OSA/ CLEO 2011
QThT6.pdf
Two-way Secure Communication Using Quantum Illumination Maria Tengner, Tian Zhong, Franco N. C. Wong, and Jeffrey H. Shapiro Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
[email protected]
Abstract: A two-way entanglement-based communication protocol resilient to high loss and noise is implemented. Even though the entanglement is lost during transmission, efficient communication that is secure against passive eavesdropping is possible. c 2011 Optical Society of America
OCIS codes: (270.5565) Quantum communications; (060.4510) Optical communications
1.
Introduction
Entanglement is a key resource in many quantum information applications. However, in general, it is fragile and does not do well in an environment of significant loss and noise. Recently the concept of quantum illumination (QI) was proposed, in which entanglement is used to enhance measurement sensitivity under high-loss, high-noise operating conditions [1, 2]. Based on the idea of quantum illumination, a novel two-way secure optical communication protocol capable of defeating passive eavesdropping was suggested [3]. Here, we report on our experimental implementation of the two-way secure communication protocol. In this QI-based communication protocol, Alice generates multimode quadrature-entangled signal and idler light beams using spontaneous parametric downconversion (SPDC). She sends the signal beam to Bob over a lossy channel while retaining the idler. Each T -sec-long transmission (one bit) from Alice comprises M = W T 1 signal-idler mode pairs, where W is the bandwidth of the signal and idler fields. Each of these modes has a mean photon number NS = NI 1. Bob encodes the desired information by modulating the received signal phase using binary phase-shift keying (BPSK). He then amplifies the signal—to compensate for loss and add a significant amount of noise—before sending it back to Alice, again over a lossy channel. Alice makes a joint measurement on the returned signal (plus noise) and the retained idler to extract Bob’s information. Assuming that an eavesdropper Eve obtains all the photons lost en route from Alice to Bob and from Bob to Alice, and that Eve has access to an optimal quantum receiver while Alice only has access to a receiver we know how to build [4], Alice still enjoys several orders of magnitude better error probability than Eve [3]. This advantage originates from the stronger-than-classical phase-sensitive cross correlation between the signal and idler created by the SPDC source, which gives enhanced sensitivity despite the fact that the entanglement between the returned signal and the idler has been destroyed by loss and noise. 2.
Experimental implementation
Our experimental implementation of this protocol is shown in Fig. 1. We generate the multi-temporal-mode entangled signal and idler by continuous-wave (cw) SPDC in a 20-mm-long type-0 phase-matched periodically poled MgOdoped lithium niobate (PP-MgO:LN) bulk crystal. The pump, at 780 nm, is focused to obtain optimal single-mode fiber coupling of the SPDC emission. This emission is broadband (∼100 nm) and, after fiber coupling, separated by a coarse wavelength division multiplexer (CWDM) into signal and idler beams centered at 1550 and 1570 nm, respectively. For ∼105 mW of pump power we have measured 179 pW of signal and 186 pW of idler at the CWDM output. The 16 nm channel bandwidth of the CWDM defines the operating bandwidth W . As an example, a bit duration T = 2.5 µs (400 kbit/s) results in M = 5 × 106 temporal modes, with a mean photon number per mode NS = 0.0006. Bob encodes his message via BPSK with a waveguide-based phase modulator. In our initial measurements we use a square-wave phase modulation at 20 kHz. The phase modulated signal is then amplified with a 40-dB-gain erbium-doped fiber amplifier (EDFA) that also injects noise into each signal mode with a mean noise photon number NB = 104 . Note that the noise is several orders of magnitude larger than the signal. The amplified signal, with noise, is returned to Alice where the signal and idler are recombined with a second CWDM. The joint measurement of the signal and idler is performed with a low-gain optical parametric amplifier (OPA) followed by direct detection of the idler [4]. The phase-sensitive OPA converts the phase modulation of the input signal to an amplitude modulation of the output idler. The OPA consists of a second PP-MgO:LN crystal that is
OSA/ CLEO 2011
QThT6.pdf
Fig. 1. Experimental setup for quantum illumination-based secure communication. DM: dichroic mirror, FC: fiber coupling, HWP: half-wave plate, QWP: quarter-wave plate, PBS: polarizing beam splitter, BS: beam splitter.
identical to that used for SPDC. After the OPA, the signal and pump are filtered to allow the weak idler (nW of power) to be directly detected. We use a low-noise InGaAs avalanche photodiode (APD) at a gain of ∼7 followed by a low-noise current amplifier. At 2 nW of input optical power, we have measured a shot-noise level that is 18 dB above the electronic noise of the InGaAs detector and current amplifier combination. At the input to the OPA we have ∼100 pW of idler power and an estimated signal of ∼200 nW embedded in 1.6 mW of noise power. With a pump power of 3 mW, giving an OPA gain of 1.0000187, we obtain ∼15 nW of generated idler. The signal phase modulation results in a ∼0.3 nW peak-to-peak modulation of the idler power (see Fig. 2). When, at the input to the OPA, either the signal or the idler is not present, or if the timing between the two is incorrect, the modulation of the generated idler disappears. The measured idler modulation depth is ∼10 times smaller than in the ideal, theoretical case due to dispersion in the fibers. In this talk we will discuss the use of dispersion compensation to improve the signal-to-noise ratio, and the implementation of passive eavesdropping to verify the security of the two-way protocol.
Fig. 2. Resulting intensity idler modulation measured by Alice’s detector from a 20 kHz square-wave phase modulation of the signal by Bob.
References 1. S. Lloyd, “Enhanced sensitivity of photodetection via quantum illumination,” Science 321, 1463–1465 (2008). 2. S.-H. Tan, B. I. Erkman, V. Giovannetti, S. Guha, S. Lloyd, L. Maccone, S. Pirandola, and J. H. Shapiro, “Quantum illumination with Gaussian states,” Phys. Rev. Lett. 101, 253601 (2008). 3. J. H. Shapiro, “Defeating passive eavesdropping with quantum illumination,” Phys. Rev. A 80, 022320 (2009). 4. S. Guha and B. I. Erkman, “Gaussian-state quantum-illumination receivers for target detection,” Phys. Rev. A 80, 052310 (2009).
