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arXiv:0908.2893v3 [quant-ph] 20 Jan 2010

Truly Random Number Generation Based on Measurement of Phase Noise of Laser

Hong Guo,∗Wenzhuo Tang, Yu Liu, and Wei Wei

CREAM Group, State Key Laboratory of Advanced Optical Communication

Systems and Networks (Peking University) and Institute of Quantum Electronics,

School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China

(Dated: January 20, 2010)

We present a simple approach to realize truly random number generation based on measurement of

the phase noise of a single mode vertical cavity surface emitting laser (VCSEL). The true randomness

of the quantum phase noise originates from the spontaneous emission of photons and the random

bit generation rate is ultimately limited only by the laser linewidth. With the final bit generation

rate of 20 Mbit/s, the physically guaranteed truly random bit sequence passes the three standard

random tests. Moreover, for the first time, a continuously generated random bit sequence up to 14

Gbit is verified by two additional criteria for its true randomness.

PACS numbers: 05.40.-a, 42.55.Px, 42.55.Ah

Random number generator (RNG) has wide applica-

tions in statistical sampling [1], computer simulations [2],

randomized algorithm [3] and cryptography [4]. Tradi-

tionally, pseudorandom number generator (PRNG) based

on computational algorithms is adopted to generate ran-

dom bits and is competent in many fields. However, it

cannot produce truly random (unpredictable and irre-

producible) bit sequence, and so may result in potential

dangers in security related applications, say, in quantum

cryptography [5]. Actually, the unconditional security of

quantum key distribution can ONLY be guaranteed when

a truly random number generator (TRNG), based on

quantum mechanical process instead of the intractabil-

ity assumption with classical algorithms [6], is available.

Distinct from PRNG, a TRNG can only be realized by

a physical way, instead of an algorithm-based way; how-

ever, a physical way does not sufficiently guarantee the

true randomness. The physically random processes, such

as radioactive decay [7], electric noise in circuits [8], fre-

quency jitter of electric oscillator [9], and those based on

laser (photon) emission/detection [10–12], can ensure the

inability of pre-estimation on random numbers and so can

be adopted as candidates to implement TRNG. In partic-

ular, those based on the detection of laser field attracted

tremendous interests in recent decade. Recently, chaotic

laser, with ultra-wide bandwidth, becomes a promising

candidate for GHz random bit generation [13, 14]. How-

ever, since the signal of chaotic laser has a periodicity

originated from the photon round trip time, it is es-

sentially NOT a truly random source. Also, we know,

the chaotic systems are deterministic that looks random

but without inherent randomness [15, 16]. Hence, we

coin this kind of physically-based, rather than algorithm-

based, pseudo RNG as physically pseudorandom number

generator (PPRNG). Thus, the true randomness guar-

anteed by physical principle(s), instead of its generation

rate, should be firstly pursued for a TRNG, otherwise,

even though with ultrahigh rate, it is just a pseudo RNG;

on the other hand, the TRNGs based on the above-

Data

Processing

BS

BS

BS

8‐bit

8-bit

ADC

Mi Mirror

Mirror

Clock

Delay

in?space

τ

VCSEL

FIG. 1: (color online). Schematic setup of TRNG based on

the phase noise measurement using delayed self-homodyne

method. BS, beam splitter; APD, avalanche photodetector

with the low (high) cutoff frequency of 50 kHz (1 GHz). ADC,

8-bit binary analog-digital-converter working at 40 MHz.

mentioned physical mechanisms [7–12] cannot offer the

bit generation rate as high as PPRNG based on chaotic

lasers [13, 14]. So far, the typical maximal random bit

generation rate is around 10 Mbit/s for electric oscillator

jitter measurement scheme [9] and 4 Mbit/s for photon

detection scheme [11]. Also, it should be noted that in

these schemes, the statistical bias and correlation for long

random bit sequence were not investigated.

In this Letter, we propose a new and simple TRNG

scheme based on the true randomness of the quantum

phase noise, which is a Gaussian random variable [17, 18],

of a single-mode vertical cavity surface emitting laser

(VCSEL). The true randomness of the quantum phase

noise is originated from the random nature of sponta-

neous emission and is guaranteed by physical principle.

It will, in the following, be shown that the random bit

generation rate of this TRNG is ultimately limited only

by the laser linewidth. In our experiment, the high final

bit generation rate reaches 20 Mbit/s with guaranteed

true randomness. Further, the true randomness is not

only guaranteed in physical principle and standard tests,

but is also verified by two additional criteria (statistical

bias and correlation) for the long random bit sequence

up to 14 Gbit, for the first time.

The schematic setup is shown in Fig.

delayed self-homodyne method is used to measure the

1 and the

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2

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?

?

-1000 -50005001000

-90

-80

-70

-60

-50

-40

-30

3dBLinewidth

~400MHz

Power (dBm)

Frequency(MHz)

Lorentzian

BeatSpectrum

FIG. 2: (color online).

amplitude) noise of the laser field is observed with (without)

the beat signal. The inset is the power spectral density of the

beat signal. (b) Autocorrelation function of the beat signal

versus time interval. In our experiment, the sampling interval

of 25 ns (40 MHz sampling rate) is chosen.

(a) The quantum phase (classical

phase noise of the VCSEL. In this case, the output al-

ternative current (AC) voltage of the avalanche pho-

todetector (APD) detecting the beat signal is Vph ∝

AC[Ibeat] = 2E(t)E(t +τ)cos[φ(t) −φ(t +τ)], where the

amplitude fluctuations of E(t) and E(t + τ) are negligi-

ble compared to the phase fluctuation corresponding to

cos[φ(t)−φ(t+τ)] [17, 18]. When the delay time is much

longer than the coherence time of laser (i.e., τ ≫ τcoh),

the phase difference ∆φ(t) = φ(t)−φ(t+τ) is a Gaussian

random variable, and thus the autocorrelation function

of the electric field of the laser is eliminated [17], i.e.,

?E∗(t)E(t + τ)? ∝ exp(−|τ|/τcoh) → 0,

where τcoh= (π∆νlaser)−1[18], and ∆νlaser is the laser

linewidth. This indicates that the electric field ampli-

tudes of the laser at different time are mutually indepen-

dent, if time interval is much longer than the coherence

time of the laser. Further, similar calculation procedure

can be applied to obtain the autocorrelation function

of the beat signal [Ebeat(t)] as ?E∗

where ∆t is the sampling interval for original random

(1)

beat(t)Ebeat(t + ∆t)?,

050100150200

-150

-100

-50

0

50

100

150

1

0

X

X

X

0

1

V(mV)

Time(ns)

10GHz

40MHz

X

FIG. 3: (color online). A 200 ns trace of the APD-detected

voltages of the beat signal (small black dots) is recorded at

10 GHz, while the random signal (big red dots) is sampled

at 40 MHz rate (25 ns interval).

obtained from the least significant bit (LSB, i.e., its parity) of

a sequence of 8-bit binary derivatives obtained by performing

subtraction between two consecutive sampled voltages (shown

in the bottom strip).

The final random bit is

bit generation. Using Ebeat(t) = E(t) + E(t + τ) and

Eq. (1), it is evident that when the sampling time ∆t

meets ∆t ≫ τ + τcoh, the autocorrelation of the beat

signal will also be eliminated. Thus, the bits extracted

from the beat signal are mutually independent and can

be adopted to implement TRNG.

In experiment (Fig. 1), a 795 nm VCSEL laser works

at 1.5 mA, a little above the threshold current 1.0 mA.

The laser linewidth ∆νlaser= 200 MHz (τcoh= 1.59 ns)

of laser is inversely proportional to the laser power, while

the classical noises are independent on it [25]. Due to

working just above threshold, the quantum phase noise

of laser dominates over the classical amplitude noise to

ensure the true randomness of generated bits. The delay

time τ is set to be about 10 ns (corresponds to 3.0 m

space delay) in order to fulfill τ ≫ τcoh. So, the self-

homodyne method with delay time τ is used to obtain the

beat signal with 3 dB linewidth of 400 MHz (detected by

an APD) and its power spectral density is shown in the

inset of Fig. 2 (a). It can be seen, from Fig. 2 (a), that

the classical amplitude fluctuation is negligible compared

to the quantum phase fluctuation within 200 MHz (the

gap is about 20 dB). Using Wiener-Khintchine theorem

[19, 20], i.e.,

Rbeat(t) =

?+∞

−∞

Pbeat(ω)exp(−iωt)dω,(2)

the autocorrelation function [Rbeat(t)] of the beat signal

is obtained from the power spectral density of the phase

noise [Pbeat(ω) in Fig. 2 (a)] and illustrated in Fig. 2 (b).

It can be seen from Fig. 2 (b) that the autocorrelation

of the beat signal can be ignored, if the sampling interval

is set as ∆t ≫ τ +τcoh. In our experiment, the sampling

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B =|0.5-p(1)|

1.5/ N

(a)

1

|a |

3/ N

(b)

FIG. 4: (color online). (a) The statistical bias (B) of the final

random bit sequence. It can be seen that B < 1.5/√N always

holds and converges to zero for large bit sequence, where p(1)

is the probability of ones in sequence. (b) The absolute value

of the first-order correlation coefficient |a1| of the final random

bit sequence. It can be seen that |a1| < 3/√N always holds

and |a1| converges to zero for large bit sequence.

rate is chosen as 40 MHz accordingly, i.e., ∆t = 25 ns, so

the bits extracted from these sampled voltages are mutu-

ally independent. These sampled voltages are digitized

by an 8-bit analog-digital-converter (ADC) shown as the

red dots in Fig. 3, and further we have confirmed that the

distribution of voltages is symmetric. Hence, we take the

least significant bit (LSB) of each sampled 8-bit voltage

as the original random bit, i.e., the parity of this 8-bit bi-

nary number, which represents whether the voltage falls

in an even or odd bin of the total 256 bins. For the non-

ideal distribution of these voltages, the probability of all

the even and odd bins of the total 256 bins are not per-

fectly equal, and the bit sequence shows a statistical bias

of the order of 10−3. For much lower bias, we perform

a subtraction between two consecutive sampled voltages

to obtain a sequence of N/2 8-bit binary derivatives as

V2− V1,V4− V3,...,VN− VN−1, where N is the total

number of the original sampled voltages. In this process,

each voltage is used only once, and thus no correlation

is introduced. After that, we adopt the LSB of the 8-

bit binary derivatives to generate the final random bits.

TABLE I: Results of Diehard statistical test suite. Data sam-

ple containing 100 Mbits is used for the Diehard test. For the

cases of multiple p-values, a Kolmogorov-Smirnov (KS) test is

used to obtain a final P-value, which measures the uniformity

of the multiple p-values. The test is considered successful if

all final P-values satisfy 0.01 ≤ P ≤ 0.99.

Statistical test

Birthday spacings

Overlapping permutations

Ranks of 31 × 31 matrices

Ranks of 32 × 32 matrices

Ranks of 6 × 8 matrices

Monkey tests on 20-bit words

Monkey test OPSO

Monkey test OQSO

Monkey test DNA

Count 1’s in stream of bytes

Count 1’s in specific bytes

Parking lot test

Minimum distance test

Random spheres test

Squeeze test

Overlapping sums test

Runs test (up)

Runs test (down)

Craps test No. of wins

Craps test throws/game

P-value

0.910531 [KS]

0.294899

0.322213

0.482575

0.749427 [KS]

0.019887 [KS]

0.079864 [KS]

0.725649 [KS]

0.293543 [KS]

0.244463

0.062188 [KS]

0.806898 [KS]

0.326209 [KS]

0.902946 [KS]

0.815876 [KS]

0.806025 [KS]

0.817356

0.805323

0.502035

0.403322

Result

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Therefore, we directly obtained the final random bit at

generation rate of 20 Mbit/s with a software-based post-

processing. Note that, the post-processing enhances the

performance of the random bits sequence by lowering the

statistical bias while not introducing any additional cor-

relations. For a TRNG, both the statistical bias and the

absolute value of the first-order correlation coefficient of

the final random bit sequence are expected to be smaller

than three standard deviations (3σ1= 1.5/√N for sta-

tistical bias [Fig. 4(a)], and 3σ2= 3/√N for correlation

coefficient [Fig. 4(b)]) with the probability of 99.7%. In

our case, both criteria are well satisfied for the final ran-

dom bit sequence up to 14 Gbit.

We continuously record a final random bit sequence of

1 Gbit, which passed three standard random tests, i.e.,

ENT [21], Diehard [22] and STS [23]. The ENT results

are: Entropy = 1.000000 bit per bit (the optimum com-

pression would reduce the bit file by 0%). χ2distribution

is 0.53 (randomly would exceed this value by 46.62% of

the times). Arithmetic mean value of data bits is 0.5000.

Monte Carlo value for π is 3.141725650. Serial correla-

tion coefficient is −0.000017. The Diehard and STS test

results are shown in Tables I and II, respectively.

It should be noted that, for a nonuniform distribution

of the probability of 256 8-bit binary derivatives, if more

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TABLE II: Results of NIST statistical test suite. Using 1000

samples of 1 Mbits data and significance level a = 0.01,

for “Success”, the P-value (uniformity of p-values) should

be larger than 0.0001 and the proportion should be greater

than 0.9805608 [23]. For the tests which produce multiple P-

values and proportions, the worst case is shown. As advised

by NIST, the Fast Fourier Transform test is disregarded [24].

Statistical test

Frequency

Block frequency

Cumulative sums

Runs

Longest run

Rank

Nonperiodic

Overlapping

Universal

Approximate

Random excursions

Random variant

Serial

Linear complexity

P-value

0.679846

0.248571

0.858032

0.816029

0.648795

0.609895

0.569334

0.565500

0.143336

0.590520

0.016388

0.029796

0.946683

0.732979

Proportion

0.9916

0.9897

0.9888

0.9907

0.9935

0.9860

0.9823

0.9916

0.9888

0.9879

0.9880

0.9865

0.9916

0.9915

Result

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

Success

than 1 bit are extracted from each 8-bit binary deriva-

tives in order to improve the random bit generation rate

(see, e.g., 5 LSBs are adopted in [14]), an additive corre-

lation in the final random bit sequence will be introduced,

even though this additive correlation is not so significant

to fail the random tests. Taking 5 LSBs for an instance,

every set of the 5 LSBs possesses a different probability

(due to the nonuniform distribution) and thus these 5 bits

from the same set are correlated to some extent. How-

ever, with this additive correlation within the same set,

both the random bit sequence of extracting 5 LSBs (with

the sampling rate of 2.5 GHz in [14]) and 4 LSBs (with

the sampling rate of 40 MHz in our case) from an 8-bit

binary number both successfully pass the three standard

random tests. This fact also indicates that the standard

random tests are only a way to examine whether the ran-

dom bit stream is “sufficiently” random, but not to judge

whether it is truly random.

We propose a new and simple approach to realize a

high-speed TRNG, which is compact and convenient to

implement. The randomness of our TRNG is physically

guaranteed by the intrinsic random nature of the quan-

tum phase noise originated from the spontaneous emis-

sion of photons. Moreover, for the first time, the true

randomness is verified by both the statistical bias and

the correlation coefficient for long random bit sequence

up to 14 Gbit. Note that, the long random bit sequence is

even more important than generation rate, because it is

the length of the random bit sequence that is required in

most applications and essentially, it is a metric for qual-

ifying the true randomness. Compared to the chaotic

laser, the intrinsic phase noise of a free-running laser is

confirmed in true randomness, which only depends on its

inherent quantum mechanical properties and does not

need the external optical feedback to laser thereby intro-

ducing photon round trip period. Although the random

bit generation rate is not as high as that of chaotic laser

scheme [13, 14], its physically guaranteed true random-

ness and high generation rate, together with its simplicity

and compactness, are attractive for applications which

need true randomness. Also, a higher generation rate is

attainable using a laser with larger linewidth and faster

data acquisition hardware.

This work is supported by the Key Project of National

Natural Science Foundation of China (NSFC) (grant

60837004). We acknowledge the support from W. Jiang,

K. Deng, X. X. Liu, C. Zhou, and G. D. Xie, and the

helpful discussions with B. Luo and J. B. Chen.

∗Corresponding author: hongguo@pku.edu.cn

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