One-step implementing three-qubit phase gate via manipulating rf SQUID qubits in the decoherence-free subspace with respect to cavity decay

We present a scheme for implementing a three-qubit phase gate via manipulating rf superconducting quantum interference device （SQUID） qubits in the decoherence-free subspace with respect to cavity decay. Through appropriate changes of the coupling constants between rf SQUIDs and cavity, the scheme can be realized only in one step. A high fidelity is obtained even in the presence of decoherence.

Decoherence-free subspace and entanglement sudden death of multi-photon polarization states in fiber channels

The construction of quantum networks requires long-distance teleportation of multi-qubit entangled states.Here,we investigate the entanglement dynamics of GHZ and W states in fiber channels.In a fiber channel,the two most important phenomena that affect polarization entanglement are polarization mode dispersion(PMD)and polarization-dependent loss(PDL).We theoretically characterize how PMD and PDL vectors affect three-qubit states.In particular,upon quantifying the entanglement at the output states using concurrence and entanglement witnesses,we reveal the occurrence of entanglement sudden death and the appearance of decoherence-free subspaces in tripartite systems.Finally,we explore the evolution of GHZ and W state with an arbitrary number of photons in a fiber network and discuss the decoherence mechanism of the 4-party cluster state.

Algorithmic cooling based on cross-relaxation and decoherence-free subspace

Heat-bath algorithmic cooling(HBAC)has been proven to be a powerful and effective method for obtaining high polarization of the target system.Its cooling upper bound has been recently found using a specific algorithm,the partner pairing algorithm(PPAHBAC).It has been shown that by including cross-relaxation,it is possible to surpass the cooling bounds.Herein,by combining cross-relaxation and decoherence-free subspace,we present a two-qubit reset sequence and then generate a new algorithmic cooling(AC)technique using irreversible polarization compression to further surpass the bound.The proposed two-qubit reset sequence can prepare one of the two qubits to four times the polarization of a single-qubit reset operation in PPA-HBAC for low polarization.When the qubit number is large,the cooling limit of the proposed AC is approximately five times as high as the PPA-HBAC.The results reveal that cross-relaxation and decoherence-free subspace are promising resources to create new AC for higher polarization.

State transfer based on decoherence-free target state by Lyapunov-based control

We propose a Lyapunov-based control approach for state transfer based on the decoherence-free target state. The expected target state is constructed to be a decoherence-free state in a decoherence-free subspace （DFS） by an external laser field I, so that the system state can be decoupled from the environment, and no more decoherence process will occur. With the decoherence-free target state, we design a Lyapunov-based control field II to steer the given initial state to the decoherence-free state of open quantum systems as completely as possible, and decouple the system state from the environment at the same time. In the end, it is verified that the state transfer control designed comes true on a A-type four-level atomic system, and the system can stay on the decoherence-free target state without coupling to environment.

Cavity loss induced generation of W states

The existence of decoherence-free subspace （DFS） has been discussed widely. In this paper, we propose an alternative scheme for generating the four-atom W states by manipulating DF qubits. The atoms are divided into two pairs and trapped in two separate optical cavities. Manipulation of atoms within DFS may generate a two-atom maximally entangled state in an individual cavity, which is a stable state. After driving the system out of DFS, the atoms will interact resonantly with the cavity field. The photons leaking from the cavities interfere at the beamsplitter, which destroys which-path information, and are finally detected by one of the detectors, leading to the generation of a W state. In addition, the numerical simulation indicates that the fidelity of the prepared state can, for a very wide parameter regime, be very close to unity.

Photoswitchable quantum electrodynamics in a hybrid plasmonic quantum emitter

The design and preparation of quantum states free from environmen-tal decohering effects is critically important for the development of on-chip quantum systems with robustness.One promising strategy is to harness quantum state superposition to construct decoherence-free subspace(DFS),which is termed dark state.Typically,the exci-tation of dark states relies on anti-phase-matching on two qubits and the inter-qubit distance is of wavelength scale,which limits the de-velopment of compact quantum chips.In the current work,a hybrid plasmonic quantum emitter was proposed,which was composed of strongly correlated quantum emitters intermediated by a plasmonic nanocavity.Through turning the plasmonic loss from drawback into advantage,the anti-phase-matching rule was broken by rapidly de-caying the superposed bright state and preparing a sub-100 nm dark state with decay rate reduced by 3 orders of magnitudes.More inter-estingly,the dark state could be optically switched to a single-photon emitter with enhanced brightness through photon-blockade,with the quantum second order correlation function at zero delay showing a wide range of tunability down to 0.02.