Entangling homogeneously broadened matter qubits in the weak-coupling cavity-QED regime

In distributed quantum information processing, flying photons entangle matter qubits confined in cavities. However, when a matter qubit is homogeneously broadened, the strong-coupling regime of cavity QED is typically required, which is hard to realize in actual experimental setups. Here, we show that a high-fidelity entanglement operation is possible even in the weak-coupling regime in which dampings (dephasing, spontaneous emission, and cavity leakage) overwhelm the coherent coupling between a qubit and the cavity. Our proposal enables distributed quantum information processing to be performed using much less demanding technology than previously

Magnetic field sensing with quantum error detection under the effect of energy relaxation

A solid state spin is an attractive system with which to realize an ultra-sensitive magnetic field sensor. A spin superposition state will acquire a phase induced by the target field, and we can estimate the field strength from this phase. Recent studies have aimed at improving sensitivity through the use of quantum error correction (QEC) to detect and correct any bit-flip errors that may occur during the sensing period. Here, we investigate the performance of a two-qubit sensor employing QEC and under the effect of energy relaxation. Surprisingly, we find that the standard QEC technique to detect and recover from an error does not improve the sensitivity compared with the single-qubit sensors. This is a consequence of the fact that the energy relaxation induces both a phase-flip and a bit-flip noise where the former noise cannot be distinguished from the relative phase induced from the target fields. However, we have found that we can improve the sensitivity if we adopt postselection to discard the state when error is detected. Even when quantum error detection is moderately noisy, and allowing for the cost of the postselection technique, we find that this two-qubit system shows an advantage in sensing over a single qubit in the same conditions

Probabilistic growth of large entangled states with low error accumulation

The creation of complex entangled states, resources that enable quantum computation, can be achieved via simple 'probabilistic' operations which are individually likely to fail. However, typical proposals exploiting this idea carry a severe overhead in terms of the accumulation of errors. Here we describe an method that can rapidly generate large entangled states with an error accumulation that depends only logarithmically on the failure probability. We find that the approach may be practical for success rates in the sub-10% range, while ultimately becoming unfeasible at lower rates. The assumptions that we make, including parallelism and high connectivity, are appropriate for real systems including measurement-induced entanglement. This result therefore shows the feasibility for real devices based on such an approach.Comment: 5 pages, 3 figure

Distributed quantum computation with arbitrarily poor photon detection

In a distributed quantum computer scalability is accomplished by networking together many elementary nodes. Typically the network is optical and inter-node entanglement involves photon detection. In complex networks the entanglement fidelity may be degraded by the twin problems of photon loss and dark counts. Here we describe an entanglement protocol which can achieve high fidelity even when these issues are arbitrarily severe; indeed the method succeeds with finite probability even if the detectors are entirely removed from the network. An experimental demonstration should be possible with existing technologies.Comment: 5 pages, 4 fig

How to evaluate the adiabatic condition for quantum annealing in an experiment

We propose an experimental method to evaluate the adiabatic condition during quantum annealing. The adiabatic condition is composed of the transition matrix element and the energy gap, and our method simultaneously provides information about these without diagonalizing the Hamiltonian. The key idea is to measure a power spectrum of a time domain signal by adding an oscillating field during quantum annealing, and we can estimate the values of transition matrix element and energy gap from the measurement output. Our results provide a powerful experimental tool to analyze the performance of quantum annealing, which will be essential for solving practical combinatorial optimization problems.Comment: 16 pages, 13 figure

Catastrophic failure of quantum annealing owing to non-stoquastic Hamiltonian and its avoidance by decoherence

Quantum annealing (QA) is a promising method for solving combinatorial optimization problems whose solutions are embedded into a ground state of the Ising Hamiltonian. This method employs two types of Hamiltonians: a driver Hamiltonian and a problem Hamiltonian. After a sufficiently slow change from the driver Hamiltonian to the problem Hamiltonian, we can obtain the target ground state that corresponds to the solution. The inclusion of non-stoquastic terms in the driver Hamiltonian is believed to enhance the efficiency of the QA. Meanwhile, decoherence is regarded as of the main obstacles for QA. Here, we present examples showing that non-stoaquastic Hamiltonians can lead to catastrophic failure of QA, whereas a certain decoherence process can be used to avoid such failure. More specifically, when we include anti-ferromagnetic interactions (i.e., typical non-stoquastic terms) in the Hamiltonian, we are unable to prepare the target ground state even with an infinitely long annealing time for some specific cases. In our example, owing to a symmetry, the Hamiltonian is block-diagonalized, and a crossing occurs during the QA, which leads to a complete failure of the ground-state search. Moreover, we show that, when we add a certain type of decoherence, we can obtain the ground state after QA for these cases. This is because, even when symmetry exists in isolated quantum systems, the environment breaks the symmetry. Our counter intuitive results provide a deep insight into the fundamental mechanism of QA

Enhanced energy relaxation process of quantum memory coupled with a superconducting qubit

For quantum information processing, each physical system has different advantage for the implementation and so hybrid systems to benefit from several systems would be able to provide a promising approach. One of the common hybrid approach is to combine a superconducting qubit as a controllable qubit and the other quantum system with a long coherence time as a memory qubit. The superconducting qubit allows us to have an excellent controllability of the quantum states and the memory qubit is capable of storing the information for a long time. By tuning the energy splitting between the superconducting qubit and the memory qubit, it is believed that one can realize a selective coupling between them. However, we have shown that this approach has a fundamental drawback concerning energy leakage from the memory qubit. The detuned superconducting qubit is usually affected by severe decoherence, and this causes an incoherent energy relaxation from the memory qubit to the superconducting qubit via the imperfect decoupling. We have also found that this energy transport can be interpreted as an appearance of anti quantum Zeno effect induced by the fluctuation in the superconducting qubit. We also discuss a possible solution to avoid such energy relaxation process, which is feasible with existing technology