Photonic quantum simulators

Journal name:
Nature Physics
Year published:
Published online


Quantum simulators are controllable quantum systems that can be used to mimic other quantum systems. They have the potential to enable the tackling of problems that are intractable on conventional computers. The photonic quantum technology available today is reaching the stage where significant advantages arise for the simulation of interesting problems in quantum chemistry, quantum biology and solid-state physics. In addition, photonic quantum systems also offer the unique benefit of being mobile over free space and in waveguide structures, which opens new perspectives to the field by enabling the natural investigation of quantum transport phenomena. Here, we review recent progress in the field of photonic quantum simulation, which should break the ground towards the realization of versatile quantum simulators.

At a glance


  1. First quantum chemistry experiment on a quantum information processor.
    Figure 1: First quantum chemistry experiment on a quantum information processor.

    a, Quantum optics experiment for simulating the energy of the hydrogen molecule in the minimal basis set. A pair of entangled photons generated via the spontaneous parametric down-conversion (SPDC) process implements an iterative phase-estimation scheme where one of the photons represents two 2×2 blocks of the 6×6 full configuration interaction matrix of H2 in the minimal quantum chemistry basis set20. The photons are coupled into free space optical modes C (control) and R (register) and manipulated by using half-wave plates (λ/2) and quarter-wave plates (λ/4) to implement single-qubit rotations around the Bloch axes, Ry and Rz, as well as Hadamard (H) and Pauli X gate (X) operations. Coincident detection events between single photon counting modules (SPCMs) D1 and D3 (D2 and D3) herald a successful run of the circuit. Panel reproduced from ref. 20. b, Plot of the molecular energies of the different electronic states as a function of interatomic distance obtained with the device to 20 bits of precision using an iterative phase-estimation procedure (IPEA) and a majority-voting scheme as a simple error correction protocol.

  2. Schematic of the photonic quantum simulation of delocalized chemical bonds.
    Figure 2: Schematic of the photonic quantum simulation of delocalized chemical bonds.

    a, Two entangled photon pairs are generated through the process of parametric down-conversion. Superimposing one single photon from each pair at a tunable beam splitter results in quantum interference, such that the measured four-photon coincidences correspond to the ground state, for example of a Heisenberg-interacting spin tetramer. Dependent on the reflectivity of the beam splitter, frustration in valence-bond states or so-called spin-liquid states can be investigated23. b, Future experiments using more entangled photon pairs may allow the study of the ground-state properties of molecular ground states, such as the delocalized bonds in benzene.

  3. Photonic quantum circuits for the simulation of quantum and quantum stochastic walks.
    Figure 3: Photonic quantum circuits for the simulation of quantum and quantum stochastic walks.

    a, The bulk-optics set-up employed to simulate a quantum-stochastic-walk transition between a pure quantum walk and a classical walk21. b, Continuously coupled waveguide arrays were also used to realize correlated-photon quantum walks22. The optical micrograph of a 21-waveguide array shows the three input waveguides on the bottom, bending into the 700-μm-long coupling region, and exiting at the top towards the output ports, where the signal is detected. The output pattern (upper inset) and a simulation of the intensity of laser light propagating in the array (lower inset) are also shown. Panel reproduced with permission from ref. 22, © 2010 AAAS.


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  1. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA

    • Alán Aspuru-Guzik
  2. Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna A-1090, Austria

    • Philip Walther

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