A scheme to generate superpositions of coherent states with variable amplitudes, phases and weighting factors is proposed. It provides experimental possibility for preparing an arbitrary Schrödinger cat state.
Superpositions of macroscopically distinct quantum states, introduced in Schrödinger’s famous Gedankenexperiment, are an epitome of quantum “strangeness” and a natural tool for determining the validity limits of quantum physics. The optical incarnation of Schrödinger’s cat — the superposition of two opposite-amplitude coherent states — is also the backbone of quantum information processing in the continuous-variable domain. Here we implement two protocols for producing and amplifying optical Schrödinger’s cats. In the first protocol we remotely prepare a high-efficiency Schrödinger cat state by applying the remote state preparation to a N00N state prepared between two parties that are separated by a lossy medium. The second protocol consists in bringing the initial states into interference on a beamsplitter and a subsequent heralding quadrature measurement in one of the output channels. The latter technique enables implementation in an iterative manner, in principle allowing creation of Schrödinger’s cat states of arbitrarily high amplitude.
Optical Schrodinger's cat (SC) is highly anticipated because of the potential of realizing fault-tolerant quantum computing, but the practical merit is only shown when the amplitude is larger than 2. However, such high-amplitude cats have not been prepared due to the limitations rooted in the existing method. Here, we demonstrate a principle that a large SC-like state can be generated by the superposition of two kittens in which two nearby coherent states interfere and grow to an enlarged coherent-like state. Further, we propose a scheme to breed the cat beyond the limitation in the former works with a high probability by realizing the superposition of two SCs in coupled waveguides. The principle and scheme demonstrated here provide a new perspective on understanding quantum superposition in phase space and a better solution for the efficient generation of SCs on chips.
Schr\{o}dinger cat states are crucial for exploration of fundamental issues
of quantum mechanics and have important applications in quantum information
processing. Here, we propose and experimentally demonstrate a method for
manipulating cat states in a cavity with the Aharonov-Anandan phase acquired by
a superconducting qubit, which is dispersively coupled to the cavity. Based on
this dispersive coupling, the qubit can be forced to trace out a circuit in the
projective Hilbert space conditional on one coherent state. By preparing the
cavity in a superposition of two coherent states, the geometric phase
associated with this transport is encoded to the relative probability amplitude
of these two coherent states. We demonstrate the photon-number parity of a cat
state in a cavity can be controlled by adjusting this geometric phase, which
offers the possibility for protecting its quantum coherence from single-photon
loss. Based on this geometric effect, we realize phase gates for one and two
photonic qubits whose logical basis states are encoded in two quasi-orthogonal
coherent states. We further demonstrate two-cavity gates with symmetric and
asymmetric Fock state encoding schemes. Our method can be directly extended to
implementation of controlled-phase gates between error-correctable logical
qubits.