in quantum computation and communication with single NV spins and superconducting qubits.     

Experiments on a Superconducting Qubit Manipulated by Fast Flux Pulses: The Issue of Non-adiabaticity

The double SQUID qubit is a superconducting interferometer (SQUID) made of two Josephson junctions and two superconducting loops. Its energy potential can be greatly modified in shape and symmetry by using two magnetic control fluxes that can change the potential from a double well to an almost harmonic single well: This feature is exploited for manipulating the qubit without resorting to the usual NMR-like techniques with microwave irradiation. The qubit machinery relies on these operations being performed non-adiabatically, realizing a transition between the two lowest-lying energy levels, at the same time avoiding excitation of upper levels, a condition that can be satisfied by using control pulses with proper risetime. We show experimental results referring to manipulation of the qubit at different rates and make a qualitative comparison with theoretical expectations.     

Effects of Energy Dissipation on the Parametric Excitation of a Coupled Qubit–Cavity System

We consider a parametrically driven system of a qubit coupled to a cavity taking into account different channels of energy dissipation. We focus on the periodic modulation of a single parameter of this hybrid system, which is the coupling constant between the two subsystems. Such a modulation is possible within the superconducting realization of qubit–cavity coupled systems, characterized by an outstanding degree of tunability and flexibility. Our major result is that energy dissipation in the cavity can enhance population of the excited state of the qubit in the steady state, while energy dissipation in the qubit subsystem can enhance the number of photons generated from vacuum. We find optimal parameters for the realization of such dissipation-induced amplification of quantum effects. Our results might be of importance for the full control of quantum states of coupled systems as well as for the storage and engineering of quantum states.     

Dispersive Charge and Flux Qubit Readout as a Quantum Measurement Process

We analyze the dispersive readout of superconducting charge and flux qubits as a quantum measurement process. The measurement oscillator frequency is considered much lower than the qubit frequency. This regime is interesting because large detuning allows for strong coupling between the measurement oscillator and the signal transmission line, thus allowing for fast readout. Due to the large detuning we may not use the rotating wave approximation in the oscillator-qubit coupling. Instead we start from an approximation where the qubit follows the oscillator adiabatically, and show that non-adiabatic corrections are small. We find analytic expressions for the measurement time, as well as for the back-action, both while measuring and in the off-state. The quantum efficiency is found to be unity within our approximation, both for charge and flux qubit readout.      

Scalable solid-state qubits: challenging decoherence and read-out

We review some basic facts about qubits and qubit processing. After a brief survey of solid-state qubits, we focus on quantized electrical circuits and superconducting qubits based on Josephson junctions. We review the general framework and indicate how the various qubits, such as the superconducting Cooper pair box charge qubit, the persistent current flux qubit, the hybrid charge-phase qubit, and the Andreev-level qubit, can be seen to appear due to different choices of design parameters. Finally, we consider multi-qubit systems and discuss some aspects of decoherence.     

Scaling of decoherence effects in quantum computers

Abstract

The scaling of decoherence rates with qubit number N is studied for a simple model of a quantum computer in the situation where N is large. The two state qubits are localized around well-separated positions via trapping potentials and vibrational centre of mass motion of the qubits occurs. Coherent one and two qubit gating processes are controlled by external classical fields and facilitated by a cavity mode ancilla. Decoherence due to qubit coupling to a bath of spontaneous modes, cavity decay modes and to the vibrational modes is treated. A non-Markovian treatment of the short time behaviour of the fidelity is presented, and expressions for the characteristic decoherence time scales obtained for the case where the qubit/cavity mode ancilla is in a pure state and the baths are in thermal states. Specific results are given for the case where the cavity mode is in the vacuum state and gating processes are absent and the qubits are in (a) the Hadamard state (b) the GHZ state.     

Coherent Cooper-Pair Tunneling Through Capacitively-Coupled Superconducting Charge Qubits

Coherent Cooper-pair tunneling phenomena through two superconducting charge qubits are studied. The maximum current through the first qubit is calculated in the presence of the second qubit capacitively coupled to it. It is shown that the effective Josephson coupling of the second qubit may increase the maximum current of the first qubit by cooperative hopping of Cooper-pairs. The modulation of the maximum current by the external current through the second qubit is also discussed.     

Measurement device independent quantum key distribution assisted by hybrid qubit

Abstract

Measurement device independent Quantum Key Distribution (MDI-QKD), is immune to all attacks on detection and achieve immense improvement with respect to quantum key distribution system security. However, Bell state measurement (BSM), the kernel processing in MDI-QKD, can only identify two of the four Bell states, which limits the efficiency of the protocol. In this paper, a modified MDI-QKD with hybrid qubit is proposed to provide a major step towards answering this question. The hybrid qubits, which are composed of single photon qubit qubits and coherent qubit, are sent to the quantum relay to perform parallel BSMs synchronously and bit flip can be easily operated to complete the whole key distribution process. The secure key rate can be improved with our modified protocol owing to the higher success probability of BSM, which is increased by adding the parity check of coherent qubit. Furthermore, though our protocol requires photon number resolving detectors, the BSM of coherent state could be instead implemented using squeezed state which makes our scheme practical with state-of-the-art devices.     

Measurements of Decoherence Times in a Josephson Charge Qubit Using Fast Pulses

We study decoherence of a Josephson charge qubit using fast pulses to perform qubit rotations. The gate charge dependence of the decoherence rate indicates that the low frequency fluctuations affecting the qubit are from charges distributed with a 1/f-type spectrum. Assuming the form S_q(ω) = α/|ω|, we find $\sqrt \alpha$ = 4 × 10^?3e, which is slightly higher than the value found from very low frequency noise measurements. PACS numbers: 03.67.-a, 75.40.+r, 85.25.Cp.     

Electrical control of a solid-state flying qubit

Solid-state approaches to quantum information technology are attractive because they are scalable. The coherent transport of quantum information over large distances is a requirement for any practical quantum computer and has been demonstrated by coupling super-conducting qubits to photons. Single electrons have also been transferred between distant quantum dots in times shorter than their spin coherence time. However, until now, there have been no demonstrations of scalable 'flying qubit' architectures-systems in which it is possible to perform quantum operations on qubits while they are being coherently transferred-in solid-state systems. These architectures allow for control over qubit separation and for non-local entanglement, which makes them more amenable to integration and scaling than static qubit approaches. Here, we report the transport and manipulation of qubits over distances of 6?μm within 40?ps, in an Aharonov-Bohm ring connected to two-channel wires that have a tunable tunnel coupling between channels. The flying qubit state is defined by the presence of a travelling electron in either channel of the wire, and can be controlled without a magnetic field. Our device has shorter quantum gates (<1?μm), longer coherence lengths (～86?μm at 70?mK) and higher operating frequencies (～100?GHz) than other solid-state implementations of flying qubits.     

Controlling qubit transitions during non-adiabatic rapid passage through quantum interference

Abstract

In adiabatic rapid passage, the Bloch vector of a qubit is inverted by slowly inverting an external field to which it is coupled, and along which it is initially aligned. In non-adiabatic twisted rapid passage, the external field is allowed to twist around its initial direction with azimuthal angle φ(t) at the same time that it is non-adiabatically inverted. For polynomial twist, φ(t) ～ Bt ⁿ . We show that, for n ≥ 3, multiple qubit resonances can occur during a single inversion of the external field, producing strong interference effects in the qubit transition probability. The character of the interference is controllable through variation of the twist strength B. Both constructive and destructive interference are possible, allowing qubit transitions to be greatly enhanced or suppressed. Experimental confirmation of these controllable interference effects has already occurred. Application of this interference mechanism to the construction of fast fault-tolerant quantum controlled-NOT and NOT gates is discussed.     

Dynamics of a Phase Qubit-Resonator System: Requirements for Fast Nondemolition Readout of a Phase Qubit

We examine the time-dependent evolution of a coupled qubit-transmission line-resonator system coupled to an external drive and a resonator environment. By solving the equation for a non-stationary resonator field, we determine the requirements for a single-shot nondestructive dispersive measurement of the phase qubit state. Reliable isolation of the qubit from the “electromagnetic environment” is necessary for a dispersive readout and can be achieved if the whole system interacts with the external fields only through a resonator that is weakly coupled to the qubit. A set of inequalities involving the resonator-qubit detuning and a coupling parameter, the resonator leakage and the measurement time, together with the requirement of multi-photon outgoing flux is derived. In particular, it is shown that to decrease the measurement time one must increase the resonator leakage. This leakage increase reduces the quality factor and decreases the resolution of the resonator eigenfrequencies corresponding to different qubit states. The consistency of our inequalities for two sets of experimental parameters is discussed. Our results can be used for optimizing experimental setups, parameters, and measurement protocols.     

Fabrication of Flux Qubit with a Mechanical Degree of Freedom

We report on the fabrication and characterization of superconducting flux qubit coupled to nanomechanical resonator vibration modes. Anisotropic and isotropic etching processes parameters of plasma created by a reactive ion etching of a CF₄ gas, were optimized to suspend one arm of the qubit. One of the beams was characterized using a magnetomotive detection scheme in the transmission regime. And suspended beams with different length coupled to a superconducting flux qubits were characterized at base temperature by performing spectroscopy measurements.     

EPR quantum key distribution protocols with potential 100% qubit efficiency

Public discussion is a useful way for quantum key distribution protocols to reveal the presence of eavesdroppers. However, to ensure better security, nearly 50% of the transmitted qubits are spent in public discussions. Consequently, the original EPR quantum key distribution protocol provides only 25% qubit efficiency and Deng et al.'s scheme delivers only 50% qubit efficiency. By bringing classical cryptographic techniques into the quantum arena, this work proposes EPR quantum key distribution protocols with a potential of 100% qubit efficiency     

Using Electrons on Liquid Helium for Quantum Computing

We describe a quantum computer based on electrons supported by a helium film and localized laterally by small electrodes just under the helium surface. Each qubit is made of combinations of the ground and first excited state of an electron trapped in the image potential well at the surface. Mechanisms for preparing the initial state of the qubit, operations with the qubits, and a proposed readout are described. This system is, in principle, capable of 10 ⁵ operations in a decoherence time.     

Landau–Zener Interferometry in a Cooper-Pair Box

In a driven system, a sequence of Landau–Zener transitions occur, and they may interfere with each other. We present experimental evidence of quantum interference effects in a superconducting charge qubit, a Cooper-pair box, where the LZ tunneling occurs at the charge degeneracy. By employing weak capacitive monitoring, we observe interference between consecutive LZ tunneling events.     

Decoherence rates in large-scale quantum computers and macroscopic quantum systems

Markovian regime decoherence effects in quantum computers are studied in terms of the fidelity for the situation where the number of qubits N becomes large. A general expression giving the decoherence time scale in terms of Markovian relaxation elements and expectation values of products of system fluctuation operators is obtained, which could also be applied to study decoherence in other macroscopic systems such as Bose condensates and superconductors. A standard circuit model quantum computer involving three-state lambda system ionic qubits is considered, with qubits localized around well-separated positions via trapping potentials. The centre of mass vibrations of the qubits act as a reservoir. Coherent one and two qubit gating processes are controlled by time-dependent localized classical electromagnetic fields that address specific qubits, the two qubit gating processes being facilitated by a cavity mode ancilla, which permits state interchange between qubits. With a suitable choice of parameters, it is found that the decoherence time can be made essentially independent of N.     

Phase qubit on a superconducting single-junction interferometer

The energy spectrum of a phase quantum bit (qubit) implemented on a superconducting interferometer with a single Josephson junction has been studied in terms of the Hamilton formalism. An analytical formula for ground level splitting in the qubit spectrum is obtained and analyzed.     

Quantum dense coding via cavity decay

We investigate a scheme for implementing quantum dense coding via cavity decay and linear optics devices. Our scheme combines two distinct advantages: the atomic qubit serves as a stationary bit and the photonic qubit as a flying bit, thus it is suitable for long distant quantum communication.