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arXiv:1008.4879 · Physical Review Letters

Ultrafast Switching of Photonic Entanglement

Matthew Hall, Joseph B. Altepeter, Prem Kumar

Published Feb 1, 2011• 116 citations
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Abstract

To deploy and operate a quantum network which utilizes existing telecommunications infrastructure, it is necessary to be able to route entangled photons at high speeds, with minimal loss and signal-band noise, and--most importantly--without disturbing the photons' quantum state. Here we present a switch which fulfills these requirements and characterize its performance at the single photon level. Furthermore, because this type of switch couples the temporal and spatial degrees of freedom, it provides an important new tool with which to encode multiple-qubit states in a single photon. As a proof-of-principle demonstration of this capability, we demultiplex a single quantum channel from a dual-channel, time-division-multiplexed entangled photon stream, effectively performing a controlled-bit-flip on a two-qubit subspace of a five-qubit, two-photon state.

DOI
10.1103/physrevlett.106.053901

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To deploy and operate a quantum network which utilizes existing telecommunications infrastructure, it is necessary to be able to route entangled photons at high speeds, with minimal loss and signal-band noise, and--most importantly--without disturbing the photons' quantum state. Here we present a switch which fulfills these requirements and characterize its performance at the single photon level. Furthermore, because this type of switch couples the temporal and spatial degrees of freedom, it provides an important new tool with which to encode multiple-qubit states in a single photon. As a proof-of-principle demonstration of this capability, we demultiplex a single quantum channel from a dual-channel, time-division-multiplexed entangled photon stream, effectively performing a controlled-bit-flip on a two-qubit subspace of a five-qubit, two-photon state.
Future quantum optical networks will require the ability to route entangled photons at high speeds, with minimal loss and added in-band noise, and---most importantly---without disturbing the photons' quantum state. Here we present an all-optical switch which fulfills these requirements and characterize its performance at the single photon level. It exhibits a 200-ps switching window, 120:1 contrast, 1.5-dB loss, and induces no measurable degradation in the switched photons' entangled-state fidelity (< 0.002). As a proof-of-principle demonstration of its capability, we use the switch to demultiplex a single quantum channel from a dual-channel, time-division-multiplexed entangled photon stream. Furthermore, because this type of switch couples the temporal and spatial degrees of freedom, it provides an important new tool with which to encode multiple-qubit quantum states on a single photon.
As the field of quantum communications matures and adopts larger network topologies, it will require switches capable of multiplexing and demultiplexing entangled photons between network nodes. Such switches will need to operate at high speeds, with low loss, and minimal signal-band noise, while also retaining the quantum state of the routed photons. We present an all-optical, fiber-based, dual-in, dual-out switch that has a switching window of 45 ps and an insertion loss of <;3.0 dB. In addition, the switch introduces minimal degradation to the quantum state of the routed entangled photons. As a test of the switch's utility, we demultiplex a single quantum channel from a 6.5-GHz dual-channel quantum data stream. The recovered quantum state exhibits a fidelity of 96.2%±1.2% with the prepared state.
In this dissertation, we describe, characterize, and demonstrate the operation of a dual-in, dual-out, all-optical, fiber-based quantum switch. This "cross-bar" switch is particularly useful for applications in quantum information processing because of its low-loss, high-speed, low-noise, and quantum-state-retention properties. Building upon on our lab's prior development of an ultrafast demultiplexer [1-3] , the new cross-bar switch can be used as a tunable multiplexer and demultiplexer. In addition to this more functional geometry, we present results demonstrating faster performance with a switching window of ≈45 ps, corresponding to >20-GHz switching rates. We show a switching fidelity of >98%, i. e., switched polarization-encoded photonic qubits are virtually identical to unswitched photonic qubits. We also demonstrate the ability to select one channel from a two-channel quantum data stream with the state of the measured (recovered) quantum channel having >96% relative fidelity with the state of that channel transmitted alone. We separate the two channels of the quantum data stream by 155 ps, corresponding to a 6.5-GHz datastream. Finally, we describe, develop, and demonstrate an application that utilizes the switch's higher-speed, lower-loss, and spatio-temporal-encoding features to perform quantum state tomographies on entangled states in higher-dimensional Hilbert spaces. Since many previous demonstrations show bipartite entanglement of two-level systems, we define "higher" as d > 2 where d represents the dimensionality of a photon. We show that we can generate and measure time-bin-entangled, two-photon, qutrit (d = 3) and ququat (d = 4) states with >85% and >64% fidelity to an ideal maximally entangled state, respectively. Such higher-dimensional states have applications in dense coding [4] , loophole-free tests of nonlocality [5] , simplifying quantum logic gates [6] , and increasing tolerance to noise and loss for quantum information processing [7] .
A crucial component of photonic quantum information processing platforms is the ability to modulate, route, convert, and switch quantum states of light noiselessly with low insertion loss. For instance, a high-speed, low-loss optical switch is crucial for scaling quantum photonic systems that rely on measurement-based feed-forward approaches. Such a device will also ideally be capable of operating on photon-number states, which can act as non-Gaussian resources. Here, we demonstrate ultrafast all-optical switching of heralded photon-number states, of up to 6 photons, using the optical Kerr effect in a single-mode fiber. A local birefringence is created by a high-intensity pump pulse at a center wavelength of 1030 nm which overlaps temporally with the 1550 nm photons in the fiber. A switching efficiency of $>$99 % is reached with a resolution of 2.3 ps, an insertion loss of 2.27±0.082.27\pm0.082.27±0.08 dB, and a signal-to-noise ratio of 32,000.