Multiqubit Phase Gate Research Articles

Broadcasting single-qubit and multiqubit entangled states: Authentication, cryptography, and distributed quantum computation

Quantum entanglement assisted with measurements provides various pathways to communicate information to parties within a network. In this work, we generalize a previous broadcasting protocol and present schemes to broadcast product and multipartite entangled quantum states, where in the latter case the sender can remotely add phase gates or abort distributing the states. We first focus on the broadcasting of product quantum states in a network, and generalize the basic protocol to include an arbitrary basis rotation and allow for multiple receivers and senders. We show how to add and delete senders from the network. The generalization also includes the case where a phase to be applied to the broadcast states is not known in advance but is provided to a sender encoded in another quantum state. Applications of broadcasting product states include authentication and three-state quantum cryptography. In the second part, we study the distribution of a single multiqubit state shared among several receivers entangled with multiqubit phase gates, which includes the graph states as an example. We show that by coordinating with the sender, the receivers can assist in performing remote, distributed measurement-based quantum computation with the Pauli-$X$ basis measurement alone. As another application of this, we discuss the distribution of the multiqubit Greenberger-Horne-Zeilinger state.

Multi-qubit phase gate on multiple resonators mediated by a superconducting bus.

We propose a one-step scheme for implementing multi-qubit phase gates on microwave photons in multiple resonators mediated by a superconducting bus in circuit quantum electrodynamics (QED) system. In the scheme, multiple single-mode resonators carry quantum information with their vacuum and single-photon Fock states, and a multi-level artificial atom acts as a quantum bus which induces the indirect interaction among resonators. The method of pulse engineering is used to shape the coupling strength between resonators and the bus so as to improve the fidelity and robustness of the scheme. We also discuss the influence of finite coherence time for the bus and resonators on gate fidelity respectively. Finally, we consider the suppression of unwanted transitions and propose the method of optimized detuning compensation for offsetting unwanted transitions, showing the feasibility of the scheme within the current experiment technology.

One-step implementation of a multi-target-qubit controlled phase gate in a multi-resonator circuit QED system

Circuit quantum electrodynamics system composed of many qubits and resonators may provide an excellent way to realize large-scale quantum information processing (QIP). Because of key role for large-scale QIP and quantum computation, multi-qubit gates have drawn intensive attention recently. Here, we present a one-step method to achieve a multi-target-qubit controlled phase gate in a multi-resonator system, which possesses a common control qubit and multiple different target qubits distributed in their respective resonators. Noteworthily, the implementation of this multi-qubit phase gate does not require classical pulses, and the gate operation time is independent of the number of qubits. Besides, the proposed scheme can in principle be adapted to a general type of qubits like natural atoms, quantum dots, and solid-state qubits (e.g., superconducting qubits and NV centers).

Multi-target-qubit unconventional geometric phase gate in a multi-cavity system.

Cavity-based large scale quantum information processing (QIP) may involve multiple cavities and require performing various quantum logic operations on qubits distributed in different cavities. Geometric-phase-based quantum computing has drawn much attention recently, which offers advantages against inaccuracies and local fluctuations. In addition, multiqubit gates are particularly appealing and play important roles in QIP. We here present a simple and efficient scheme for realizing a multi-target-qubit unconventional geometric phase gate in a multi-cavity system. This multiqubit phase gate has a common control qubit but different target qubits distributed in different cavities, which can be achieved using a single-step operation. The gate operation time is independent of the number of qubits and only two levels for each qubit are needed. This multiqubit gate is generic, e.g., by performing single-qubit operations, it can be converted into two types of significant multi-target-qubit phase gates useful in QIP. The proposal is quite general, which can be used to accomplish the same task for a general type of qubits such as atoms, NV centers, quantum dots, and superconducting qubits.

Fast multiqubit phase gate in circuit QED beyond the rotating-wave approximation

We propose a two-step approach to realize a multiqubit phase gate in circuit QED beyond the rotating-wave approximation. The multiqubit gate can be implemented in a few nanoseconds, which is much faster than that under the rotating-wave approximation. Also the operation time of the gate is independent of the number of qubits. Due to the absence of the resonator degree of freedom in the effective Hamiltonian, our scheme is insensitive to the initial state of the resonator and immune to the leakage of photons. Numerical simulation shows that the scheme is robust against the main decoherence sources.

Single-step implementation of a multiple-target-qubit controlled phase gate without need of classical pulses.

We propose a simple method for achieving a multiqubit phase gate of one qubit simultaneously controlling n target qubits, by using three-level quantum systems (i.e., qutrits) coupled to a cavity or resonator. The gate can be realized via one operational step, without need of classical pulses, and by a virtual photon process. Thus, the gate operation is greatly simplified and decoherence from the cavity decay is much reduced, when compared with previous proposals. In addition, the operation time is independent of the number of qubits and no adjustment of the qutrit level spacings or the cavity frequency is needed during the operation.

Implementation of a multiqubit phase gate with one qubit simultaneously controlling n qubits in the ion-trap system

In this paper, we present a scheme for implementing a multiqubit phase gate with one control qubit simultaneously controlling n target qubits in the ion-trap system. In our scheme, there is no ener...

Implementation of a multiqubit phase gate with one qubit simultaneously controlling n qubits in the ion-trap system

In this paper, we present a scheme for implementing a multiqubit phase gate with one control qubit simultaneously controlling n target qubits in the ion-trap system. In our scheme, there is no energy exchange between the internal and external degrees of freedom in the course of operation, and the vibrational mode is only virtually excited. The system is insensitive to changes in the vibrational motion. The proposed scheme is experimentally feasible based on the present ion-trap techniques.

One-step implementation of a multiqubit phase gate with one control qubit and multiple target qubits in coupled cavities

We propose a one-step scheme to implement a multiqubit controlled phase gate with one qubit simultaneously controlling multiple qubits with three-level atoms at distant nodes in coupled cavity arrays. Selective qubit-qubit couplings are achieved by adiabatically eliminating the atomic excited states and photonic states, and the required phase shifts between the control qubit and any target qubit can be realized through suitable choices of the parameters of the external fields. Moreover, the effective model is robust against decoherence because neither the atoms nor the field modes are excited during the gate operation, leading to a useful step toward scalable quantum computing networks.

REALIZATION OF 1 → n CONTROLLED PHASE GATE IN CAVITY QED

A scheme for realizing a multi-qubit phase gate with one control qubit simultaneously controlling n target qubits in a cavity QED system is proposed. The operation time of the gate is independent of the number n of qubits. The realizability of the gate with current technology is also discussed.

Multiqubit tunable phase gate of one qubit simultaneously controllingnqubits in a cavity

We propose how to realize a multiqubit tunable phase gate of one qubit simultaneously controlling $n$ qubits with four-level quantum systems in a cavity or coupled to a resonator. Each of the $n$ two-qubit controlled-phase (cp) gates involved in this multiqubit phase gate has a shared control qubit but a different target qubit. In this proposal, the two lowest levels of each system represent the two logical states of a qubit while the two higher-energy intermediate levels are used for the gate implementation. The method presented here operates essentially by creating a single photon through the control qubit, which then induces a phase shift to the state of each target qubit. The phase shifts on each target qubit can be adjusted by changing the Rabi frequencies of the pulses applied to the target qubit systems. The operation time for the gate implementation is independent of the number of qubits, and neither adjustment of the qubit level spacings nor adjustment of the cavity mode frequency during the gate operation is required by this proposal. It is also noted that this approach can be applied to implement certain types of significant multiqubit phase gates (e.g., the multiqubit phase gate consisting of $n$ two-qubit cp gates which are key elements in quantum Fourier transforms). A possible physical implementation of our approach is presented. Our proposal is quite general and can be applied to physical systems such as various types of superconducting devices coupled to a resonator and trapped atoms in a cavity.

Modularization of a multi-qubit controlled phase gate and its nuclear magnetic resonance implementation

Quantum circuit network is a set of circuits that implements a certain computation task. Being at the center of the quantum circuit network, the multi-qubit controlled phase shift is one of the most important quantum gates. In this paper, we apply the method of modular structuring in classical computer architecture to quantum computer and give a recursive realization of the multi-qubit phase gate. This realization of the controlled phase shift gate is convenient in realizing certain quantum algorithms. We have experimentally implemented this modularized multi-qubit controlled phase gate in a three qubit nuclear magnetic resonance quantum system. The network is demonstrated experimentally using line selective pulses in nuclear magnetic resonance technique. The procedure has the advantage of being simple and easy to implement.