Abstract: Various embodiments of the present invention are directed to methods for generating an entangled state of qubits. In one embodiment of the present invention, a method for preparing an entangled state of qubits comprises providing a probe and N non-interacting qubits, each qubit comprises a linear superposition of two basis states. The probe is transmitted into an interaction region that separately couples the probe to each of the qubits and produces a number of different probes. A linear superposition of states is output from the interaction region, each state in the linear superposition of states comprises a tensor product of entangled basis states and one of the different probes. The linear superposition of states is projected into one of the entangled states by measuring the state of the probe.

Abstract: The states of matter system (110) having only ones basis state that couples to an excited state can be entangled using measurements of photons during transitions from the excited state. High efficiency of entanglement operation can be achieved by repeating the measurements after performing bit flips on the matter systems (110). High efficiency of entanglement operation can be achieved using non-absorbing parity measurements on the emitted photons so that measured photons can be subsequently manipulated and measured to near-deterministically produce entangled states. Such entanglement operations can be employed to construct cluster states suitable for simulating arbitrary logic networks.

Abstract: Nonlinear elements can efficiently implement quantum information processing systems such as controlled phase shifters, non-absorbing detectors including parity detectors, quantum subspace projections, non-absorbing Bell state analyzers, non-absorbing encoders/entanglers, and fundamental quantum gates such as CNOT gates. The non-absorbing detectors permit improvements in the efficiency of a probabilistic quantum gate by permitting reuse of the same photonic resources during multiple passes through the probabilistic gate.

Abstract: Various embodiments of the present invention are directed to methods for determining a phase shift acquired by an entangled N-qubit system represented by a NOON state. In one embodiment, a probe electromagnetic field is coupled with each qubit system. The phase shift acquired by the qubit systems is transferred to the probe electromagnetic field by transforming each qubit-system state into a linear superposition of qubit basis states. An intensity measurement is performed on the probe electromagnetic field in order to obtain a corresponding measurement result. A counter associated with a measurement-result interval is incremented, based on the measurement result falling within the measurement-result interval. A frequency distribution is produced by normalizing the counter associated with each measurement-result interval for a number of trials.

Abstract: We propose a practical method to generate cluster states for quantum computers. The qubit systems can be NV-centers in diamond, Pauli-blockade quantum dots with an excess electron or ion traps with optical transitions, which are subsequently entangled using a so-called double-heralded single-photon detection scheme. The fidelity of the resulting entanglement is extremely robust against the most important practical errors such as detector loss, light collection efficiency and mode mismatching. The cluster states are generated efficiently using a modified probabilistic teleportation protocol.

Abstract: Various embodiments of the present invention are directed to methods for determining a phase shift acquired by an entangled N-qubit system represented by a NOON state. In one embodiment, a probe electromagnetic field is coupled with each qubit system. The phase shift acquired by the qubit systems is transferred to the probe electromagnetic field by transforming each qubit-system state into a linear superposition of qubit basis states. An intensity measurement is performed on the probe electromagnetic field in order to obtain a corresponding measurement result. A counter associated with a measurement-result interval is incremented, based on the measurement result falling within the measurement-result interval. A frequency distribution is produced by normalizing the counter associated with each measurement-result interval for a number of trials.

Abstract: Various embodiments of the present invention are directed to methods for generating an entangled state of qubits. In one embodiment of the present invention, a method for preparing an entangled state of qubits comprises providing a probe and N non-interacting qubits, each qubit comprises a linear superposition of two basis states. The probe is transmitted into an interaction region that separately couples the probe to each of the qubits and produces a number of different probes. A linear superposition of states is output from the interaction region, each state in the linear superposition of states comprises a tensor product of entangled basis states and one of the different probes. The linear superposition of states is projected into one of the entangled states by measuring the state of the probe.

Abstract: Nonlinear electromagnetic elements can efficiently implement quantum information processing tasks such as controlled phase shifts, non-demolition state detection, quantum subspace projections, non-demolition Bell state analysis, heralded state preparation, quantum non-demolition encoding, and fundamental quantum gate operations. Direct use of electromagnetic non-linearity can amplify small phase shifts and use feed forward systems in a near deterministic manner with high operating efficiency. Measurements using homodyne detectors can cause near deterministic projection of input states on a Hilbert subspace identified by the measurement results. Feed forward operation can then alter the projected state if desired to achieve a desired output state with near 100% efficiency.

Abstract: Nonlinear electromagnetic elements can efficiently implement quantum information processing tasks such as controlled phase shifts, non-demolition state detection, quantum subspace projections, non-demolition Bell state analysis, heralded state preparation, quantum non-demolition encoding, and fundamental quantum gate operations. Direct use of electromagnetic non-linearity can amplify small phase shifts and use feed forward systems in a near deterministic manner with high operating efficiency. Measurements using homodyne detectors can cause near deterministic projection of input states on a Hilbert subspace identified by the measurement results. Feed forward operation can then alter the projected state if desired to achieve a desired output state with near 100% efficiency.