First solid state quantum computer was built using transmons (cooper pair boxes). The operation of the computer is limited because of using a number of the rigit cooper boxes working with fixed frequency at temperatures of superconducting material. Here, we propose a novel architecture of quantum computer based on a flexible wire circuit of many coupled quantum nodes containing controlled atomic (molecular) ensembles. We demonstrate wide opportunities of the proposed computer. Firstly, we reveal a perfect storage of external photon qubits to multi-mode quantum memory node and demonstrate a reversible exchange of the qubits between any arbitrary nodes. We found optimal parameters of atoms in the circuit and self quantum modes for quantum processing. The predicted perfect storage has been observed experimentally for microwave radiation on the lithium phthalocyaninate molecule ensemble. Then also, for the first time we show a realization of the efficient basic two-qubit gate with direct coupling of two arbitrary nodes by using appropriate atomic frequency shifts in the circuit nodes. Proposed two-qubit gate runs with a speed drastically accelerated proportionally to the number of atoms in the node. The direct coupling and accelerated two-qubit gate can be realized for large number of the circuit nodes. Finally, we describe two and three-dimensional scalable architectures that pave the road to construction of universal multi-qubit quantum computer operating at room temperatures.
First solid state quantum computer was built using transmons (cooper pair boxes). The operation of the computer is limited because of using a number of the rigit cooper boxes working with fixed frequency at temperatures of superconducting material. Here, we propose a novel architecture of quantum computer based on a flexible wire circuit of many coupled quantum nodes containing controlled atomic (molecular) ensembles. We demonstrate wide opportunities of the proposed computer. Firstly, we reveal a perfect storage of external photon qubits to multi-mode quantum memory node and demonstrate a reversible exchange of the qubits between any arbitrary nodes. We found optimal parameters of atoms in the circuit and self quantum modes for quantum processing. The predicted perfect storage has been observed experimentally for microwave radiation on the lithium phthalocyaninate molecule ensemble. Then also, for the first time we show a realization of the efficient basic two-qubit gate with direct coupling of two arbitrary nodes by using appropriate atomic frequency shifts in the circuit nodes. Proposed two-qubit gate runs with a speed drastically accelerated proportionally to the number of atoms in the node. The direct coupling and accelerated two-qubit gate can be realized for large number of the circuit nodes. Finally, we describe two and three-dimensional scalable architectures that pave the road to construction of universal multi-qubit quantum computer operating at room temperatures.
The first realization of solid state quantum computer was demonstrated recently by using artificial atoms — transmons in superconducting resonator. Here, we propose a novel quantum computer based on quantum electrodynamic cavity coupling many quantum nodes of controlled atomic ensembles. The quantum computer contains quantum memory and processing nodes. For the first time, we find the optimal practically attainable parameters of the atoms and optical scheme of the computer for realization of the multimode quantum memory for the photonic qubits with efficiency close to 100%. Then we reveal self modes for reversible transfer of the qubits between the quantum memory node and the processing nodes. Also, we find a realization of iSWAP gate via direct coupling of two processing nodes with an operation rate accelerated proportionally to the number of atoms in the nodes. Collective dynamic and static blockade mechanisms are proposed for realization of $\sqrt {iSWAP} $ quantum gate in multi atomic ensembles. A large number of the two-qubit gates can be simultaneously realized that opens a possibility for parallel quantum processing in the proposed quantum computer.
A design is proposed for scalable solid-state quantum computing, which is based on collectively enhanced magnetic coupling between nitrogen-vacancy center ensembles and superconducting transmission line resonators interconnected by current-biased Josephson junction superconducting phase qubit. In this hybrid system, we realize distant multi-qubit controlled phase gate operations and generate distant multi-qubit entangled W-like states, being indispensable resource to quantum computation. Our proposed architecture consists of solid-state spin ensembles and circuit QED, and could achieve quantum computing in a solid-state environment with high-fidelity and scalable way. The experimental feasibility is discussed, and the implementation efficiency is demonstrated numerically.
Creation of quantum computer is outstanding fundamental and practical problem. The quantum computer could be used for execution of very complicated tasks which are not solvable with the classical computers. The first prototype of solid state quantum computer was created in 2009 with superconducting qubits. However, it suffers from the decoherent processes and it is desirable to find more practical encoding of qubits with long-lived coherence. It could be single impurity or vacancy centers in solids, but their interaction with electromagnetic radiation is rather weak. So, here, ensembles of atoms were proposed for the qubit encoding by using the dipole blockade mechanism in order to turn multilevel systems in two level ones. But dipole-dipole based blockade introduces an additional decoherence that limits its practical significance. Recently, the collective blockade mechanism has been proposed for the system of three-level atoms by using the different frequency shifts for the Raman transitions between the collective atomic states characterized by a different number of the excited atoms. Here, we propose two qubit gate by using another collective blockade mechanism in the system of two level atoms based on exchange interaction via the virtual photons between the multi-atomic ensembles in the resonator. Also we demonstrate the possibility of three qubit gate (Controlled SWAP gate) using a suppression of the swap-process between two multi-atomic ensembles due to dynamical shift of the atomic levels controlled by the states of photon encoded qubit.
After years of academic development, the circuit quantum electrodynamics is entering the age of applications. This thesis was realized in this context of creating tools to bridge the gap between an astounding academic quantum system, superconducting circuits, and a grand goal, the universal quantum computer. A likely blueprint for quantum processors consists in the assembly of a large number of elementary modules arranged in a network.In this experimental thesis, a possible node for such a network, the quantum node, was developed and fabricated using state-of-the-art techniques for 2D superconducting microwave circuits. This node was first used to implement a novel sequential readout method for a superconducting qubit. This experiment, first proposed in 2013 by Sete et al., potentially allows for faster, more accurate read out of superconducting qubits. The read out of qubits is one of the several bottlenecks limiting the development of fault-tolerant superconducting quantum computers, which made this project both useful as a demonstration of the quantum node and for applications. This novel readout method achieves readout performances close to state-of-the-art of superconducting qubit readout even though the chip was not optimized for that purpose.During this work, we also contributed to two other experiments engineering quantum measurement and dissipation with superconducting circuits. First, a dedicated circuit was developed to demonstrate a new form of quantum measurement: the multiplexed photon number measurement. In that experiment led by A. Essig and Q. Ficheux, a superconducting transmon quantum bit is read out at multiple frequencies simultaneously to extract more than one bit of information about the number of photons contained in a microwave resonator coupled to that quantum bit.Second, we contributed to the experimental demonstration of the exponential suppression of bit-flips in a qubit encoded in a Schrödinger Cat state of a microwave mode. This experiment led by R. Lescanne and Z. Leghtas, demonstrates the improvement by a factor 300 of the lifetime of a qubit thanks to the autonomous error correction realized through the engineering of the dissipation of a microwave mode.
Superconducting circuit quantum electrodynamics is the main architecture for implementation of quantum processors. This work presents a proposal to miniaturize quantum memory devices by combining the quantum memory qubits encoded directly in the first two Fock states ${|0\ensuremath{\rangle},|1\ensuremath{\rangle}}$ and the bus channel in the same specially designed multimode cavity. One key feature of the resonant modes used for quantum memory is that the microwave current is localized, which facilitates the storage and retrieval of information in superconducting qubits while preventing crosstalk from other resonant modes. Frequency design and distinct coupling strengths in this kind of device are analyzed with electromagnetic simulations and numerical calculations. The quantum memory properties, e.g., storage speed, fidelity, and stability, are examined in four cases: (1) memory operation of single-qubit state, (2) disturbing effects, (3) coupling operation, and (4) entangled states storage. Decoherence time of the quantum processor can be extended to more than one order of magnitude longer than that without the memory. By integrating transmons with optimally designed coplanar waveguide multimode quantum memories, the present study could provide an optimistic perspective for the development of large-scale superconducting quantum processors.
Superconducting circuit QED is a promising approach for building a quantum computer. In order to realise superconducting circuits at a sufficient scale for useful near-term applications, an architecture with an extensible design is required which implements good connectivity between qubits, and allows for selective readout and control of the qubits without introducing detrimental crosstalk or decoherence. This thesis describes the development of a new coaxial circuit QED architecture that fulfils this requirement of extensibility by incorporating out-of-plane wiring into the sample holder. Single-qubit unit cells consisting of a transmon qubit and readout resonator with coaxial geometries are fabricated on opposing sides of a substrate, and selective control and readout of the qubits is achieved via a capacitance to coaxial wiring built into the device enclosure. Unit cells of qubit and resonator can be arranged in a 2D array without modification of the wiring scheme. A single-qubit unit cell of this architecture is used to implement dispersive circuit QED, and a full characterisation of the Hamiltonian is performed. The device is shown to have parameters comparable to those found in other approaches, such as a coupling between qubit-resonator of ~100 MHz and a coherence time of order ~10 µs. The extension of this scheme to 2D arrays of qubits is then presented, and realizations of two-qubits gates are demonstrated with fidelities all above 87% on a four qubit device. Further evaluations are performed on multi-qubit devices, including a characterisation of the drive isolation of the mode-matched drive ports, finding values in the range of 50 dB and 30 dB for measurement and control respectively. Similarly, the cross coupling between circuits is shown to have values ~2% of the coupling within a unit cell. The effective circuit temperatures are measured, finding typical values of ~100 mK, and the techniques of spin-locking and T2 spectroscopy are employed to probe the noise environment. Finally the architecture is extended to incorporate frequency tuning of qubits with gradiometric SQUID loops by way of off-chip flux bias lines (FBLs). These lines are used to tune qubits with a signal isolation of >99%. Furthermore, the ability of these FBLs to dynamically control the qubit frequency is shown by demonstrating switching of the frequency on a nanosecond time-scale, and parametric driving over a frequency range of gigahertz.