Petr Marek

Generating superposition of up-to three photons for continuous variable quantum information processing

We develop an experimental scheme based on a continuous-wave (cw) laser for generating arbitrary superpositions of photon number states. In this experiment, we successfully generate superposition states of zero to three photons, namely advanced versions of superpositions of two and three coherent states. They are fully compatible with developed quantum teleportation and measurement-based quantum operations with cw lasers. Due to achieved high detection efficiency, we observe, without any loss correction, multiple areas of negativity of Wigner function, which confirm strongly nonclassical nature of the generated states.

Noise-powered probabilistic concentration of phase information

Amplifying a signal usually also amplifies the noise. A quantum-state amplifier is now demonstrated that can actually decrease uncertainty about the state’s phase. Counterintuitively, the concept involves the addition of thermal noise.

Elementary gates for quantum information with superposed coherent states

We propose an alternative way of implementing several elementary quantum gates for qubits in the coherent-state basis. The operations are probabilistic and employ single-photon subtractions as the driving force. Our schemes for single-qubit phase gate and two-qubit controlled phase gate are capable of achieving arbitrarily large phase shifts with currently available resources, which makes them suitable for the near-future tests of quantum-information processing with superposed coherent states.

Coherent-state phase concentration by quantum probabilistic amplification

We propose a probabilistic measurement-induced amplification for coherent states. The amplification scheme uses a counterintuitive architecture: a thermal noise addition (instead of a single-photon addition) followed by a feasible multiple-photon subtraction using a realistic photon-number-resolving detector. It allows one to substantially amplify weak coherent states and simultaneously reduce their phase uncertainty, which is impossible when using a deterministic Gaussian amplifier.

On the distillation and purification of phase-diffused squeezed states

Recently it was discovered that non-Gaussian decoherence processes, such as phase-diffusion, can be counteracted by purification and distillation protocols that are solely built on Gaussian operations. Here, we make use of this experimentally highly accessible regime, and provide a detailed experimental and theoretical analysis of several strategies for purification/distillation protocols on phase-diffused squeezed states. Our results provide valuable information for the optimization of such protocols with respect to the choice of the trigger quadrature, the trigger threshold value and the probability of generating a distilled state.

Implementation of a quantum cubic gate by an adaptive non-Gaussian measurement

We present a concept of non-Gaussian measurement composed of a non-Gaussian ancillary state, linear optics and adaptive heterodyne measurement, and on the basis of this we also propose a simple scheme of implementing a quantum cubic gate on a traveling light beam. In analysis of the cubic gate in the Heisenberg representation, we find that nonlinearity of the gate is independent from nonclassicality; the nonlinearity is generated solely by a classical nonlinear adaptive control in a measurement-and-feedforward process while the nonclassicality is attached by the non-Gaussian ancilla that suppresses excess noise in the output. By exploiting the noise term as a figure of merit, we consider the optimum non-Gaussian ancilla that can be prepared within reach of current technologies and discuss performance of the gate. It is a crucial step towards experimental implementation of the quantum cubic gate.

Emulating quantum cubic nonlinearity

Unitary non-Gaussian nonlinearity is one of the key components required for quantum computation and other developing applications of quantum information processing. Sufficient operation of this kind is still not available, but it can be approximatively implemented with help of a specifically engineered resource state constructed from individual photons. We present experimental realization and thorough analysis of such quantum resource state, and confirm that the state does indeed possess properties of a state produced by unitary dynamics driven by cubic nonlinearity.

Probabilistic Cloning of Coherent States without a Phase Reference

We present a probabilistic cloning scheme operating independently of any phase reference. The scheme is based solely on a phase-randomized displacement and photon counting, omitting the need for non-classical resources and non-linear materials. In an experimental implementation, we employ the scheme to clone coherent states from a phase covariant alphabet and demonstrate that the cloner is capable of outperforming the hitherto best-performing deterministic scheme. An analysis of the covariances between the output states shows that uncorrelated clones can be approached asymptotically. An intriguing feature is that the trade-off between success rate and achieved fidelity can be optimized even after the cloning procedure.

Noise-resilient quantum interface based on quantum nondemolition interactions

We propose a quantum interface protocol based on two quantum-non-demolition interactions (QND) arranged either in sequence or in parallel. Since the QND coupling arises naturally in interactions between light and a macroscopic ensemble of atoms, or between light and a micro-mechanical oscillator, the proposed interface is capable of transferring a state of light onto these matter systems. The transfer itself is perfect and deterministic for any quantum state, for arbitrarily small interaction strengths, and for arbitrarily large noise of the target system. It requires an all-optical pre-processing, requiring a coupling stronger than that between the light and the matter, and a displacement feed-forward correction of the matter system. We also suggest a probabilistic version of the interface, which eliminates the need for the feed-forward correction at a cost of reduced success rate. An application of the interface can be found in construction of a quantum memory, or in the state preparation for quantum sensing.

Vacuum as a less hostile environment to entanglement

We derive sufficient conditions for infinite-dimensional systems whose entanglement is not completely lost in a finite time during its decoherence by a passive interaction with local vacuum environments. The sufficient conditions allow us to clarify a class of bipartite entangled states which preserve their entanglement or, in other words, are tolerant against decoherence in a vacuum. We also discuss such a class for entangled qubits.